Hot water issues in the apartment and plumbers are stumped?

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. Rarely water might get hot or 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 water the sink 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 cartridge so we replaced the entire faucet set including the rough in valve. I'm not sure how that would effect the other fixtures, but regardless it did not help.

Also we noticed that the shower handle was installed backwards once again (part of the reason the plumber thought it was the cartridge) i.e. the valve is off when it is supposed to be hot and vice versa, but 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?

We've already called several of the best plumbers around, and no one seems to know what's going on. The possible explanations make sense individually but not together. The water heater can't be broken because there CAN be hot water. If something is wrong with the way things were piped, why would the problem only occur now after so many years? The fact that the sink gets hot when the shower is on is what I think confuses most people. I will try playing around with the temperature limiters, but I assume it will not help, because again, why would one fixture affect the other ones?

I hope someone can provide some sense or help us troubleshoot the problem. We've already spent quite a deal of time and money and our tenants are getting annoyed. Thanks!

UPDATE

We had changed the shower handle cartridge last year per a plumber's suggestion and it seemed to resolve the situation but apparently it didn't. Right now, for some strange reason, one fixture is still affecting what comes out of another fixture. For example, the sink will not dispense hot water until the shower hot water is turned on, and then the shower gets cold if the sink hot water is turned on. Like I said before, we had a few plumbers come already last year and they didn't seem to know what the issue was.

It looks like some of you responded with some insightful tips that sound more plausible. Since the building is old, could this be pipe related (the hot water is also yellow looking)? Or does is it more likely to be capacity related (not enough hot water for everyone trying to use it), which doesn't quite seem plausible since there IS hot water, it's just that for some reason one fixture is impacting another. Or could it something with some sensor, valve, or just has to do with the design of the plumbing? The last one doesn't seem likely as the other units don't seem to have this problem, and it wasn't reported before by previous tenants. Last thing is that this and the unit below both report that the water takes a long time to get hot and that sometimes the water gets cold suddenly, and may have to wait for it to become hot again.

Answer 1

Very odd, but here's a thought on how to troubleshoot. Shut off water pressure into your water heater. Test each one of your hot water taps. If any of them work, something is mixing cold water into your hot water pipes. Some one-handle faucet and shower fixtures can do this if their internal seals are worn out. Find it and fix it. If they all only work when water pressure is supplied to your water heater, then the problem is likely the water heater itself.

Are the other units on the same piping? If so, the mix may be caused by a recent change in their plumbing. Maybe you can engage them in the troubleshooting.

Answer 2

I have been dealing with the exact same thing in a larger building for several years. After 3 days, two different plumbers and a ton of money we finally figured out that all of our Moen shower valves had the wrong cartridge in them. We were putting in 1220 the brass cartridge and we really needed 1225 the plastic one that is 1/8" longer. Cold water was getting into that 1/8" gap and bleeding into the other apartments. The further down the line, the colder it was for the next tenant.

Answer 3

Yeah, that's weird. Thoughts:

Hot water heaters have hot and cooler ends, due to thermodynamics. The hot water should be pulled from the hot end, but that might not be done right, either due to internal weirdness or connecting it backwards.

Maybe the hot water heater has multiple coils, and one burnt out?

I suggest measuring the temperature of the hot water output pipe. That could at least rule out the pipes as a cause.