A plumbing crossover can let cold water enter the hot line through a mixing valve or a recirculation connection. A heater may still produce hot water while a tap runs cooler than expected. Comparing temperatures at different fixtures helps a technician narrow the cause.
Record which fixtures are affected
Note whether the problem begins at one shower, affects several taps, or changes when another fixture is used. If it started after a shower valve, recirculation pump or water heater was serviced, report what changed and when.
Ask the technician to isolate possible connections
The technician can check the heater outlet temperature and compare it with fixture readings. They may isolate suspect valves or recirculation connections to see whether the hot side returns to normal. Leave isolation and adjustment to the technician so the diagnosis reflects how your piping is connected.
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