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Our new house has a hot water dedicated recirculation system that was installed with only a single check valve, a Euroblock spring check valve that is supposed to prevent the hot water from backflowing into the cold inlet to the water tank/heater. (Not the purpose of this post, but separately, there was no check valve installed on the recirculation line after the last fixture, which results in cold water backflow into the hot line when the recirc pump is off.)

Initially the plumber did not install any expansion tank for thermal expansion, so not surprisingly the pressure relief valve on the hot water heater started leaking pretty quickly. The plumber then installed an expansion tank on the cold inlet line, but placed it BEFORE the check valve from the recirculation system. Based on information I've seen online (I'm not an expert), this isn't correct, as the check valve closes the hot water system and to relieve the thermal pressure, the expansion tank should be between the check valve and the water heater. However, once the expansion tank was put in (the wrong) place, the pressure relief valve on the water heater stopped leaking, which seems to indicate that the thermal expansion is somehow being alleviated.

Does anyone have any idea how an expansion tank placed before the check valve could be alleviating thermal expansion? Is it possible that the check valve itself is not sealing completely (and can this be tested)? I would like the plumber to reinstall the expansion tank in the correct location, but he insists that he has solved the problem as the PRV is no longer leaking. Honestly, as long as the system is not stressed, I'm fine leaving it - I just can't wrap my head around what's going on.

I can post pictures if helpful. Thanks all for your advice.


Licensed plumbing contractor
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San Francisco Bay Area
Any diagram of dedicated recirculation and pure logic dictates check valve needed at the cold water line in let of the water. Depending where the return line taps into the WH determines if any second check valve would be needed.
Returns into the bottom of the WH tank do not require any extra check.
A check valve is NOT needed after your described "last fixture" unless you can illustrate your situation.
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