Yes, I was just thinking about that as a possibility. The bathroom is over the garage, and I can probably trace the pipes to see which lines they come from as it has a suspended ceiling. (another project, sheetrocking the ceiling, sigh). The bathroom tile was done at least 15 years ago, possibly longer so the fixture has been there for a while. Someone may have messed with the plumbing in the garage. Weird.

Hi, About 20 years ago i read about a valve that you could turn the cartridge inside the valve and reverse the hot and cold. I never worked on one but maybe someone knows more about it. Just trying to ring a bell.

Good Luck Woodbutcher

I’ve just discovered that one of the two sinks in the vanity also has the hot and cold supplies reversed under the sink. The supplies to the shower are visible in the garage, but we have to remove more ceiling tiles to see where they are connected so we can swap the hot and cold.

Update: I had a plumber out and he was able to reverse the valve in the shower fixture to correct the problem. interesting.

Update 2: problem persisted, we had to replace the cartridge.