Stopping Low Head Drainage
Sprinklers that Spit Air
What Is Low Head Drainage?
Water that flows onto the sidewalk or curb after the sprinklers turn off, but then stops after a few minutes, is due to a phenomena called "low head drainage". This occurs when the sprinkler system is installed on a sloped area. The slope does not need to be very high, a change of elevation of less than a foot will often create low head drainage. After the sprinklers are turned off, the water in the pipes drains out through the lowest sprinkler heads and is replaced with air. The easiest way to tell if you have low head drainage is to watch the sprinklers when you turn them on. If they spit and spew lots of air when the valve is turned on, then you have a low head drainage problem. Obviously the water that drains out of the pipes is wasted. The spewing and spitting of air every time you turn on the sprinklers also puts a lot of stress on the pipe and sprinklers.
Leaking Valve or Low Head Drainage?
If you have water flowing from a sprinkler head continuously, even when the sprinkler system is off, then the problem is a leaking control valve. The primary difference between low head drainage and a leaking valve is that low head drainage results in water flowing from the lowest sprinklers for a while after they run, but the drainage stops after the pipes are fully drained. (It may take several hours for the water to drain out of the pipes.) If a valve is leaking the water will run out of the lowest sprinkler head all the time, 24 hours a day, every day. A typical indication that the problem is a leaking control valve is moss or algae growing on the sidewalk due to the constant flow of water. Another common sign is puddles of water around the lowest sprinklers that never dry out. To fix the leaking valve you must disassemble the valve, clean it, replace any bad parts, then reassemble it. See How to Repair a Irrigation Solenoid Valve for instructions. It is possible and common to have problems with both a leaking valve and low head drainage. Fix the valve first, then check for low head drainage.
How to Stop Low Head Drainage:
To fix low head drainage you need to have special anti-drain check valves installed at the sprinkler heads. These check valves prevent the water from draining out of the pipes through the lowest sprinklers. In most cases these check valves are built into the sprinkler head. The anti-drain check valve is an optional feature available when you purchase the sprinkler. This check-valve option is available on all major brands of sprinklers. The anti-drain check-valve closes and holds the water in the pipes when the sprinkler system is off. These built-in check-valves don't cause any drop in performance of the sprinklers, so they don't have any impact on your sprinkler system design. Many major brands of sprinklers can be retrofitted with a new internal check valve, although the retrofit kits are hard to find. Most pros simply buy a new head with the check valve feature and replace the old sprinkler with the new one.
You can also buy separate check valves that can be installed on the pipe under existing sprinkler heads. These are a lot harder to install, you need to dig up the sprinkler head, remove it, install the new check valve on the riser pipe, then screw the old sprinkler back into the new check valve. Then comes the hard part- the length of the check valve you just added makes the sprinkler sit about 3 inches higher than it was before! So now you need to lower the sprinkler head. There is also another catch to the retrofit type check-valves; they do result in a drop in the sprinkler's performance. They typically create a drop in water pressure at the sprinkler inlet of 2 to 5 PSI. In addition to the performance drop the retrofit devices are often more expensive than simply replacing the sprinkler with a model that has a built-in check valve.
Where do you buy sprinklers with check valves or retrofit check valve? Check valves and sprinklers with built-in check valves are often not available at discount stores or big box hardware stores. You will probably need to get them from a local irrigation specialty store or online.
A tentative change to the language of the State of California's Water Conservation Laws will make the use of low-drainage check valves mandatory for new sprinkler systems starting in the year 2010, and may also require retrofit of many older sprinkler systems with check valves. Expect other States where water conservation is an issue to also implement this requirement.