Shower water gets hot when the toilet is flushed

plumbing question. Why would the water get hot when the toilet is flushed in the bathroom

Cold water is being used to refill the toilet and the flow of cold water to the shower fixture is reduced.


I know that. Shouldn’t there be independent water supply ?

The bathroom is served by one hot and one cold supply. Flush the toilet and you starve the shower of cold water. If the shower valve is not a modern thermostatic valve you will get more hot water than cold water.


Is there a specific name for that

A specific name for what

When that happens

Yeah it’s called crappy water pressure.


Thanks you for all your help

Your welcome Joseph.

If they would have hit the water heater first with the supply line. You wouldn’t have had that problem. Been there done that

Thermostatic Mixing Valves
Mixing valves adjust the temperature of the water that comes out of the hot water heater before it reaches the faucet. They automatically mix the correct amount of hot water coming out of the water heater with cold water to help maintain a safe water temperature for washing and showering.

Sounds like they need a Delta Scald Guard shower valve, or so replumbing.

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:thinking: Dear, Kids,“Don’t flush the bloody toilet” when I am taking a shower!, or maybe I should install a thermostatic mixing valves prior any faucet. :thinking:Hum. … Which ones cheaper? I got it… a thermostatic mixing valve :grinning:

I concur Spence. A Delta 36004 Scald Guard Single Lever Cartridge would do the trick. Great work!

How’s inspecting going in Iowa these days?

What happens is the cold supply feeding the shower valve loses pressure and can no longer push the same amount of water (flow) through the shower valve, so now there is more hot water than cold. If you took the shower cold supply and you ran it (without the toilet) back to the same point where the water heater taps into water main, you would not have the same pressure loss when toilet is flushed. However, this is not done because it is expensive and would take more space for the pipe. Usually only one cold supply feeds the bath. So if you install a pressure compensating shower valve it will adjust the hot side for the pressure drop on the cold side, keeping the water temperature almost the same when the toilet is flushed.

A pressure balance valve such as the Moentrol would resolve this problem.

Yes. I’m taking a shower so don’t flush! lol