enter image description hereI had someone come over my house and winterize the sprinklers. He explained to me what he did: He said he closed the main valve to the sprinkler system, hooked up a compressor and blew out all the water from sprinkler pipes. He also, opened up the air valve so air can go in but water can not. After that I noticed the water is leaking slowly from the air valve. I called the same guy again and he told me the water is leaking because the main valve to sprinkler system is bad and I need to replace the same.

Can someone please point out if water is leaking from the air valve while the main sprinkler valve is closed, does it need to be replaced. Please note that while the air valve was in the closed position and main sprinkler valve open, no water leaked. Also, while keeping the main sprinkler valve closed, if I close the air valve too, water starts coming out of something connected above the air valve.

  • 3
    A labelled picture would help especially to figure out what the "something connected above the air valve" that is leaking. It's definitely possible though that the valve(s) have just started leaking now that they've been operated, even if they haven't leaked before. Could be that a gasket is damaged or there is sediment, rust, or something similar keeping it from fully closing.
    – gregmac
    Oct 24, 2011 at 16:46
  • added a picture.
    – Varun
    Oct 25, 2011 at 16:03

3 Answers 3


Without a picture I am guessing, but it sounds like the "something connected above the air valve" is your pressure vacuum breaker. A pressure vacuum breaker prevents water from flowing from the outside back into your main water line (and then into your drinking water). It will allow water from inside your water main to travel through it with the water pressure from your house forcing open the pressure vacuum breaker (while water traveling in the reverse direction closes the valve and prevents contamination of your drinking water).

It does sound like the sprinkler water main has a very slow leak. And I imagine a slow leak from your sprinkler water main is not enough to completely open the pressure vacuum breaker and it is just exiting at this point (when the air valve is closed).

  • Jeff, I have attached the picture for your reference. Thanks.
    – Varun
    Oct 25, 2011 at 16:03

it's not an air valve you have. This is called an anti-siphon valve. On the inside, there should be a shut-off on the pipe coming out of the house. Make sure that is shut off. Again, this should be on the inside of your home and is typically a lever valve.

I'm not a plumbing expert or anything, but have owned 3 homes and in each home I either fixed or installed irrigation systems. I learned through college of hard knocks.

Consider the situation where you have a garden hose attached to that spigot you have labeled as "air valve" and the hose is used to fill a pool and is left in the pool. The anti-siphon valve is suppose to stop water from flowing back into your water supply inside the house thus contaminating drinking water.

The air sound I think you hear is that you've removed all the water outside leaving an unbalanced pressure. Some irrigation systems have a "check valve" inside with a pressure relief screw. If you have one of these, you can turn the screw to release the pressure.

Hope this helps.

As an update: Then what I would do is make sure the inside lever is still closed and open the spigot you have labeled as an air valve and if you have the underground valves for your sprinklers and they have a manual switch, flip it to see if the "leak" goes away. I had this scenario before. I blow out my own sprinklers each year. DO NOT leave the mechanical switch override open though. Only do this to see if the leak stops. If it stops then you have a simple pressure imbalance is all.

  • Yes, there is a lever valve inside the house and it is in shut off position. I don't think I mentioned about air sound in my question.
    – Varun
    Oct 25, 2011 at 14:34
  • BMitch, What I have is 2 sprinkler valves - outside the house - downstream to the above assembly. So, are you saying I should turn them on to see if the leak stops? And if it does, there will be water again in sprinkler pipes; which was the condition before I got the water blown out. If you are saying this will prove a simple pressure imbalance, how do you think it will resolve it?
    – Varun
    Oct 26, 2011 at 3:06
  • no, you don't want to put water in the now empty lines. Underground sprinklers typically have two underground control boxes. One for the front yard & one for the back. Open each box to locate the electro mechanical valves. These valves typically have a small lever like switch on them so you can manually turn them on. Since the water is shut off inside the house, you can safely turn these switches to their ON position. Do this for all valves and see if it fixes the problem. Obviously if you hear a rush of water, turn it off immediately and re-confirm inside shutoff is off.
    – Eric
    Oct 27, 2011 at 2:14
  • incidentally, if you have an air tank you can blow out your sprinklers yourself by hooking it to the outside spicket (labeld air valve in your photo), turn off the water from the inside and then open each sprinkler zone with an air tank full of air.
    – Eric
    Oct 27, 2011 at 2:16

I believe the same situation happened to my pressure valve as well. After I shut off the main water valve inside the house, I opened up the pressure valve by unscrewing the 2 bolts. Then, I could see some rust and dirt that made the valve not close fully when the main water valve was opened inside my house.

So, I cleaned up the rust and the dirt before reopening the water main valve inside my house. No more water leak from the pressure valve anymore.

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