Symptoms: When using shower, cold water soon (2 minutes) will supply warm or hot water. i.e without turning the hot valve on, and only turning cold valve on, the water out of showerhead gets warm or hot.
The shower has an American Standard single handle valve. I changed the cartridge but situation is the same. The old cartridge was missing one o-ring by the ceramic disk anyways.
This problem is only seen in one bath (with single handle), other baths don't have this issue. And none of the faucets in the kitchen or vanity sink have this issue. This bath is the closets one to the water heater unit.
It was suggested that I have a "cross over" problem, typically caused by single handle faucets. There are two such single handle faucets in the building. One upstairs vanity sink faucet and one right below it, ie the shower having this problem.
Isolate vanity sink: I closed off both cold and hot supply lines to the vanity sink faucet but the problem remains. While doing this test, I noticed that hot line supply to the vanity sink was turn all way on. I lowered that.
I checked the water heater and noticed that temp is set to "VERY HOT", touched the cold supply pipe and it was warm. I lowered that to "hot". Cold supply pipe is no longer warm to touch.
One more data point, there seems to be some temp fluctuations in all the showers. Water temp is adjusted, a few minutes later, suddenly the temp drops to very cold, requiring increase in hot supply, then it is too hot, requiring decrease in hot supply.
It was also suggested that the washer has a mix valve in it that could cause this sort of cross over.
Any ideas ?