FWIW...Not saying this is the case but, if the "flapper" valve is an offset pivot disc design, it would be pressure balanced to where it would not take that much force to open it even with 20 psig water pressure against it.

The designer and/or installer of the system should be able to provide details on the valve used.

Last edited by Bill D.; 11/13/17 07:38 PM.

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