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When installing a water powered back-up sump pump can the discharge of the water-powered back-up sump pump be connected by a Y to the discharge of the primary sump pump? I read that it shouldn't according to the Uniform Plumbing Codes and IAPMO PS119. Also what is the proper distance between the back up sump pump float and the primary?

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You "can" do anything, but the entire point of a backup system is in the event the primary fails. What would happen if the primary discharge tube gets clogged or otherwise prevents discharge? Codes/guidelines are usually there for a reason, even if they can be annoying. – Jacob S Jul 30 '13 at 19:02

Put check valves on both discharge lines upstream of any "Y" connection to prevent back feeding through the unused pump.

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