Fixtures will not run hot water unless another fixture is on

Question

We have a unit upstairs in a 4-plex. The other units don't have any issues, but hot water recently stopped working in this apartment. Sometimes water might get warm but goes cold again. When the plumbers came and we played around with the fixtures we found out that the sink in the bathroom would get hot if the shower was on. When we turned off the shower/tub valve the sink water lost heat. I believe it was the same with the kitchen sink, but I could not figure out when or how to get the shower hot. The plumbers were also dumbfounded but one thought something was wrong with the shower valve and wanted to replace the cartridge. Logically, I'm not sure how that would effect the other fixtures, but I don't know what the problem is either. Another plumber didn't know what to do.

I noticed that the shower valve was installed backwards i.e. the valve is off when it is supposed to be hot and vice versa. That is in line with what the first plumber said, but again, how could that be causing, or allowing the other fixtures to get hot water? won't reversing the valve still mean that the other fixtures will still be dependent on that valve when they should be independent from it?

-Frustrated Apartment Dweller

Answer 1

No other fixtures are "dependent" on the shower valve. What may be happening is what is called "crossover", hot water crossing over into the cold water line (or vise-versa) due to a defective single-handle mixing control. It sounds like your plumber may have been right, as this would tend to not have a noticeable effect when the shower is flowing. When the shower is static, you turn the hot on at a different faucet, cold water is pulled through the defective shower cartridge to mix with the hot in the supply line, resulting in luke warm water.

Troubleshoot by positively shutting water supply to the hot water side of your system and checking for flow at a hot water tap, or have a plumber replace the shower mixing valve