Since it’s only the hot water that seems to be involved, it’s possible that a prior owner installed a return line going back to your water heater from the bathroom area. As you know, you only have hot water in the water heater unless you open a tap or valve. The water in the piping cools over time. Since the tap can be quite a way from the water heater, someone may have wanted to get hot water faster for their morning shower or to mix a little hot water in with the cold water going to refill the toilet and cut down on the amount of condensation on the outside of the toilet tank on hot, humid days. This can be a problem in some parts of the country.
The way a system like this works is there is a small pump just past the water heater on the supply side and a return line from the bathroom. The pump keeps hot water going to the bathroom area and the return line lets it go back to the water heater for reheating. There has to be a “one way” valve in the regular supply line to the water heater to keep the warm water coming back from the bathroom area from going into the supply piping as well as the pump and extra piping that would not be in a more normal system. With the pump and water running in 2 pipes, one to the bathroom and one returning, this might be a little noisy.
Let us know what you find as you look further.
Henry in MI
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