You can fix this
Likely cause: the water heater, not the shower valve
A shower that fades colder over minutes with nothing else running is usually running short of hot water. Check the heater before the valve.
Why this happens
A pressure-balancing valve reacts to sudden pressure changes, not to a slow fade. A gradual drift colder usually means the hot water supply itself is running short. A. O. Smith lists insufficient hot water supply as one of the signs a water heater needs repair or replacement.
How to fix it
- Run the hot water at a sink for the same length of time. If it also fades, the heater is the cause.
- Check whether someone else was using hot water at the same time, like a dishwasher or washing machine.
- Check the water heater's age on its data plate. A. O. Smith puts average life at 8 to 12 years.
- Follow the water heater troubleshooter linked below.
If this does not fix it, tell the plumber
- The shower drifts colder over several minutes even when nothing else runs.
- Whether hot water at the sinks fades the same way.
- The water heater's fuel type and age from its data plate.
Build the full list: what to tell the plumber.
Start this troubleshooter over
References (every fact above comes from one of these pages)
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