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If you are on a well then adding an expansion tank will be useless as the existing well tank will take care of any expanded water
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You may have some inadvertant crossover between a cold water supply and the hot water in that Heater B system. It could be a single-control faucet that is bypassing hot to cold. Try shutting off the supply to all of the faucets on that system, and then see if you get the hot water back flowing out of the B heater when you irrigate. My guess is that it will stop.
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Well... I replumbed the 2 HWHs in true serial and everything is fine. I'm going to guess my quasi-parallel system - Tank B - was overpowering the only fixture it was infrequently serving. Thanks, topic closed.
