Selecting a New Pump for a Old Irrigation System:

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.

If you need a pump for a brand new irrigation system, see the article on Selecting A Pump For A New Irrigation System.


Replacing it with the Same Exact Pump:

Just a quick tip on swapping out pumps, then we’ll move on to selecting a new one.

Make sure the new pump really is the exact same pump as the old one.  Many pumps are individually custom built for a specific flow, which means even if you get the same pump model, it may not have the exact same performance.   The pump should have a serial number on it that hopefully will allow the manufacturer to get the exact same pump specifications from their records.  So get the serial number off the pump as well as the model number.   I suggest you take photos of all the name plates and labels you can find on the pump, as well as the pump from every angle.  Take lots of photos of the pump and bring them with you to the pump dealer to help their tech department figure out what actual pump customizations you might need.


Getting a Whole New Pump:

We will assume from here on that you want a  whole new pump model, maybe even a different brand.

Warnings:

  • Do not select a replacement pump based on horsepower alone. Two pumps can have the same exact horsepower and produce radically different flows and pressures.  We’ll discuss this more in later articles.
  • Keep in mind that if your old pump is over 5 years old it is likely worn, and as pumps begin to wear out their performance decreases. So while you may be thinking you need a bigger pump because the irrigation isn’t working as good as it should, this may not be true.  You may just need a newer, less worn out pump!  When you install a new pump you may find it performs much better than the old worn out one did, even though they are both exactly identical.

Here’s the basic procedure to follow if you’re selecting a pump for an existing irrigation system.

1. Decide on the type of pump.  

Figure out which type of pump best fits your needs, end-suction centrifugal, submersible, turbine, jet pump, etc.  This will be covered in detail by the next article in this series: Types of Water Pumps

2. Determine your flow and pressure requirements.

If you have the original irrigation design the plans should tell you this information. If not, you will need to figure out what the requirements are for your irrigation system by “reverse engineering” it.

How to Reverse Engineer the Flow: Calculating the flow demand is easy but time consuming, you look to see which nozzles are installed in the existing sprinklers on a single valve circuit, look up the flow requirement (GPM) for each sprinkler and nozzle (use the sprinkler manufacturer’s website), then add up the flow requirements (GPM) of all the sprinklers that operate at the same time.  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.) 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 pressure is much easier a problem to deal with than too little pressure.  If you get a pump that is too big you can just throttle the pressure down and the only damage done is perhaps a little electricity wasted.  If you get too small a pump and it doesn’t create enough pressure you are screwed, you have to remove the pump and get a bigger one!  So it you are guesstimating at a pressure, guess HIGH!  If you aren’t sure choosing between a 50 PSI or 60 PSI pump, get the 60 PSI one!

  • 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 pump is going to be installed, turn on the system, and the gauge will tell you the pressure in the existing system.   You may need to cut the pipe and install a tee fro the gauge to connect to, or possibly you can drill a hole in it, thread the hole with a tap, and then screw the gauge into the hole.  Remember that if your existing system uses an old pump the pressure may be lower due to pump wear.  You might want to increase the pressure by 10 PSI to compensate.
  • 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.
  • Guesstimate It.  If you really can’t figure out the pressure needed you can use a guesstimate.  Most of you are likely to use this method.  This 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 .  To guesstimate the pressure needed take the largest distance in feet between adjacent sprinkler heads, and add 30 to it.  Which sprinkler heads to measure?  Generally you should look at all your sprinkler heads that are next to each other.   Find the two that are the farthest apart, but still are closer to each other than they are to any other sprinklers.  It’s not uncommon to have two heads that are way further apart than all the others.  If so you can choose to ignore those two and use two other sprinklers that are a more typical distance apart.
    So example, lets 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 others so let’s ignore it and use the next greatest distance, which is 35′.   So then 35′ + 30 = 65 PSI of pressure guesstimated for the pump.  Understand there may be a dry spot between those two heads that are 50′ apart (there probably already is a dry spot there!) but we don’t want to overkill with the pump either.  That 50′ distance is really a design error on the original sprinkler system and there should be another sprinkler head in the middle of that space.
    So the formula is: spacing in FEET + 30 = Pump PSI pressure for the pump.
    If the resulting total is less than 50, use 50.   Do not use a value less than 50 PSI !
        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!
    No guarantees, this method gives you a guesstimate!  You understand that you are taking a risk using this.  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.

3. Miscellaneous Stuff to Go with a New Pump Installation. 

You may need to add some new pipe and other equipment to the existing irrigation system to accommodate the new pump. For example if you are switching a irrigation system from city water to water from a lake or stream you will probably need to add a intake pipe or manifold, as well as an intake screen and filter.  You may require pump control valves (ie; anti-cycle valves,) Filters, and/or new mainline pipe to attach the new pump into the old irrigation system.  Each one of these new items causes some additional water pressure loss, so you need to add maybe 5 PSI more pressure to the PSI value you previously calculated above for each of them.   For long lengths of new pipes the pressure loss can be calculated using the pressure loss calculators on the Landscape Irrigation Formulas page of this website.  Example: If you’re just adding a short intake pipe (20 feet or less) and a filter this is going to require about 10 PSI additional pressure (5 PSI for the filter plus 5 PSI for the intake pipe.)  So if you had previously determined you needed 65 PSI to run the sprinklers, you would add the 10 PSI to that value, resulting in a new pressure requirement of 75 PSI.

4. Start Shopping. 

Research the available pump models and select a pump model that meets the minimum pressure and flow requirements of your irrigation system.

Now that you know what the process is, let’s move on and start learning about pumps.  First up there are lots of different types of pumps used for irrigation systems: centrifugal, submersible, turbine, jet, etc..  Let’s determine which is the best type for your irrigation system.  Go to  Types of Water Pumps.