Anti-siphon valves are sprinkler valves used in an irrigation system to prevent water sprinkler water from backing up into the potable water system. Regular maintenance on your valves can prevent water leakage. It is always good to do a weekly sprinkler check by turning on each station manually and electronically to see if you have any leaks or malfunctions. You will notice as you are doing this that your valves will spout water occasionally. As long as it is not continuous this is fine.
The anti-siphon valve is used to prevent back siphonage into the main water system. This may occur when the water system fails and loses pressure. Then the water will tend to be siphoned back to the potable water. So a one directional valve or check valve is installed. This is part of the anti-siphon valve. Water can go out but can not come back in. If it does get siphoned back into the system, as soon as the valve loses pressure, a poppet is activated that keeps the water from flowing back into the potable system. The poppet drops down to seal the valve and open a vent on top of the valve, which lets the contaminated water flow out.
While the sprinklers are on, you may notice that suddenly there is water gushing out of the vent on top of the valve and then it stops. This means that there was a loss of pressure in your system and the valve is operating normally by letting the water gush out. When pressure returns, if the sprinkler system continues to operate normally, then you just lost pressure for a moment, and this is normal. Pressure fluctuation could come from the use of a fire hydrant nearby or the city adjusting the system.
If the valve is gushing water from the vent of recently installed sprinklers, and little water is coming from the sprinklers, this may mean a bad installation. The valve likely is not 6 inches higher than the highest sprinkler. If this is the case then you must raise the valve and replumb it.
Water Line Break
If water is gushing from the vent and not getting to the sprinklers, but you have pressure at the house, there may be a main sprinkler line break. Shutting the sprinklers off and trying other stations will get the same gushing water leaking from the vent. This indicates a break. By leaving the sprinkler line on, you can find the break in the line by looking for where it is bubbling up from the ground. Once found you then need to turn the water off and repair the break.
If water gushes from the vent and will not stop, but you have pressure on the other sprinkler lines, this indicates a clogged valve. You need to turn the water off to the sprinklers and open the valve up to unclog it.