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I'm experiencing a problem where I'm getting hot water out of one bathroom sink (the closest to the water heater), but luke warm water at best in all other places. I crawled under the house to follow the pipes, and I found that the branch line leading to kitchen is barely hot, whereas right before that branch on the hot water pipe, it was too hot to hold onto.

What could be causing this? Thinking it might be a pressure issue I turned off the water heater, drained the tank, refilled and reheated it, but the same problem persists. If it was just one faucet causing the problem I can see how it might be a bad mixing valve, but how is it that I just have one reliable hot tap, and the others are barely warm? We just had our water heater replaced earlier this year, so it's not even a year old. The hot line coming out of the tank is definitely hot.

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    Try shutting off the valves feeding the fixture that gets good hot water (or the one right where the "too hot to hold / just warm" transition takes place, if not the same) and see if the others improve. Alternatively, shut off all but one, and reopen until the problem recurs, then shut off that one and see if the problem stays solved - then replace that one.
    – Ecnerwal
    Commented Feb 15, 2021 at 3:14

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Failing back flow preventers in a shower or kitchen thermostat faucet could be the reason. At that faucet, the cold water could be pushed into the hot water pipes, since the cold water normally has a slightly higher pressure. The back flow preventers are upstream of the mixing area which is upstream of the opening valve, i.e. this effect is significant even if the faucet is not in use. In a normal non - thermostat faucet, the mixing area is downstream of the opening valves. The hot water back flow preventer is usually the first to fail due to higher temperature.

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