Backflow valve started blowing water out bottom of valve when sprinkler system shut down.
Sprinkler system shut off and water started flowing out bottom of backflow valve. When run sprinklers no water comes out bottom, only when sprinklers stop. What could be problem with backflow valve?
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You have an RPZ, and they will vent from the bottom of the device when a valve shuts off suddenly. The venting will cease, unless there are other factors.