Shower with only hot water on, flush toilet, it gets hotter

[MIBamse]

Hello
We have a problem with luke warm water in the shower in the morning. The shower water is not warm enough to mix it with cold water, so we only turn on the hot water faucet and leave the cold water faucet off.
My wife is not happy about the water not being very warm so she flushed the toilet, and the water got scolding hot!
How is that possible if we are not running any cold water in the shower?

Cold water entering my hot water pipe? How do I trouble shoot that the best?

Thank you for looking at my question

 

[Reach4]

Interesting. Try closing either the hot or cold stop valve for your lavatory, kitchen sink, washing machine, or whatever else uses both hot and cold. That should eliminate each from being the cause of the crossover.


One more thing: do you maybe have a mixing valve at your water heater? I think the crossover idea is most probable, but if you have one of those mixing valves we could think how that could fit into your symptoms. I don't see it so far.

 

[Sponsor]

 

[Terry]

Do you have a single handle shower faucet? Can you post a picture? 800 pixels or less.

 

[MIBamse]

Do you have a single handle shower faucet? Can you post a picture? 800 pixels or less.

No, two handles, one for cold and one for hot. Old shower...

 

[hj]

There are a few possibilities, but would have to be there to do some testing to see which one is the cause. Turn on the sink's hot water, while the shower is running, and see what happens.

 

[Jadnashua]

Water will flow along the path of least resistance. That can depend on the length of the pipes, how many changes of direction it has, and other things. By flushing the toilet, there was less cold that could cross-over into the hot, so the shower got hotter. Now, finding where that cross-over is will be the challenge.

Usually, it takes a single-handle valve to allow crossover. So, for any of those that may be in the house, one by one, shut off the cold supply to it and see if that makes any difference. Once you find the offending one, you'll need to repair it. NOte, some people mix their supply into the washing machine, so check that, too.