Zone valves are the individual electrically or manually controlled valves that operate the sprinklers or drip. You have one valve per zone (the system of sprinkler heads or drip lines that are controlled by the valve). Valves are located aboveground (manual and some electric valves) or underground in valve boxes (electric valves only).
For underground installations, the valve box should be dry inside; if it's flooded, check for leaks. Occasionally, an electric valve sticks open — usually due to grit lodging in the solenoid port. Piece of cake to fix. First, turn off the water. Then remove the solenoid (the thing with the wires attached to it) and gently rinse any grit out of the port located in the valve body. The other thing that could cause a valve to stick is a punctured diaphragm (no birth-control jokes, please). Remove the top of the valve, inspect the rubber diaphragm for holes or tears, and replace it if it's defective.
Every now and then a solenoid goes bad. You can test for trouble by removing the solenoid and applying 24 volts from the transformer of your controller (not high voltage at an outlet!) to see whether it clicks on. Replace it if it doesn't. (If electrical stuff makes you nervous, call an irrigation company to do this test for you.)
If ground squirrels or gophers have filled the valve box with soil, clean it out. While you're in there, turn the valve on (use the manual bleed control that's on the valve, which is a little lever or screw), and then adjust the flow control (usually a cross-shaped handle on top of the valve) so that the sprinklers are getting enough water. Flow controls usually are set to wide open, but in some cases, you may want to adjust them down to reduce flow over the whole zone.
Another part of the system that sometimes causes problems is the wiring that connects the valve solenoid to the controller. The weak points are the waterproof connectors that join the control wires coming from the controller (these wires come into the valve box from underground) to the solenoid wires. If the connectors fail — or if the connection is made improperly with electrical tape, wire nuts, or duct tape — replace with proper connectors.
MJEff Sometimes underground wiring is damaged due to careless digging or burrowing animals; both can be a serious problem. (You'll know when you have this problem, because one or more valves won't turn on from the controller.) Unless you see suspicious activity aboveground, such as a gopher mound or recent digging, finding the problem can be mighty challenging. Instead of tearing up the whole yard, call an irrigation company that can locate the fault with special electronic equipment.
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