Why does my hot water fluctuate

The hot water temperature in the shower / bathtub fluctuates

In any case, it sounds like cold water is going into the hot water circuit. I would like to schedule a maintenance window where the hot water supply is completely shut off (including the return line / circulation pump) and you can access the units that share this circuit.

Hypothesis: water continues to flow from the hot water taps. possibly cold water as you are no longer supplying the system with hot water. (Start with the top floor; otherwise, this test will be confused by all of the water in the pipes above you.)

If the water continues to flow, you have a leak or transition from the cold water system to the hot water system. Your ears, a stethoscope, and / or selective shutoff of cold water valves could isolate this. I've seen bad faucets cause this problem in residential buildings, but appliances (especially washing machines) can too. Problems are most likely to appear first on the lower floors, where pressures are higher.

I suspect you will find that hot / cold lines are crossed (or improperly connected) at one point in the building. When a resident runs hot water, the line will be filled with cold water from this intersection point and not from the water heater. Your hot water system eventually pulls the cold water back to the heater, but it does mean that cold water is coming out of the hot water pipes at times!

If the water stops, if you can drain the entire hot water system and no water is flowing at all, you have determined that there is no leakage or crossover, at least when the connected taps are closed. Before turning the hot water back on, I am inclined to test these faucets. All faucets leave some cold water in a drained hot water system, but only when the faucet is open - and it shouldn't be too much.

The last resort, after everything has been turned back on, is to monitor the temperatures of both the "hot" and return lines on the water heater for a while. The draining hot water should have an almost constant temperature. The return line should just be a little cooler, but still constant; If the "hot" temperature (outlet side) drops quickly when someone is using hot water, the heater itself may have a problem, e.g. B. a broken dip tube (or some other "short circuit" between hot and cold).