I have 2 waters on a boat, 1 is 50L and the other is 35L. I have no choice because of space !
I have tried plumbing them in series using the smaller tank as a feeder into the larger tank but I got back pressure at the taps, quite a lot actually and even with the water system turned off, hot water would come from the cold taps for quite a long time.
Can you help me on this subject.

Warren

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I got back pressure at the taps

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Pleaswe explain what you mean by "back pressure". Are you talking about some sort of suction?

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hot water would come from the cold taps for quite a long time.

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Are you saying that after you close the cold water tap hot water continues to run? Can you be more descriptive? Back to you, Tom

Install a check valve that will take care of hot water back flow. Also, make sure you have installed Pressure Relieve Valve on EACH water heater tank. Back to you. Milo