Sprinkler systems need flow and pressure to operate. Sometimes you might need to know the flow and pressure requirements of an existing sprinkler system. For example the flow and pressure are needed if adding on to the existing irrigation system or adding or replacing a pump. This article contains instructions on how to determine those values. This article uses the USA measurements GPM for flow and PSI for pressure, however conversion to metric is easy using standard conversion formulas.
How to Reverse Engineer the Flow (GPM)
This is going to take some time and a lot of crawling around on your hands and knees… sorry, that’s just how it is.
Look to see the sprinkler brand and which nozzles are installed in the existing sprinklers on a single valve circuit. Write them all down. You may have to look very close to find the nozzle number, it is usually imprinted using tiny text on the nozzle itself. It may be right next to where the water comes out, or it may be on the top of the nozzle. It is often hard to see. Each sprinkler may have a different nozzle, so you will need to look at every one of them and write them all down. Rotor type sprinklers (streams of water that rotate) tend to have numbered nozzles, #1, #3, #9, etc.. For rotors you often need to pull the pop-up riser up from the sprinkler body to see the nozzle outlet and the number. Spray type sprinklers (steady spray like a shower head) tend to have a number followed by a letter that indicates the arc, like 10F, 12H or 15Q.
Look up the flow requirement (GPM) for each sprinkler and nozzle using charts you should be able to find on the sprinkler manufacturer’s website. Add all the GPM values together to determine the total GPM for the valve circuit. For older discontinued sprinkler models you may have to contact the manufacturer’s consumer help department and ask them to email you performance charts.
The sprinkler manufacturer’s website will probably give you a chart that shows different flow values for the nozzles depending on the “PSI”. To determine the PSI to use measure the distance between adjacent sprinklers in feet. Measure several and determine the average distance. Now use that average distance between sprinklers as the “PSI” value– but do NOT use a value less than 30. Example: If the average distance between adjacent sprinklers is 45 feet, use 45 PSI to find the GPM in the chart. If the average distance between sprinklers is 25 feet, use 30 PSI. Do not use less than 30 PSI!
Repeat for each valve circuit.
Assuming you run one valve circuit at a time, your flow requirement for the pump will be the same as the single valve circuit with the highest flow value (GPM.) If you run more than one circuit at a time, add together the GPM values for the ones you run together as they essentially become one big circuit when run together.
To recap, calculate the total GPM of each group of sprinklers that run at the same time. The flow (GPM) value to use when determining the pump size that of the group of sprinklers that has the highest total flow (GPM).
Determining the pressure requirements can be difficult. First off, pump pressure in the USA is measured in either “Feet of Head” (Ft.Hd.) or “PSI”. (The rest of the world uses “bars” but this article is USA based. ) We’ll talk more about this later, but for now you can use either “Ft.Hd.” or “PSI.” You can switch back and forth between these two values using simple conversion formulas:
____ PSI x 2.31 = ____ Feet of Head (ft.hd.)
____ Feet of Head x 0.433 = ____ PSI
How to Reverse Engineer the Pressure
(3 different methods)
One quick rule of thumb- too much water pressure is much easier a problem to deal with than too little pressure. If you are uncertain at all, always go with the higher value. It is very easy to throttle down the pressure if you have too much, if you don’t have enough pressure it is very hard and expensive to increase the pressure. So if you are looking at a sprinkler head and asking yourself: “should this sprinkler head use 30 PSI or 35 PSI, I’m just not sure?”, the answer is easy, use the higher pressure, 35 PSI.
Method #1: Measure It with A Gauge.
If the irrigation system is still operational install a pressure gauge on the pipe as close as possible to where the water source, turn on the system, and the gauge will tell you the pressure the existing system uses. You may need to cut the water supply pipe and install a tee for the gauge to connect to, or possibly you can drill a hole in the pipe, thread the hole with a 1/4″ NPT thread tap, and then screw the gauge into the hole (most gauges have 1/4″ male NPT thread connections on them, but you might want to make sure before you drill and tap!)
If the irrigation system uses a pump to run it you typically would measure the pressure within a foot or two of the pump outlet. Remember that if you do have an existing older pump the pressure you measure may be lower than it should be due to the pump being tired, old, and worn out. You might want to add another 10 PSI to the pressure you measure at the pump to compensate for age.
Method #2: Calculate the Pressure losses.
If you can’t take a pressure reading on the existing system, the next best method is to completely redo all the calculations for the sprinkler system. Let’s be honest, this is too difficult and time consuming for most people, but it is the best way. If you want to try it you need to read through the Sprinkler System Design Tutorial to learn how pressure requirements for a sprinkler system are calculated. You should be able to then reverse engineer your existing sprinkler system by calculating the pressure loss in each section of pipe, each valve, each sprinkler head, etc. to figure out the PSI it requires to operate. You may have to dig up some pipes to determine what size they are. It is a lot of work, you will have to basically learn how to design irrigation systems to do it. Seriously, it is unlikely you are going to do this unless you are an engineer or just very anal-retentive… let’s move on.
Method #3: Guesstimate It.
If you really can’t figure out the pressure needed you can use a guesstimated pressure value. Most of you are likely to use this method. Obviously this method is not optimal and there are no guarantees it will work but it’s a lot less effort that the first two methods above. The idea here is to guesstimate high as previously mentioned in the introduction. Not sure? Add 5 PSI! Really feeling uncertain? Add 10 PSI. You get the idea. Instead of measuring with a pressure gauge, the guesstimate method is going to require you to grab a tape measure (which most people have) and you won’t need to cut into any pipes or dig anything up.
FYI: This guesstimate method is based on some hydraulic principles that define the water pressure needed at a sprinkler head in order to shoot water a specific distance, along with some very rough assumptions of how much pressure is typically needed to move water through all the various pipes, valves, etc.
Guesstimate formula: Sprinkler spacing in FEET + 30 = guesstimated PSI required at water source.
If the resulting total is less than 50, use 50. Do not use a guesstimated water pressure value less than 50 PSI !
To guesstimate the pressure needed take the largest distance in feet between adjacent sprinkler heads, and add 30 to it. If the answer is less than 50, use 50. Do not use a pressure lower than 50, very few sprinkler systems will function well at a pressure below 50 PSI. Which sprinkler heads to measure? Generally you should measure and write down the distance between all your sprinkler heads that are next to each other. It’s not uncommon to have two heads that are way further apart than all the others. If so you can choose to ignore that set of sprinklers. The examples below will help this make more sense.
Example: let’s say you have 6 sprinklers and you measure the distances between the adjacent ones and you get 50′, 32′, 29′, 33′, 35′, and 29′. In this case 50′ is way out of line with the other values so let’s ignore it and use the next greatest distance, which is 35′. So then 35′ + 30 = 65 PSI. So in this case 65 PSI is the guesstimated pressure required for the system. Understand there may be a dry spot between those two heads that are 50′ apart (there probably already is a dry spot there!) That 50′ distance is really a design error on the original sprinkler system and there should probably be another sprinkler head in the middle of that space.
More Guesstimate Method Examples:
Sprinklers 15 feet apart: 15 + 30 = 45 PSI. This is less the 50, so use 50 PSI. Sprinklers 35 feet apart: 35 + 30 = 65 PSI. Use 65 PSI. Sprinklers 45 feet apart: 45 + 30 = 75 PSI. Use 75 PSI.
Remember, 50 PSI is the minimum!
Remember: No guarantees, this method gives you a guesstimate! You understand that you are taking a risk using this. Buying a pump? It is strongly recommended that if you guesstimate the pressure you buy your pump from someplace with a generous return/exchange policy. You may need to return or exchange the pump.
If you have an irrigation system that currently has a pump on it, and everything has worked good right up to the point the existing pump died, then your best bet is to replace the pump with the exact same model as the old one. This is a much, much easier process than trying to find a different make or model of pump. However if the old pump wasn’t really getting the job done, or isn’t available anymore, this is a good time to reevaluate your pump needs. Perhaps there is a better pump out there for your system. This tutorial will teach you how to do that.
There are numerous types of pumps designed for various purposes. Pumps commonly used for irrigation fall into two broad categories: Displacement Pumps and Centrifugal Pumps. Within those categories there are sub-categories that further define the type of pump. This tutorial will focus on those types of pumps most often used for irrigation.
STOP! If you mess this up you can ruin your pump! Read this! Most do-it-yourself sprinkler design tutorials are not suitable for rural irrigation systems that use pumps!!! Be very careful if you are also looking at one of them for guidance. The methods they use to determine water supply are little better than taking a wild guess. You’ll understand after reading this page.
If you are going to use this tutorial (and you should!) you need to use ONLY this tutorial. If not, you are going to be in big trouble. A large number of questions I get from people who are confused are caused because they are trying to mix together this tutorial with some other guide or tutorial, or maybe it was advice they got from some well-meaning, but clueless, salesman. Even many professional irrigation contractors do not understand the hydraulics of systems using pumps! Use this tutorial exclusively and save yourself a lot of grief and get a professional quality sprinkler system. Please, please, please! Thank you. You are helping save my sanity. Now on with the tutorial.
Sorry this is such a long page, but pumps are tricky. There is a lot you need to know. You can really mess up big time with a pump and an irrigation system if you do it wrong. So please be patient, read carefully, then reread it all again. Try working the examples, it helps the old brain to kick in!
None of that domesticated city water for you! You have your own pump and water source! That may be a well, river, pond, lake, or the ocean. (Ocean water? Are you growing seaweed?) Unfortunately, pumps are tricky and so you’re going to have to do a bit more work than you would if you had a “City Slicker” water supply. If you have a well, you have probably been told at some time or another that you have a “XX GPM well”. Do NOT rely on that figure! That is most likely the capacity of your well, not the output of your pump, and there is a big difference between the two. There may also be a GPM & PSI noted someplace on the pump, pump panel, literature, or the box the pump came in. Same deal. Don’t rely on these numbers; they assume optimum conditions that exist pretty much only in the pump company’s test facility. You’re going to have to do a little research, do some tests, probably get wet, and do a few calculations. I don’t recommend calling your pump and well company and asking them for the GPM of your well or water system. While some have given me a correct figure, more have given me an incorrect one! It’s not that they want to lie to you, it’s just that there are several terms here that can be easily confused. If the figure they give you is too high, your irrigation system will not work, period. If it is too low you will burn up your pump. I have written a complete tutorial on pumps and related equipment. I strongly suggest the Pump Tutorial if you would like to know more about your pump system, or if you want to know more of the reasoning behind what follows on this page. Click here to go to the Pump Tutorial.
Planning to buy a pump or install a well, but don’t have it yet? First, if you have a well, you will need to know the GPM of the well itself. That’s not how much you will pump out of it, that’s the maximum the well will provide. Your well driller should have measured this when the well was drilled, if not you will need to have the well tested by a well drilling company. They will install a temporary pump in the well to test the output. Then you need to select a random Design Flow and Design Pressure. I suggest 20 GPM per acre and 50 PSI, as those are good starting values. Example: If you have a 2.5 acre mini-ranch you would want to use 50 GPM at 50 PSI. Now proceed with a trial irrigation design using those values. Once you have finished the trial design, you will have discovered whether those are good values for your sprinkler system. If not, what would be? Redesign it if you have to. I know, that’s a lot of work, I agree, but it is far easier to redesign it on paper than to try to fix it after it is installed! Once you have established a good Design Flow and Design Pressure it’s time to shop for a pump. Remember the Design Pressure is at the irrigation connection point, the pump will also need additional pressure to lift the water from the well or pond (see the pump tutorial.) After you have your pump installed and running, test it using the Wet Method below. Then create your final sprinkler design based on those results. I know this sounds like a lot of extra effort, but the result will be an almost perfectly matched pump and sprinkler system. It will be worth the effort! You should absolutely read the Pump Tutorial.
So-called “Sprinkler Pumps” and packaged pumps. Several retail hardware store chains sell what they call “sprinkler pumps”. They are also sold on a number of web sites and, of course, Ebay. At least one of them even calls their pump a “High Pressure Sprinkler Pump.” Sorry, but not by any stretch of the imagination! Typically these pumps do not provide enough water pressure for a standard sprinkler system, especially if you are on a hilly site or pumping from a water source that is more than 10 feet below the pump. (See the paragraph above for suggested pressures.) These pumps will operate small sprinkler heads on a level lot, using water from a shallow well. Few people outside of Florida have this situation. These pumps are really best suited for use as booster pumps. In general, they do not produce enough water pressure for automatic sprinkler systems. Also watch out for packaged pumps that say they are rated X GPM and X PSI on the box (insert any values for “X”.) Often these two performance figures are both the maximums possible for the pump, which may be accurate, but is also misleading. When a pump is operating at it’s maximum GPM, it will also be at the minimum PSI. There is an inverse relationship between the two values. With pumps (and this applies only to pumps) as the output pressure (PSI) goes up, the output flow (GPM) goes down. Often the figure they give you on the box is the maximum possible for each value. (Read the Pump Tutorial.) Just be very careful. Design your sprinkler system FIRST, then buy your pump! If your calculations say you will likely need a 1.5 HP pump, and you find a 1/2 HP pump at the store that says it will do the job, be very suspicious.
Measuring Pump Output:
By the way, this looks complicated but it’s really pretty simple if you take it step-by-step! There are two ways to do this; The Dry Method and The Wet Method. I suggest you read through both of the methods, and try both of them if possible.
The Wet Method is more accurate (but only when done correctly.) The advantage to the Dry Method is that it gives you a chance to check the condition of your pump. If the Dry Method results in a much higher GPM and flow than the Wet Method, then it probably means your pump is worn out. Consider replacing your pump.
If you think you may be getting a new pump in the next 5 years, wait until you have the new pump installed before designing your sprinkler system! A new pump will likely have much better performance than the old one. If you designed your sprinkler system for the old pump then it will not work well with the new one, and may even damage the new pump!
Submersible Pumps. Many people have submersible pumps. In those cases the pump is located down inside the well, rather than on top of the well as in the diagram above. All the calculations work the same regardless of where the pump is.
First you need the horsepower of your pump. This may be stamped on the well, on the pump, on the pump panel, or your pump company should have a record of it. You may notice a GPM and PSI stamped on the pump plate also. I’ll say it again because it’s important: don’t rely on these numbers!
(A) Enter your pump horsepower on the Design Data Form.
What if you don’t have a well? If you don’t pump out of a well, substitute river, lake, pond, spring, mud-puddle, or whatever for “well” in the following procedures. In this case “water level when pump is running” is the lowest expected water level in your pond, stream, etc. (i.e.; the level the water would be in a really dry year.) You obviously also don’t have a “top of well”, so for a submersible pump when the tutorial mentions the “top of well” you would use the highest possible water level of your water source. If the pump is mounted above the water level (non-submersible), then you will need to substitute the actual pump location for “top of well” in this tutorial.
When using a non-submersible pump (any pump not below the water level) it is very important that the pump be installed as close to the water surface level as possible. Pumps are made to push water, not to pull it. The farther and higher the pump has to pull the water, the less efficient the pump will be. Some pumps work better than others in this situation, but in general expect trouble if the pump is more than 10 feet higher than the water surface. The higher your elevation is above sea level, the closer the pump needs to be to the water level. Therefore, in Denver, Colorado, the “Mile High City”, you need to have your pump very close to the water surface (or better yet, use a submersible pump.) Also, avoid long intake pipes between the water and the pump. Long intake pipes/manifolds can also hurt the pump performance. Plus, a long intake pipe is more likely to have a leak in it, and a leak in the intake pipe can cause your pump to lose prime. Losing the pump prime is a real pain in the rear, and if you automate your system it can cause serious damage to the pump. The bottom line here is to keep the pump as close to the water as possible.
Now you need to find out the “Dynamic Water Depth” of the water in your well. Your pump company may have a record of this, however you really should have the well “sounded” to get a new reading, especially if the well is more than 5 years old. Water levels often drop over time. As a last resort you can use the pump depth or well depth, but if you do, you may experience expensive pump problems later. Better to sound the well now. This is something your pump company can do for you and in most cases, is relatively easy and inexpensive. If you don’t have a well (you have a pond, creek, etc.,) use the lowest “dry year” water level of your water supply.
As you can see from the diagram above, the Dynamic Water Depth is the distance in feet between the top of the well and the water level in the well when the pump is running (dynamic means moving, as in the water is moving when the depth is measured.) It is important that the pump be running when this is measured. This is because when the pump runs, the water level in the well drops. The distance it drops is known as “draw-down”. The further the pump must lift the water, the more energy it takes. So as the water level gets deeper, the pump will produce less water pressure (water energy) at the outlet. That energy (water pressure) is what runs the sprinklers, so you must have an accurate measurement of it. You need to measure the Dynamic Water Depth, not just the depth of the water table.
As you know, water flows downhill. When the pump runs it pulls water out of the well. This causes the water level to drop, and then water flows into the well from the surrounding soil. How far the water level drops depends on how hard it is for the water to move into the well from the soil. Some wells have very little draw-down, others may have 50 feet or more of draw-down. Don’t worry if you don’t understand, it will all come together later!
(B) Enter your “Dynamic Water Depth” on your Design Data Form.
Now you need to enter the elevation difference between the top of your well and the highest point in the area to be irrigated. That is, how much higher (or lower) is the highest point in the irrigated area than the top of the well. The best way to do this is to use a laser level and a tape measure. Place the laser level at the high point of the irrigation system and shoot a level beam toward the well. Then use the tape measure to measure the distance from the laser beam to the top of the well. That is the elevation difference. You may need to make several stepped measurements, or you may prefer to just make an educated guess at the elevation difference rather than try to measure it. Also, if the well is higher than the irrigated area the distance will be a negative number.
(C) Enter your elevation difference on your Design Data Form.
Add the elevation difference to the Dynamic Water Depth of the well (or subtract if it’s negative). This number (in feet) is called “elevation head” and is a measure of the height the pump must push the water to get it to your irrigation system.
(D) Enter your elevation head on your Design Data Form. B + C = D (Dynamic Water Depth + elevation difference)
One more thing needs to be factored in at this point, which is your Design Pressure. The Design Pressure is the amount of water pressure that is needed at the inlet of the irrigation system in order for the system to operate. Design Pressure is measured in PSI (pounds per square inch), but for this formula we need the pressure as measured in feet of head. To convert PSI to feet of head we simply multiply PSI times 2.31.
PSI x 2.31 = Feet Head (ft.hd.)
Well that’s all fine and dandy, but what IS our Design Pressure you ask? Good question! Guess what? “Guess” is the operative word here. You’re going to need to take an educated guess at this number. For most situations I recommend that you use a Design Pressure of 50 PSI. This is a good number that works with most small to mid-size irrigation systems. For sprinkler systems with large radius sprinklers (over 35′ between sprinkler heads) you will need a higher Design Pressure. A good rule of thumb for large systems is to take the distance you would like to have between sprinkler heads in feet, and add 15 to it to get a reasonable Design Pressure. For example, if you want to put the heads 50 feet apart, you will need a design pressure of 65 PSI (50 + 15 = 65). Don’t be surprised if your system won’t pump 65 PSI, most residential pump systems aren’t designed to supply more than 50 PSI. As a side note keep in mind that higher Design Pressures result in lower flows, so the higher the pressure, the more valves you will need. I do not recommend spacing sprinkler heads farther apart than 50 feet without having a professional design the system! It gets very tricky. Even most City parks now keep the spacing between sprinklers at 55 feet or less.
(E) Enter your desired “Design Pressure (PSI)” on your Design Data form. For most of you this will be 50 PSI as discussed above. Remember this number is not written in stone! You may want to try adjusting it up and down.
Now we need the “Design Head” so multiply Design Pressure (PSI) times 2.31 to convert it to feet head. This is just a conversion from one type of pressure measurement (PSI) to another (Feet Head). Pump calculations always measure pressure in Feet Head. Example: 50 PSI x 2.31 = 115 ft.hd. (rounded down from 115.5)
(F) Multiply your Design Pressure by 2.31 and enter it as “Design Head” on your Design Data Form.
Now we put all these numbers together to get our “total pressure head”. Total pressure head is the “elevation head” plus the “design pressure head”, all in feet of head.
elevation head (ft hd) + design pressure head (ft hd) = total pressure head (ft hd)
30 ft. Dynamic Water Depth is measured in well with the pump running. The high point of yard is 10 ft. higher than top of well. 50 PSI Design Pressure, which equals 115 feet of design pressure head.
Total Pressure Head = 30 + 10 + 115 = 155 ft. hd.
Pumping from a lake. The low water level is 20 ft. below the high water level. The lowest point of the irrigation system is 10 ft. higher than the high water level. 45 PSI Design Pressure (104 feet head).
Total Pressure Head = 20 + 10 + 104 = 134 ft. hd.
Yet another Example:
For a small park, pumping from a canal. We use the canal bank as our “top of well” level. The low water level in the canal is 8 ft below the top of bank. The irrigation system is downhill from the canal, 25′ below the top of bank. 65 PSI Design Pressure needed for large turf sprinklers, which equals 150 feet of design pressure head (65 * 2.31 = 150 ft hd).
Total Pressure Head = 8 – 25 + 150 = 133 ft. hd.
(G) Calculate your “Total Pressure Head” and enter it on your Design Data Form.
Now for the flow formula:
Multiply the pump Horsepower times 2178 (a constant value, see note at bottom of this page) and then divide by the Total Pressure Head in feet.
Horsepower x 2178 / Total Pressure Head (feet) = GPM (the “Design Flow”)
So for the first example above with a 2 h.p. electric pump:
2 h.p. x 2178 / 155 ft. hd. = 28 GPM Design Flow That’s it! The GPM resulting from the above formula is your “Initial Design Flow”. You will need the Initial Design Flow and Design Pressure values later in the tutorial.
(H) Calculate your “Initial Design Flow” and enter it on your Design Data Form.
Caution: When designing a sprinkler system with a pump you want to keep the actual flow of each valve zone as close to the “Design Flow” as possible without exceeding the Design Flow. This is to keep the pump from cycling on and off as it tries to match the demand of your irrigation system. Don’t worry about valve zones now, we’ll have more on that later. Just remember this: “Valve Zone GPM must be between 80% and 100% of Design Flow”. You may want to write that down someplace. Technical note: In order to simplify the pump formula I have factored a pump efficiency of 55% into the value of the formula constant (2178).
Pressure Up, Flow Down?!!! Wait a minute here! It seems like what this formula says is that if the pressure goes down the flow goes up! That just doesn’t make sense. What gives? O.K., by popular demand, here’s an answer to this little dilemma. It’s one of those obscure, hard to understand hydraulic principles I was yapping about earlier. Sit back. Grab a nice soothing cup of tea or whatever. Put on some soft, relaxing background music. Ready? Here we go… I know it doesn’t SOUND logical, but believe me it IS correct. You’re thinking if more pressure is added then more water will be moved, which is true, but you’re talking about ADDING pressure, not using the AVAILABLE pressure! And that is the key to the problem. Remember we are measuring your AVAILABLE pressure and flow, based on the current conditions at your house (or whatever).
If we were planning to add a big pumping system then we would use a different approach. The correct way of looking at it is “how much water can we move with the pressure that your existing pump can supply?” It takes energy to move water, and the water pressure is the energy that is used. As the water is moved, the energy is used up. Therefore, the water pressure goes down. The more water is moved, the more of the existing pressure is consumed moving it. As the flow (GPM) of the water increases, the pressure (PSI) of the water must decrease! (This is because the pressure is used up moving the water.) Does that make sense? Well, even if you don’t get it, that’s O.K. It takes most engineering students 2-3 months of college level class work before they fully understand hydraulics, so don’t feel bad if it still doesn’t make sense. You’ll just have to believe me that it’s correct (and don’t worry, it is!)
Wet Method (Also called the “Bucket Method”)
This is the most accurate way to test your pump. BUT… Before we start into this I need to warn you that you must follow the instructions below exactly.Do not skip any steps, do not shortcut. If you do not follow these steps exactly you will get a false reading, your sprinkler design will not work, and you probably will destroy your pump! Understand?
Sprinkler systems need pressurized water to operate. Without the pressure, the water doesn’t shoot out of the sprinklers! Think of water pressure as the “energy” that moves the water. Just measuring the water flow with a bucket is not good enough, we must also measure the water pressure at the same time, as we need that pressure to make the sprinklers spray water. If you had a city water supply we would measure the “Static Water Pressure”. This is the pressure when the water isn’t moving. The amount of water available on a city water system is determined by the size of the pipe supplying the water. For a pumped system the amount of water available is determined by the size of the pump. When using a pump we must measure the “Dynamic Water Pressure” also. Dynamic water pressure is the pressure of moving water. Dynamic pressure will always be lower than static pressure. This is because when the water is moving friction is created with the edges of the pipe. This friction consumes energy (remember, pressure is energy) so the pressure drops. The faster the water moves, the more friction, so the pressure is reduced even more. When pumping water there is an inverse relationship between flow and pressure, if you want to get more pressure, then you are going to get less water flow. If you just turn on a faucet and measure the flow into a bucket the pressure falls to zero (the water just falls into the bucket, therefore no water pressure.) That results in a higher than normal flow (lower pressure = higher flow.) To make matters worse, most people measure the flow from a standard faucet. The small size of the faucet restricts the flow, adding more error to the measurement. If you’re lucky the errors balance each other out, but most of the time that isn’t going to happen. Add in a little sloppy measuring, and the results can be off by 20% or more. That can be enough to cause your pump to cycle, which will result in pump failure. Replacing pumps is a major expense, so let’s do this thing right. It takes more time and effort, but it will be worth it. Need to know what size your pipe is? How to find the size of a pipe.
Pump and Pressure Tank Example #1:
The light green colors are the new pipes added for the test. Later you can use this new pipe for the supply pipe (mainline) for your sprinkler system supply tap, so installing this isn’t a waste of money. When the pipe from the pump goes into the tank and then another pipe comes out and goes to the house you should make your tap after the tank as shown. Note that the pressure tank may not be near the pump, especially in cold winter areas where the tank is often installed in the house basement. The new piping can be vertical if you wish, and you can add ells to the last section to get the pipe over the top of the bucket. The 8″ straight lengths of pipe before and after the pressure gauge are important, they must be 8″ long and there can’t be any ells in these sections. The purpose is to avoid creating turbulence in the water that could affect the pressure gauge accuracy.
Pump and Pressure Tank Example #2:
When the pressure tank only has a single pipe going into it you can tap the pipe anywhere. You can tap it between the pump and pressure tank as shown, or after the pressure tank. Most pressure tanks are set up this way. Note that the pressure tank may not be near the pump, especially in cold winter areas where the tank is often installed in the house basement. The new piping can be vertical if you wish, and you can add ells to the last section to get the pipe over the top of the bucket. The 8″ straight lengths of pipe before and after the pressure gauge are important, they must be 8″ long and there can’t be any ells in these sections. The purpose is to avoid creating turbulence in the water that could affect the pressure gauge accuracy.
Wet Method Test Step-by-Step:
Follow the instructions below exactly!
1. Select the location where you will connect the sprinkler system into your water system (your “tap location”). See the pump and pressure tank example diagrams above. In some (very few) pump systems you must tap the pipe after the pressure tank as in Example #1 (see example #1 above.) However, most systems are like Example #2, so you can tap into the water supply line anywhere you want. In this case it is often best to tap as close as possible to the pump. This will usually give you a higher pressure, which means a better sprinkler system (and often less expensive, too!) In some cases you may want to connect your new irrigation system into an existing pipe somewhere a considerable distance away from the pump, such as a faucet near a garden. This is fine, go ahead and try it. The problem is that often the existing pipe is not large enough. You may find that you don’t get a very high water pressure due to the small pipe. If this happens you can confirm the problem by making another tap near the pump and testing there also. If you get a much higher pressure and flow when you test near the pump, then the existing pipe is too small.
2. If there is already an outlet on the pipe at your desired tap location you may use it for the test, provided the outlet is the same size as the pipe coming from the pump. If there is not already an outlet you will need to cut the pipe and install one as follows. Measure the size of the pipe. Go to the hardware store and purchase a compression tee that will fit the pipe. Have the sales person show you how it works. I suggest using a metal compression tee rather than plastic. The side outlet of the compression tee should be the same size as the pipe you are tapping into. At your tap location, cut a short section out of the pipe out and install the compression tee.
Note, sometimes it is necessary to brace the compression tee in place to prevent it from moving and slipping off the pipe. In some situations compression tees will not work. In that case you may need to install a threaded or glue in place (pvc pipe only) tee. If you have the time and skill to install a threaded or glued tee, that is always a better option than a compression tee.
3. Install the parts shown in green as per the Wet Method Pump Output Test sample drawings above as follows:
Start with an 8″ long pipe section installed on the (compression) tee outlet, then another tee with a 0-100 PSI pressure gauge on it, then another 8″ long pipe section, then a ball valve, then add a temporary PVC pipe (maximum of 4 feet of pipe with 3 ells) so that you can fill a 5 gallon bucket to measure the water flow. All the pipes and the valve should be the same size as the pipe you cut to make the tap. Do not use a garden hose as it will restrict the flow and give you a false low flow reading! I strongly suggest using metal pipe and fittings between the tap and the ball valve. I also suggest using a brass (or bronze) ball valve. This ball valve will become the shut-off valve for your future sprinkler system. The last pipe section going to the bucket after the ball valve is temporary and can be plastic. See the example drawings above. The rest of this test process is going to dump a lot of water on the ground, so now is a good time to figure out where that water will go, before you’re up to your knees in mud!
4. Get a “5 gallon bucket”. Since most 5 gallon buckets actually hold more than 5 gallons of water you probably will need to “calibrate” it and mark the actual 5 gallon volume level on the side of the bucket. Fill it with 5 gallons of water using an accurate measuring container to measure the water, then mark the water level on the bucket with a marking pen so you can easily see it. Empty the bucket.
5. Open the ball valve and allow the water to flow freely from it for at least 5 minutes so the flow from the pump can stabilize. ( If your pump is manually controlled you will have to manually start it.) Assuming you have a pressure controlled pump like most are, the pump should start by itself and run continuously during this time.
If the pump shuts off by itself when running with your test outlet full open you have an unusual problem, probably too much restriction in the piping. First check to see if there is any kind of restriction in the pipe (I’ve found rocks, toy cars, rags, rats, fish, roots, etc. in pipes). If not, the pipe from your well may be too small. Best to call a pump company for help at this point as something is seriously wrong with the pumping system.
6. With the pump running, watch the PSI reading on the pressure gauge, and slowly start closing the ball valve until the pump shuts off. The water pressure shown on the gauge will increase as you close the valve. (As the flow from the pump is reduced the pump produces more pressure.) Make a mental note of the pressure when the pump shut off.
7. Reopen the ball valve and wait for the pump to start again. Now slowly close the ball valve again until you find the point at which you get the highest possible pressure reading on the gauge without the pump shutting off. When you find this “balance point” the pump should run continuously and the pressure should remain more or less constant. You will need to close the valve a little, then wait, then close it some more to do this.
What you are determining is the exact point where your pump produces the highest possible combination of pressure and water flow. Do not turn off the pump or adjust the ball valve from now until you finish the rest of the steps! In order to tell when the pump shuts off you may need someone to help out by standing close to the pump and signaling to you. The water coming out of the valve may be loud enough that you can’t hear the pump running. If you have a submersible pump it will be even harder to hear. Try removing one of the plugs in the top of the well so you can hear better. Another method is to see if you can feel the vibration of the water running in the pipe where it exits the well. Water will not be flowing through this pipe when the pump is off.
If you absolutely can’t tell if the pump is running you will need to just watch the flow and pressure. Adjust the valve until you find a flow where the pressure stays constant for at least 15 minutes.
If you don’t have a pressure switch on your pump you still use the same method for the test. Instead of listening for the pump to shut off, you will just have to play with the flow until you reach a good balance between flow and pressure. You will notice as you close the valve that there is a point where you have to severely reduce the flow to make the pressure go higher. Keep your pressure below this level, it is not good for the pump to restrict the flow too much.
The pressure reading is going to be your design pressure. If it is too low, you may have problems with your sprinkler design. For most situations I recommend that you use a Design Pressure of 50 PSI. This is a good number that works with most small to mid-size irrigation systems. For sprinkler systems with large radius sprinklers (over 35′ between sprinkler heads) you will need a higher Design Pressure. A good rule of thumb for large systems is to take the distance you would like to have between sprinkler heads in feet, and add 15 to it to get a reasonable Design Pressure.
For example, if you want to put the heads 50 feet apart, you will need a design pressure of 65 PSI (50 + 15 = 65.) Don’t be surprised if your system won’t pump 65 PSI, most residential pump systems aren’t designed to supply more than 50 PSI. Most people who use sprinklers with a radius over 45 feet need to buy a new high-pressure pump or add a booster pump in order to create enough pressure for the sprinkler system. Should you desire sprinkler heads that are farther apart than 50 feet apart you should have a professional design the system. Wide spacing between sprinklers gets very tricky as the water distribution just doesn’t remain uniform very well at long distances. Even most City parks now keep the spacing between sprinklers at 55 feet or less.
Most pumps have a pressure switch on them that turns the pump on and off at preset pressures. Typical settings are 35 PSI on and 45 PSI off. It works just like the heater and thermostat in your house do, only it measures water pressure rather than temperature. When you open the valve, the water flows out and this causes the water pressure to drop. When it reaches the preset “on” pressure the pump turns on. As you close the valve the pressure climbs. When it reaches the preset “off” pressure the pump shuts off. This creates a problem if you want to get 50 PSI and the pump turns off at 45 PSI! Fortunately, the pressure at which the pump turns on and off can be adjusted. If your pump is turning off at less than 50 PSI you may want to have the settings adjusted. You may be able to increase the off setting by 5 PSI, maybe a little more or less. Sometimes this requires the installation of a new pressure switch on the pump, most can only be adjusted within a limited range. (If you need to replace the pressure switch, have a pump professional do it. You can damage the pump if you use a switch with a pressure range that is too high. Your pump may not be designed to pump higher pressures! )
How do you adjust the pressure setting? Somewhere on the piping, usually near the pressure tank, there is a tee in the pipe and a small box is sitting on a short pipe above the tee. The box has electrical conduit running to it. This is the pressure sensor. It has an adjustment screw inside the box cover. I can’t tell you much more than that since the location of the screw will vary depending on the model. You may be able to find instructions for the pressure switch online by searching for the manufacturer’s website. Newer pumps often have solid state pressure sensing. Depending on the model you may be able to change the settings using the keyboard. Other models do not have a keyboard and must be hooked up to a programming device (often a laptop, tablet, or smartphone) to change the settings. You may need to call your pump company for help.
A. Write down the pressure gauge reading on the line “Design Pressure (PSI)” on your Design Data form.
8. Now measure the flow coming out of the pipe without adjusting the ball valve. The pressure gauge must stay at the “Design Pressure” and the pump must continue running while you measure the flow. Put your 5 gallon bucket under the flow and time how many seconds it takes to fill the bucket to the 5 gallon mark. Repeat this 3 times to make sure the the results are accurate (all three measurements should be about the same). Divide 300 by the number of seconds it takes to fill 5 gallons into the bucket to get the GPM. (300 / seconds to fill 5 gallon bucket = GPM) Example: 10 seconds to fill the 5 gallon bucket, therefore 300 divided by 10 seconds equals 30 GPM.
B. Write down the flow (GPM) you measure on the line “Initial Design Flow” on your Design Data Form.
You’re done measuring. You can close the valve now and shut off the pump if it is locked on. Leave the ball valve in place as you will connect your new sprinkler system to it.
A Few Other Important Items to Note
Caution: When designing a sprinkler system with a pump you want to keep the actual flow of each valve zone as close to the “Design Flow” as possible without exceeding the Design Flow. This is to keep the pump from cycling on and off as it tries to match the demand of your irrigation system. Don’t worry about valve zones now, we’ll have more on that later. Just remember this: “Valve Zone GPM must be between 80% and 100% of Design Flow”. You may want to write that down someplace.
Do you have enough water available from your pump?
You are going to need about 20 GPM of water to irrigate 1 acre of grass with sprinklers. One acre is equal to 43,560 square feet (or 4047 square meters.) Therefore, if you have a 2 acre grass yard you will need to have 40 GPM of water available in order to water it. If you have shrubs, they typically only use 1/2 as much water as grass, so 20 GPM would water 2 acres of shrubs. If you don’t have enough water you will either need to find a larger water supply, or reduce the amount of area watered. Another option is to plant shrubs and use drip irrigation on them. With drip irrigation you only water the area the plant foliage actually covers. Therefore, if the plants only cover half the actual ground area, you only need half the water.
There are only so many hours in the day to water. The amount of water needed varies with the climate, these estimates of the water quantity required per acre are typical for warm summer areas where most sprinkler systems are installed (daily high temperatures in summer over 90 degrees F., 32 degrees C.) These values assume you would water as much as 10 hours per day. If you are able to water more time per day then you will be able to irrigate more area.
Minimum pipe size:
If the pipe between the pump and the tap point for the irrigation system is longer than 10 feet (for submersible pumps measure from where the pipe exits the well) then the pipe must be as large or larger than the following:
Initial Design Flow
Minimum Pipe Size
0 – 5 GPM
5 – 15 GPM
15 – 30 GPM
30 – 40 GPM
40 – 70 GPM
70 – 100 GPM
100 – 160 GPM
If the pipe is smaller than the above minimum sizes you will need to replace it to avoid the possibility of water hammer which can damage your pump, pressure tank, and household plumbing, not to mention the sprinkler system. If your design pressure is over 50 PSI use one size larger pipe than what is shown.
Example: 55 PSI design pressure and 35 GPM Design Flow require a 2″ pipe (one size larger than chart says because design pressure is over 50 PSI).
Lots more on pumps…
I strongly suggest you take a look at my Irrigation Pumping Systems Tutorial. There’s a lot more information on pumps, pump controls, and pumping from wells, lakes, rivers, etc. in the Irrigation Pumping Systems Tutorial.
Q. My irrigation pump runs fine when the system is operating, but after it turns off it cycles on for 5 seconds every 10 minutes or so.
A. If you are using a pressure switch and pressure tank to turn the pump on and off my first guess would be that you have a water leak in your irrigation system. The water leaks out, which cause the water pressure to drop, then the pump kicks on and recharges the pressure. Then the pump shuts off again. That would cause exactly this symptom.
Knowing the problem is the easy part. Finding the leak, that could be harder to do. It could be a zone valve that isn’t turning off all the way or it could be a leaky pipe. You can narrow the search area a little, the leak will be someplace in the pressurized part of the system, that is, between the pump and the zone control valves. Start by looking for obvious dripping, then look for someplace that seems wetter than it should. If it is a leaky zone valve then the water will be leaking through the valve into the sprinkler zone pipes and will dribble out at the lowest sprinkler head. So look at the sprinkler heads. There will be a small “swampy” area around the lowest sprinkler head that is controlled by that valve.
Every now and then I come across something interesting related to irrigation and take a photo. Nothing important about them, just interesting to me for some reason or another. So I share them.
This pump is located at Nicholas Flat in the Coastal Mountains above Malibu, California. It was probably used by a former ranch to irrigate a large nearby meadow. A creek is located in a gully in the bushes behind the pump. A large intake pipe is still back there going down toward the creek, not visible in this photo. There is a nearby pond that still exists, the pipes may have originally extended from the pond, although the location seems rather far from the pond to have been pumping pond water. I suspect the pump drew water from a sump in the creek bed. Sumps were (and still are) created by simply piling sandbags in the creek to dam up enough water to keep the pump intake submerged.
On the right is the gas engine that powered the pump. To the left in the grass is the flywheel and the actual pump, laying on it’s side. before it was vandalized the pump was mounted on the far side of the engine, near where the pipe with the “tee” on it is. A large belt connected the small drive wheel on the engine to the large flywheel on the pump. I find it interesting that the galvanized tee on the pipe behind the engine has not rusted, but the pipes apparently were not galvanized, or at least not as well as the tee!
Nicholas Flat is now within the boundaries of Leo Carrillo State Beach, within the Santa Monica Mountains National Recreation Area. There are a number of great, easy hiking trails in the area. The pond makes a great hike destination, the trail is an old ranch road, short and level with lots of interesting things for kids to look at. Suitable for strollers. The pond is created by a natural rock outcropping that forms a dam. Scrambling up an easy climb to the top of the rock outcrop rewards you with a great view down the canyon to the Pacific Ocean. The far side of the rock outcrop is the top of a high cliff, so keep kids supervised! Also lots of poison oak if you go off-trail. Just a warning if you have small kids keep an eye on them. The trail head is at the end of paved Decker School Road.
Q. I am currently in the process of converting my entire lawn irrigation system into an electronically controlled system using a control and relay setup for the pumps. I currently have two centrifugal pumps that pump water from a pond about 150 yards away. The system has seventeen zones and I have already ordered all the valves needed as well as the controller and pump relay and am in the process of installing it all. I am concerned with the fact that if one of the valves fail to open then I may have a problem with too much pressure and would like to know what kind of setup you suggest in order to overcome this. I researched pressure relieve valves and such but I feel that a flow sensor combined with a high pressure sensor to turn the pumps off would be the safest route in order to minimize damage to the pumps. How could this be done to cut pumps off if there is too much pressure or no flow at all?
A. They make flow sensors that use paddle wheels, they can actually measure the flow rate in the pipes in GPM or cu ft/min. They are a great way to go for this. They require that you have a fancy irrigation controller that can work with them, so you may need to return your controller and upgrade it. The irrigation controller measures the flow and compares it to the pre-programmed flow that should be present in the system for the valve that is currently open. The controller then makes a decision based on that flow. If the flow is too low or too high it can shut down the pumps or close a master valve that shuts off the water to the entire system.
The sensor needs to be installed in a tee on a straight length of pipe. The length of the straight pipe should be 5x the pipe diameter before the sensor and 5x after it. This is to reduce water turbulence in the pipe caused by turns, the turbulence can cause inaccurate pressure readings.
You can also use a pressure sensor and pump logic controller to turn off the pumps at high pressure or very low pressure. You should be able to get what you need at a specialty pump supplier. The sensor is a bit different from the typical pressure switch. A standard switch turns the pump on at low pressure and off at high pressure. The logic controller is basically used as a detector and timer. The timer would only turn off the pump if the high pressure was present for maybe 4 minutes or so. It is normal to have a pressure spike as the system changes from one valve to another, you don’t want the pump to shut off during the switch of valve zones. You also need a delay to allow the pump to start up, since there will be no pressure until it gets going (so the switch would never allow the pump to start!) The pressure sensor also needs to be on a straight pipe section like the flow sensor.
If you wnat to use a pressure sensor you should also do a quick test to make sure your pumps are capable of producing a high enough pressure to detect. Some pumps don’t produce very much increased pressure, even at no flow. So you need to make sure your pump will, if it doesn’t you need to use a different method of detecting no flow, like a flow sensor. Run the pump as normal with the smallest valve circuit open and check the pressure. Now shut off the valve and watch the pressure (don’t let it run for more than 3-4 minutes without flow! Don’t want to overheat the pump.) Ideally you want to see a pressure increase of 5 or more psi. The more pressure increase you have the less likely you are to get a false alarm caused by a small pressure spike.
If the pump doesn’t produce enough pressure to measure the increase at no flow you will need to use a flow switch to detect flow. A flow switch is nothing more than a paddle that sticks down into the pipe. When the water is flowing it presses against the paddle and the switch opens/closes (depending on how you have it set.) It’s very simple. Unfortunately flow switches also break pretty easy, so they have to be frequently replaced. That’s why I don’t use them as my first choice.
Q. I have a shallow well that was drilled this summer and a centrifugal pump pulling up about 15 gallons/min (HAPPY!). The problem, it will only produce somewhere around 30psi (sad!). Am I able to add a booster pump to this setup to produce more psi or should I just forget it and go for a submersible pump? Obviously the booster pump would save me $…
A. You can add a booster pump but it is tricky. The flow range of the booster pump needs to match that of the existing well pump. Using two pumps will probably use considerably more electricity than a single new pump, especially if it is a submersible. Submersibles are by nature more efficient than a centrifugal pump at the top of the well and now you are adding the friction drag of two pumps rather than one. I can’t tell you how much the electricity cost difference would be, that’s beyond my knowledge level. But ongoing electricity cost is certainly something to look at.
Essentially when you couple two pumps together they are going to have to play nice with each other. You don’t want one to over-power the other and do most all the work while the other just causes drag. Plus you need to deal with the wiring issues and how you will start the two pumps. Hopefully they would both stay primed so, in most cases, you could start them both together using the irrigation controller connected to a relay connected to the pumps. You might need two relays if the pumps exceed the capacity of the relay.
Finally you will need to deal with figuring out if and how you will handle problems such as the malfunction of one of the pumps. If one burns out the drag created by the burned out pump could very quickly burn out the other. Hopefully you would quickly notice the problem, since the irrigation system would not work well at all if only one pump was running. But what if you were on vacation when it happened?
You probably should get a local pump professional who knows his/her stuff and has experience with two pump systems to help you if you use two pumps.
Basically if you want to keep this a simple do-it-yourself project I’m thinking buying a new submersible for your well would be the better way to go.
Standard solenoid irrigation valves don’t work well with a typical rain barrel. The standard solenoid valves used for most irrigation systems simply need more pressure than you have available from a typical gravity fed rain barrel. The higher pressure requirement for the valve is a function of the hydraulics that makes the valve operate. You either need more pressure or you need a different type of automatic valve. If you want to create more pressure you need to raise the height of the rain barrel. For every foot you raise the rain barrel you will create 0.433 PSI. The minimum operating pressure of most irrigation valves is at least 15 PSI, that means the barrel needs to be 34 feet above the height of the valve. That is simply not practical in most cases! Now you understand why those water towers you see in some communities are so tall.
Motorized rain barrel valves:
Mechanical motor-operated ball or butterfly type valves will open at any water pressure, unlike solenoid irrigation valves. This makes them a good solution for rain barrels irrigating by gravity alone.
One of the least expensive solutions is a combination timer and valve made for garden hoses. The Toro #53746 Battery Operated Hose End Timer is an example of this type of timer/valve. There are likely other brands available as well. This Hose End Timer uses a motorized ball valve to control the water flow. Most of the time the hose end timer gets the job done when used with a rain barrel… but this is a low end market product and be aware that the quality is low. It may very well quit working after a year or two. On the other hand you can buy and replace a lot of these for the price of a full blown commercial quality motorized valve and timer unit like the ones they use on home floor heating systems.
A more expensive, but more reliable and longer lasting method, is to use a motorized ball valve made for home hydronic heating systems. Make sure you get a motorized ball valve, not a heat motor valve, unless you really want to use lots of power and take several minutes to open or close. Most of these hydronic valves are 24VAC and thus directly compatible with standard irrigation timers. So when using a motorized heating valve make sure the motor operates on 24VAC and a amperage your timer can handle. The hydronic valves come in a variety of voltages and amperages. Irrigation timers only work with low amperage 24VAC valves. To find these motorized valves do a search for “hydronic zone valve”. Be sure to note the connection types for the valves, most are made to connect to PEX pipe or be soldered onto copper. You may have to install adapters to fit them to your irrigation system pipes or tubes.
Use Emitters That Work Well at Very Low Pressures
Drip Emitter & Tube Selection
Most people use drip irrigation with their rain barrels, so this article assumes you are using drip irrigation. (If you want to use sprinklers you will probably need a lot more water pressure, and therefore a larger pump.) The best emitters for the very low pressures in a rain barrel fed system are the most simple emitters, such as those commonly called a “flag emitter” or “take-apart emitter”.
Another popular choice for emitters when using a rain barrel is the adjustable flow emitter/bubbler. These use more water and are even less uniform than the Flag Emitters, but they are particularly good for watering pots of various sizes as you can adjust the flow needed for each pot.
Stay away from the higher cost emitters and those labeled as “pressure compensating” as they tend require higher pressures to operate efficiently.
Use 1/2″ tube if you can and keep the drip tube lengths short. Smaller diameter tubes (especially 1/4″ tube!) and longer tube lengths both restrict the water flow and lower the water uniformity between plants. To put it another way; if you use long, small tubes the plants closest to the rain barrel may drown from too much water, while the plants at the far end of the tube may not get any water at all.
Gravity fed drip systems from rain barrels are going to have less uniform water distribution. That’s just the way it is, with minimal water pressure it is very hard hydraulically to maintain uniformity. To make the best out of a bad situation you use larger diameter tubes and keep the barrel as close as possible to the plants so the tubes are not too long. If that won’t work for you, then the alternative is to use a small pump to create more water pressure. Most people just elect to be content with the low uniformity.
If you want to test the uniformity of your drip system it is very easy to do, simply build your drip system and attach it to your rain barrel. Then place a disposable plastic cup under each emitter and run the system for a few minutes. All the cups should have about the same amount of water in them. If the amount of water in the cups varies greatly then the uniformity is pretty bad. If the uniformity is bad enough that you think it will create uneven watering you can do a simple test to see if more pressure will help by hooking your drip system up to a garden hose. Be careful, the garden hose will provide more pressure than you need, so turn the valve on slowly and don’t turn it on all the way. Empty out the cups and run the test with the cups again. Usually the higher pressure from the garden hose will result in more uniformity between the water in the cups.
Using a Pump for your Rain Barrel
The best way to automate a rain barrel irrigation system may be to not using a valve at all! Consider using a small pump placed on your rain barrel outlet hose. A pump is often the best solution as it may provide the added benefit of more water pressure. But it’s not cheap to add a pump.
Selecting and Installing a Rain Barrel Pump
Make sure the pump is rated for enough flow to supply your emitters, and enough lift to get the water needed for your irrigation over the top of the barrel. Add the flow rate of all the emitters together to determine the flow rate needed for the pump. For example if you have 15 emitters that are rated at 1gph (gph means “gallons per hour”) then the pump will need to supply at least 15 gph. If the barrel is 5 feet tall then the pump will need to lift the water 5 feet just to get it out of the barrel. But you need to do more than get the water out of the barrel. You need pressure to move the water efficiently through the tubes and push it out through the drip emitters. That requires another 45 feet of elevation. So add your rain barrel height to the elevation needed to power the drip system. 5 + 45 = 50 feet. So you want a pump with the capacity to move 15gph of water and lift it 50 feet.
Some pumps are rated using PSI (pounds per square inch of pressure) output value rather than feet of lift. A simple formula converts feet to PSI. Just multiply feet x 0.433 to get PSI. So a pump with a 50 feet of lift becomes 22 PSI. (50 feet * 0.433 = 22 PSI) So if the pump is rated in PSI it needs to produce 22 PSI.
If you can find one the right size, a submersible pump is the easiest and best method. Unfortunately most are made to be fountain pumps or sump pumps and they don’t create enough water pressure to efficiently operate drip emitters. If you find one that will work for you, attach your irrigation hose to the pump, put the pump in the bottom of the barrel, and run the tube up over the top of the barrel. You will need a air vent at the high point on the tube near the top of the barrel (above the maximum water level) to prevent water from siphoning out of the barrel through the tube when the pump is not running. You can buy an air vent from any drip irrigation store. Or… a very simple and cheap way to create an air vent is to add a drip emitter on the hose at the top of the barrel, so that the water from the emitter drips back into the barrel and is not wasted. When the pump turns off, this emitter will allow air to flow back into the tube and the air will stop the water from siphoning out.
If you don’t use a submersible pump then the pump will be attached to an outlet at the bottom of the rain barrel. Make sure the pump is bolted or screwed down to a firm surface or it will jump all over the place when it runs. The tube from the pump outlet will need to be looped up above the top of the barrel and an air vent (or emitter as described above) installed at the high point to prevent all the water in the barrel from draining out through the pump when the pump is off.
Example of a pump: Surprisingly the best pumps for rainbarrels are not ones made for irrigation uses, but rather industrial pumps. For example the Little Giant 35-OM pump is made for high pressure applications like commercial carpet cleaners, but it produces good pressure at a low flow, a combination that is great for small drip systems. Here is the performance chart for this pump:
40 gph at 70 ft hd
60 gph at 65 ft hd
80 gph at 58 ft hd
100 gph at 54 ft hd
120 gph at 45 ft hd
140 gph at 30 ft hd
Controlling the Rain Barrel Pump:
The pump can be turned on and off by using a timer. A simple lamp or other household electricity timer will often work for an extreme low cost option, however lamp timers are pretty limited. Most timers of this type will only turn on and off the pump once a day, and do it every day. Most people don’t need to water daily, so this could waste water. If you do use a simple timer make sure it is rated for a voltage and amperage that is equal to or higher than the input of your pump.
If you want to use a standard irrigation timer to control the pump you will need to buy a pump relay unit. Irrigation timers output 24 VAC, most pumps use 120 VAC. So the pump can’t be connected directly to the irrigation timer. A relay is used to allow the pump to be turned on by the timer. Make sure the relay is rated for the correct voltage and amperage for your pump. Instructions for installing and wiring the pump relay should be provided with the pump relay.
Almost any major maintenance problem in an irrigation system will cause a unusual pressure level or flow level in your irrigation system. Therefore pressure and/or flow monitoring is a good way to detect problems. Most of the time the response to a abnormal pressure or flow level would be to shut down the system, or possibly to shut down the current valve zone and try another one. Irrigation systems are typically shut down using what is called a master valve. A master valve is a single valve located at the water source that can shut off all the flow of water into the irrigation system. For more details see my article on master valves. On systems with a pump you will probably want to shut off the pump. Sometimes, as with booster pumps, you will need to both shut down the pump and close a master valve.
So what problems might an abnormal pressure or flow indicate? A very low pressure may indicate that perhaps the pump is broken (if you have a pump), an intake screen is clogged, a filter is dirty, a valve failed to open, or a pipe has broken. Abnormally high pressure could be the result of a valve not opening when it should, a dirty filter (if the pressure is measured upstream of the filter rather than downstream) or some obstruction in the pipes. Low flow could indicate a valve failed to open, a filter is dirty, or that a pump isn’t working as it should. High flow could indicate a broken pipe, a broken sprinkler, or a valve that is stuck open. In most cases monitoring either flow or pressure is sufficient as opposed to monitoring both.
How to Monitor Your Irrigation System
There are a number of different ways to detect and respond to abnormal pressure or flows. Following are a few or these. If you would like to suggest other methods, please contact me. I realize this is not an exhaustive list.
Use a Smart Irrigation Controller that has a Sensor Input and Response Feature:
This is probably the easiest way to add pressure detection and response. It is also what I consider to be the preferred method, as it is reliable and gives you the most control. Some high-end irrigation controllers can use an electronic sensor hooked up to the mainline pipe to monitor the water in the irrigation system. Some of these controllers use flow sensors, some use pressure sensors, some can use both types. These controllers with advanced features are typically sold as Smart Controllers and are expensive compared to ones typically found on a residential irrigation system. Prices for these controllers typically start around $300.00 and go up into the thousands for ones that handle dozens of stations. But then you get a lot more with them too. They are sold through professional irrigation supply stores, both online and locally.
WARNING: Be sure the controller will do exactly what you want BEFORE you purchase it! Not all controllers marketed as “Smart Controllers” have these sensor input features, many only work with specific types or even models of sensors, and some controllers may not provide the response options you want or need. You need to research the controller carefully. Don’t rely on a simple check list of features! “Sensor input” can mean almost anything, you need details! I have seen controller feature lists where the unit sounded fantastic and ultra flexible, only to discover after closer examination that the actual response features don’t do what I need or want. Read the actual owner’s manual (most controller manufacturer’s have them available on their websites) to see what the true capability of the controller is. Read the sections of the manual on how to hook up the sensor, then there will also be a separate section on how to program the sensor you should look through. Some controllers allow for time delayed responses, some don’t. If you have a pump you will almost always need a time delay feature to bypass the sensor when the pump is starting up. Even those controllers that do allow you to add delay times may not allow as much or little time as you need. It is critical that you do as much research as possible before you go to the expense and effort of purchasing, installing and programming the controller.
For example, I have a Rainmaster Eagle Smart Controller on my own irrigation system, as well as using it on the majority of the commercial systems I design. This particular Smart Controller has flow sensing capabilities, but it does not have built-in pressure sensing capability. It does have a delayed response allowing delays of 1-6 minutes, but only in one minute intervals. It will also allow the use of one additional simple on/off type sensor (most controllers have a circuit for this type of very simple sensors. A simple rain switch is an example of this type of sensor.) It has an audible “chirp” alarm that alerts you that a sensor response has been activated. While this particular controller meets my needs, it certainly will not meet everyone’s. Almost every major irrigation company makes a Smart Controller, and each has different features and capabilities. Be sure you are using up-to-date resources when checking out models. Smart Controller models are introduced each year, and often the capabilities of existing models change from year to year, so it is hard to keep up with them.
When using a controller with a pressure and/or flow sensor you start by installing the actual sensor on the mainline pipe. The method varies with the brand and model of sensor, most are pretty easily installed. The sensor is wired to a special terminal on the irrigation controller. Typically the wire used must be a special shielded communications cable, rather than standard irrigation valve wire. Consider installing communications cable in PVC conduit to protect it, as it is very sensitive to even the smallest nicks from shovels, animals digging it up, or rodents chewing on it. Most pressure sensors work by sending a reading of the current pressure to the controller every few seconds. A typical flow sensor has a small paddle that turns as the water flows through the pipe. Flow sensors normally send a signal based on the amount of flow, for example they might send a signal each time 5 gallons of water has flowed past the sensor. The controller then interprets that data from the sensor and responds. In most cases you will pre-decide what the response will be when you set up the controller. For example; if you have a system with a pump, you could program the controller to shut down the irrigation system if the pressure was below 10 PSI for more then 2 minutes during the set irrigation period. The 2 minute qualifier (delay) for shut down would allow the pump time to pressurize the system during start up and also avoid “false alarms” caused by brief dips in pressure.
Using a Simple Pressure Switch with a Pump Operated System:
This method is for those with pumps. What I am describing here is for emergency shut off only. I’m assuming you already have something set up to turn on or off the pump during normal irrigation operation. That might be a standard pressure tank with a pressure switch to control it. Or you may be using the pump start feature on the irrigation controller to actually start and stop the pump using a 120v relay. The new pressure switch we are talking installing in this case is used only to detect pressures that indicate a problem and turn off the pump. So if all is hooked up properly, in the event of blockage or no water going into the irrigation system the pressure will drop and the new pressure switch will shut the pump off.
This method requires that your irrigation system is leak free and can hold pressure for days between irrigations. If the system is not leak free see #4 below.
1. Make sure you have a really good quality spring-loaded check valve on the irrigation mainline pipe. The check valve goes someplace after the pump, but before the pressure switch. A good quality check valve is needed to keep the water from leaking backwards out of the system through the pump. Typically the self-priming feature of the pump is not good enough by itself to do this, you need a separate check valve.
2. You will need to use a pressure switch that works backwards from normal ones used for household water systems, since you want the switch to shut off the pump at low pressure (standard switches used on household water systems turn on the pump at low pressure.) Some switches can be wired to work either way, others can’t. Keep in mind that the low end on many common pressure switches in around 25-30 PSI. That might be a bit higher than you want for a low end shut off, especially if your system will be operating at less than 45 PSI. You don’t want accidental “false” shut offs since the only way to get the system back on will be to manually start the pump and hold it on until the pressure is back above the shut-off level.
3. There a problem to be dealt with. The problem is that valves close slowly, taking as much as a minute or two to close after the controller tells them to. At the end of the last irrigation cycle a typical controller closes the last valve and immediately shuts off the pump. But it takes the valve several seconds up to a minute or two to actually close. During this closing period the system will depressurize. With no pressure in the system the pump will not restart for the next irrigation cycle, because the low pressure shut-off switch is detecting low pressure and shutting off the power to the pump. There are two ways to deal with this.
A. You can fool the controller into keeping the pump running after the last valve circuit has finished watering. Your controller needs to have the capacity for one extra valve on it to do this, so if you have 10 valves you will need a controller with 11 stations. The last station on your controller needs to not have a valve attached to it. Program 1 minute of time on that last station. Now the controller thinks it is operating one last valve, so it keeps the pump running. That will keep the system pressurized while the final valve closes. If one minute is not enough time for the final valve to close then add another minute of run time to that last empty station.
B. Some controllers have a built in delay feature that keeps the pump running after the last valve closes. This feature keeps the pump start circuit energized, which keeps the pump running for a minute or two after the last valve is signaled to close. This gives the valve time to close before the pump is shut off. Some less expensive controllers have this feature. But typically only high-end controllers have this feature, so this method isn’t very practical. If you are going to buy an expensive controller you might as well forget about using a pressure switch and use a Smart Controller and a sensor to shut the system down, as described in the first section of this article.
4. Often a small leak will cause the system to depressurize between irrigation runs. This can be a major problem. The pump will not start if the pressure is low, the low pressure switch is going to shut off the power to it.
If the leak is very small you can install a pressure tank, just like on a typical house water system. Assuming a small leak, the tank keeps the system pressurized. But that only works with a very small leak and it can take a huge pressure tank to supply enough water to keep the system pressurized. If your system has a larger leak you will need to find and repair the leak. If you can’t get the system leak free, you will need to take a different approach, as described below.
You can use a timer to over-ride the low pressure switch, and allow the system to start even with no pressure. You will need a “Time Delay Relay”. The time delay relay needs to be the type that allows the power to flow when energized, then shuts it off after a minute or two of delay. It needs to have an automatic reset. You then install the relay on a bypass wire around the low pressure switch. That way the pump can start even when the pressure switch is “off” due to low pressure. You will need to work with someone knowledgeable when ordering the time delay relay to be sure you get the correct relay, as they make many different kinds.
Using a Pump Controller with a Sensor:
This is essentially the same method as the Smart Controller method I described earlier. Only the “smarts” are in the pump controller rather than in the irrigation controller. Some of the newer digital pump controllers (don’t get confused here, we’re talking about a separate pump controller, not the sprinkler controller) are programmable, they are simply a small computer that operates a relay that starts and stops the pump. You hook them up to a pressure sensor, also to the irrigation controller, and to any other sensor you want (wind, rain, temperature, light, flow, you name it.) Then you can program them to do just about anything using that information input. They can turn off the pump if a low pressure occurs for more than x number of seconds, turn off the pump if a high pressure occurs for x number of seconds, turn on the pump at a given time of day, etc. Pretty much any input you want can cause the pump to turn on or off. The capability depends on the brand and model of the pump controller. The downside is it takes electronics know-how to set the thing up and someone tech savvy to program it. Typically you hook up a laptop to the pump controller to program in the logic, then once it is programmed it runs by itself. The laptop just gives you an interface that is easier to work with. I really can’t give you much more details beyond that, this type of pump control is beyond my expertise, I just have seen pump system experts use them to do amazing things.
Q. We live on a river. I would love to plant some interesting things on the bank below our home but with the price of water these days I would love to be able to pump some river water up to do the job. Do you think that that is something we could do without spending a fortune? It would be great to have a soaker system.
A. First, you must have the right to take water from the creek, river,. pond, etc.. This almost always means you need to talk with the US Fish & Game Department, State regulators, and possibly the Environmental Protection Agency (or equivalent agencies for whatever country you are located in.) If you take water from a creek or pond or any other natural body of water in the USA without checking on the legal rights and requirements you can get into a lot of hot water, fast. The fines penalties and restitution costs can be enormous. So before you do anything, start doing some calling around. Be safe, not sorry. If you don’t know who to call, try calling the local County or Parrish Planning Department, they should be familiar with the agencies that regulate water and be able to point you to the right people.
Yes, from a physical standpoint it is not difficult to pump the water. The cost depends on how fancy you make it. My parents had a cabin on a river in Oregon. They simply had a small portable pump that sat on a concrete block and was chained to a tree. One end of a 15′ garden hose was attached to the pump intake, the other end of the hose had a piece of window screen tied around it to create a home-made filter and keep out small fish and junk. The end of the hose with the screen filter was tied to a concrete block and dropped into the river. The pump outlet was attached to a second garden hose, this one was 150 feet long. A long extension cord went from the pump to the power outlet at the cabin. They put a sprinkler on the end of the hose, placed the sprinkler where they wanted water, then plugged in the pump. Simple, cheap. You could easily semi-automate that by simply plugging the pump’s power cord into a timer to turn it on and off.
A fancier system is certainly possible. The pump still needs to be portable in most cases. The pump has to be mounted less than 8 feet above the water level (the closer the better.) You need a pad of some sort to put the pump on, but it is best if the pump can be easily moved, especially if the water level fluctuates in the creek or floods. There is also the possibility of using a submersible pump. A submersible should not sit on the bottom of the stream if there is a lot of mud and silt in the water that would get sucked into the pump. If you have a floating dock or a pier an alternative is to place the pump on it (or hang it below the dock in the case of a submersible pump.) Submersible pumps are often strapped to the side of pier pilings. Be sure to read installation instructions for the pump, many pumps have very specific positioning requirements, some submersibles must be installed inside a special sleeve.
You can get about as fancy as you want- using automatic controls to start and stop the pump and also to open and close multiple irrigation valves. Many irrigation controllers have built in circuitry that will start and stop the pump for you using a electrical relay. If you do it yourself, and you need only something similar to my parent’s small pump you could probably install a pump for around $200.00. The price can go up fast as you get bigger and fancier, $1000.00 is not an out of line figure for a pump system capable of watering an acre or so of yard. The wiring for the pump automated controls is a bit tricky, so most people would want to have that part done by a electrician. How much that costs depends on the length of wire needed to reach the pump. One option to look at when you get to larger irrigation systems is a pre-constructed pump unit. This consists of the pump and all of the needed controls for it pre-installed and pre-tested on a metal frame. You just hook up the pipes and wires to it and turn it on.
You may also need a storage tank for the water, especially if you have a small water supply (like a creek.) That way you could pump a small flow continuously from the creek to fill the tank. Once in the tank the irrigation water would either be pumped out of the tank to the irrigation system by a second pump, or if the tank can be located 30′ or so higher than the level of the irrigated area, you could use gravity flow from the tank. (If you want to use sprinklers the tank would need to be at least 60 feet higher to create enough pressure for a small sprinkler.) The tank will probably need to be a lot larger than you think. Typically they are 5,000 gallons or larger. To find out what size tank you will need you need to determine how much water it will take to irrigate your area. See How to Estimate Irrigation Water Quantity Needed for instructions on estimating your water requirements.
One last word of warning before you start: PLAN FIRST, BUY LATER! Don’t run out and buy an “irrigation pump” first! Most pumps sold with the description “irrigation pump” are designed to operate a single sprinkler on the end of a hose. You need to design the irrigation first, then you will now how much water volume AND water pressure the pump will need to produce. The Sprinkler System Design Tutorial takes you through the process of irrigation system design and finding the right pump size. It’s at https://www.irrigationtutorials.com/sprinkler00.htm
Q. I’m designing a pump system from a lake and have read and understand your calculation of FT HD needed for pump selection but it seems that the upstream (uphill) pipe diameter would be a factor in the calculation. I was going to use larger pipe to reduce pipe resistance and valve pressure drop but it seems to me the weight of the additional water (back pressure) would be higher for a larger diameter pipe than a smaller one. It must be easier to push water up a 3/4″ column than a 1 1/2 inch column. You mention nothing about this. Excluding pipe resistance, does the pipe diameter play a roll in taxing the FT HD required? Rephrased – Does a larger diameter column of water have any effect on the static pressure or force required to move it?
A. The short answer is that the larger pipe would be better because there would be less pressure loss in the pipe. This is due to less “friction loss” as the water flows through the larger size pipe. The larger amount of water in the bigger pipe has no impact on the water pressure. A smaller pipe may create more friction loss however, so it can be worse than a larger pipe. To find out, you need to calculate the friction loss in the different sizes of outlet pipe based on the flow and pipe size. See the Friction Loss Calculators to calculate the friction loss in pipes.
More detailed answer:
One of the really hard to grasp principles of hydraulics is the relation of volume of flow, pressure, and the weight of water. Odd as it seems a larger pipe will actually be easier for the pump. It’s not the volume of water, but the height it is lifted that matters. In a way this is a variation on the old saying “which weighs more, a pound of feathers, or a pound of lead?” Obviously both weigh a pound! This version could be phrased “which is easier for the pump, 5 GPM in a 1/2″ pipe or 5 GPM in a 2” pipe? Neither because 5 GPM is still 5 GPM regardless of the pipe size! Yes, you would need more power if you were actually lifting more water, also we would need more power to lift the water higher, but neither is not what is happening. The amount of water nor the height we are lifting hasn’t changed.
The other issue here is flow through a pipe. This is the issue that actually makes the smaller pipe potentially worse than the larger. Because the smaller pipe is smaller it is harder to force the water through it. The resistance of the walls of the smaller pipe causes pressure loss as water flows through. this is commonly called “friction loss”. How much friction loss occurs depends on the flow rate and pipe size. Both higher flows and smaller pipes sizes result in greater friction loss. This is the only reason a smaller pipe would be worse than the bigger pipe. How much worse is dependent upon the actual flow rate and pipe size.
As a general rule (ie: not always true, but is most of the time) the pipe size of the pump outlet is almost always smaller than the size of pipe that will provide optimal flow from the pump. In other words, if a pump has a 1″ threaded outlet, it is very likely that a 1 1/2″ pipe would be attached to the 1″ outlet for use as the outlet pipe. Pump manufacturer’s tend to use smaller size inlets and outlets to save money.
More technical answer:
Think about feet of head. As discussed in the Pump Tutorial, the number of feet of water depth determines the water pressure. So 80 feet of water depth equals a pressure of 80 ft. hd. This pressure will be the same regardless of the pipe size. The water pressure at the bottom of an 80′ high 1/2″ pipe is exactly the same as the water pressure at the bottom of an 80′ high 6″ pipe, even though the 6″ pipe holds a lot more water. A pump actually works by creating water pressure. So for the pump there is no difference between pumping into either size pipe, the water pressure required to move the water into the bottom of both pipes is the same. Now the pressure lost as water moves through the two pipes will be different. Assuming a high rate of flow, a lot more pressure will be lost due to friction in the smaller pipe. So for that reason using a larger pipe will be better. Depending on the flow, however, it may be only very marginally better. To find out you need to calculate the friction loss in the outlet pipe based on the flow and pipe size. See the Friction Loss Calculators to calculate the friction loss in pipes or tubes of various types.
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