Don’t Install Valves Downstream of an Anti-Siphon Valve

Q.  I have manual shut-off valves installed downstream from my electronic anti-siphon valves.  I installed them to turn off the water to parts of my yard where I grow annuals and only need to water for a few months out of the year.   I would really appreciate it if you would explain why valves downstream cause the anti-siphon valve backflow prevention to fail.

A.  If there are some sprinklers that are not shut off by the downstream valves (ie; there is always a sprinkler that will be on when the anti-siphon valve is on) then you should be fine.  The key to this is that when the anti-siphon valve is closed the water remaining in the pipe downstream of the anti-siphon valve MUST become depressurized.  Depressurizing normally occurs when you shut off the anti-siphon valve and the remaining water pressure in the downstream pipes is released through a sprinkler.   But if you have a valve downstream of the anti-siphon valve it will trap pressurized water in the pipe between the anti-siphon valve and the downstream valve and not allow it to “depressurize”.  Note that sprinkler heads with built-in check valves will also hold the water pressure in the pipe.  That is why when using anti-siphon valves you should remove the check valve from at least one of the sprinklers on each valve circuit (normally you would remove it from the sprinkler on the circuit with the highest elevation.)  the check valves are easy to remove from the sprinklers, normally you just unscrew the sprinkler cap and lift out the riser assembly.  You will see a rubber washer attached to the bottom of the riser assembly, pull it off.  That rubber washer is the check valve seal, with it removed the check valve won’t work.   Now reassemble the sprinkler.

How an anti-siphon valve works:
The  anti-siphon valve works by use of a little air vent that is located on the downstream side of the actual valve.  Look at the anti-siphon valve you will see there is a large cap directly above the water outlet of the valve, the air vent is under this cap.  If you look closely at the lower perimeter of the cap you will see holes or slits that allow the air to move in and out of the vent.  When the anti-siphon valve is turned off the pressure drops in the pipes downstream from it as the remaining water flows out of the sprinklers.  When the pressure drops the little air vent drops open and lets air into the pipe right behind the valve.   This air goes into the pipe and breaks any siphon effect (“anti-siphon”) so that sprinkler water can’t be drawn backward through the valve into the potable water supply.

(Water from the sprinkler pipes can be siphoned back into the water supply system when pressure is lost in the water supply system.  For example, the water company might depressurize their pipes to make repairs.  It doesn’t happen frequently, but it does happen.  When the pressure drops the flow reverses and water from the sprinkler pipes, along with dirt and other yucky stuff, can be sucked in through the sprinklers and then into the water supply system.  When the pressure returns that dirty sprinkler water may go back into the sprinkler system, but it may just as easily go to your kitchen or bathroom sink.  So why wouldn’t the closed anti-siphon valve stop this from happening?  After all the purpose of a valve is to stop water from flowing through it when it is closed, right?  Yes, of course, if the valve is a manual valve.  But electric solenoid valves are “directional” valves.  What that means is they are designed to stop the flow when the water is flowing in one direction only.  When the water flows backwards they don’t fully close!)

What the downstream valve does:
If you have another shut-off valve after the anti-siphon valve, then the water on the downstream side of the anti-siphon valve will stay pressurized even when the anti-siphon valve is closed.  This water pressure holds the little air vent in the closed position so it can’t let in air, and therefore the siphon effect is not broken.  This means the anti-siphon part of the valve will not work.  Even worse, when the little vent is held closed for days at a time due to the constant downstream pressure, it eventually just sticks in the closed position.  Then even if the pressure drops the anti-siphon won’t work.

My Friend or Irrigation Person Says This is All Just Something  YOU Made Up!
Unfortunately, this wrong practice of installing valves after an anti-siphon valve is pretty common in the irrigation industry.  I’ve been called some pretty ugly names over this issue.  Fortunately for me, you don’t have to take my word for it.  Tell your friend/buddy/pal to read the box the anti-siphon valve came in.  It says right on it “do not install valves downstream” or something similar.  If you don’t have the box or it didn’t come in one, then go to the manufacturer’s website and find the anti-siphon valve installation instructions.  You will find that same warning.  Here’s a sample from Rainbird if you want to check for yourself:  Rainbird Anti-siphon Valve Operation Manual. See the section that starts with the heading “CAUTION”.