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!

Is there a recirculation pump on the hot water piping ?

i don't believe so.

FYI there is one shared water heater for all the units and the shower handle is a single handle Delta Monitor 13/14.

How many pipes go up from the water heater to the apartments? I wonder if they fed 2 apartments with a single line, and the problem actually exists in the first floor apartment.

Could you be more specific or are you just brainstorming ideas? Please keep in mind the unit CAN get hot water and that the other units are working fine.

If the tenants turn on the cold water to a slow stream early in the morning is the water coming out warm instead of cold? Depending on the configuration of the plumbing, it is possible that you have a cross connection in a lower unit that is only affecting the upper units. Does each unit have a set of valves that control distribution? If the lower apartments can be shut down and the upper apartment tested individually, you could narrow the search quickly. If the problem doesn't occur with just the upper, then turn on apt#1 and see if the water goes cold, then turn off #1 and test #2 and so forth. My first though was a bad check valve in a hot water recirculation system as well, but since you said it doesn't have one, and the shower valve has been replaced, it is most likely a shower valve or a laundry hookup done incorrectly.

Run the hot water in the shower for the 1st floor apartment. Get the hot side of its valve hot. It should stay hot for a bit. Now(attempt to) run the hot water in the 2nd floor apartment. If the 1st floor valve cools rapidly, there's your cross-connection.

Do you know if the toilets have anti sweat valves? They can create a cross connection. Just another idea to thro out there.