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I bought a house that has a bad DIY sprinkler system. I am trying to figure out how to attach an air compressor to the system so I can blow it out. The below photo is the only external place on the house where it appears it would even be possible to attach the compressor. The cap appears to be something you could unscrew, is about 2" in diameter, and says something on it about "Max pressure 150 PSI".

Any ideas?

Photo of sprinkler system pipe and cap

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That is a Vacuum Breaker which is in place to prevent water from siphoning back from the sprinkler system into the water supply. It closes under water pressure and opens to the atmosphere under vacuum.

enter image description here

You theoretically could spin the top off and using a rubber ball fitting, use an air hose to apply pressure, however; the inflow side of the circuit will have a manual or electric zone control valve preceeding it that could be damaged by air pressure being forced into its outlet. They're usually a rubber diaphragm servo action valve that is only meant to block pressure on the inlet side and probably won't take kindly to having it forced through them backwards. So, it would be best if you could arrange an adapter that would seal against the inlet seat and the top so the air pressure only gets applied to the outlet of the vacuum breaker.

enter image description here

Zone Control Servo action diaphragm valve.

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If you did use this for access, the supply pipe is still full of water. Is there something to connect to in the basement/crawl space? That would facilitate clearing all exposed lines. –  bcworkz Oct 6 '13 at 18:52
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As @bcworkz states, blowdown from the inlet side of the control valve (wherever the manual shutoff in the supply line is) with all control valves manually on would probably be the best. Due to the design, the air pressure will open the control valves, close the vacuum breakers and push the water out the sprinklers. Depending on municipal ordinance, there also may be a double backflow preventer (checkvalves) between the supply line shutoff and the control valves that also needs to be water free. You probably don't want to exceed 50psi doing this. –  Fiasco Labs Oct 6 '13 at 19:09
    
This is a rental property so I can't check the basement at the moment. I'll look for access down there when I get a chance and let you know. Thanks for the very helpful responses! –  jfritz42 Oct 7 '13 at 3:34
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