This one needs a licensed pro
Likely cause: a valve with no pressure balancing
An older plain mixing valve has nothing to offset a flush. The fix is a new valve in the wall, which is a plumber's job.
Why this happens
Symmons explains that pressure-balancing valves exist to react to changing water pressure when someone flushes a toilet or runs cold water elsewhere. A plain mixing valve does not react at all, so every drop in cold pressure sends hotter water to the showerhead. Symmons adds that thermostatic valves give finer control and may work well where supply swings are dramatic.
What to do right now
- Test the water with your hand or a thermometer before anyone steps in.
- Ask the household not to flush or run water while someone is showering.
- Check that your water heater is set no higher than 120F, as CPSC recommends.
- Ask a licensed plumber about replacing the valve with a pressure-balancing or thermostatic valve.
What to tell the plumber
- The temperature swings hard when a toilet flushes or another tap runs.
- The valve brand if you can find one, and a photo of the handles and trim.
- Whether there is an access panel behind the shower wall.
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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