A tempering valve blends hot water with cold water before the mixed water reaches part or all of the building. If the valve sticks or its supplies change, several taps can run cooler or hotter than expected even when the heater is operating.
Compare the fixtures on the mixed line
Note whether temperature changes affect one shower or several taps. Tell the technician if the problem began after water heater work or a supply shutdown. Leave the adjustment to the technician, particularly if the water is hotter than normal.
Ask for a temperature check after service
A technician can compare temperatures at the heater and downstream of the valve. Ask which fixtures the valve serves and what temperature was measured at a representative tap after the work. Share the heater and valve labels if you can see them without opening equipment.
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