Automatic Valves for Rain Barrels:
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.