I own a building that was originally a single family home but was divided into 3 units (long before I purchased it). There are 2 water heaters that service the 3 units. For the past few months the tenants in the top floor unit have been complaining that they aren't getting enough hot water.

I went over the other day and confirmed that after the bathtub is filled about 4 inches, the water starts to go cold.

I recently replaced both water heaters (yes they are plumbed correctly), but wasn't sure which one was servicing the upper unit, so I did a little test. I shut off the valve coming out of #1 and the hot water still flowed in the unit in question (in fact it was hotter than normal). Then I shut off #2 and the SAME THING HAPPENED--they still got hot water.

Yesterday I had a plumber come out and look at the situation. Ninety-three dollars later, the only thing he could say is that it looked like I may have a cross connect between the hot and cold somewhere inside the walls--it is impossible to trace the lines once they leave the basement. But he couldn't explain the situation where when one or the other of the hot water heater valves is shut off I get water that is hotter than when they are both on.

BTW there is no third water heater (in case you were going to ask, as several people have). We've looked all over.

Any other ideas? We're trying to avoid having to tear up the walls trying to get at this problem.

If you have a single handle mixing valve in the bathroom in question it may be the culprit. To troubleshoot this, run the water from a 2 handle fixture and either put it in the bathtub so that you can measure it or measure it another way, (5 gallon buckets) just so you can verify that the water is the desired temp as it is coming out. Of course it will cool as it is being transferred but it will be hot coming out if the tub valve is the problem. If that works you need to change the cartridge in the tub valve. Please let me know what you find.
Darrel

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Originally Posted by theburg

I own a building that was originally a single family home but was divided into 3 units (long before I purchased it). There are 2 water heaters that service the 3 units. For the past few months the tenants in the top floor unit have been complaining that they aren't getting enough hot water.

I went over the other day and confirmed that after the bathtub is filled about 4 inches, the water starts to go cold.

I recently replaced both water heaters (yes they are plumbed correctly), but wasn't sure which one was servicing the upper unit, so I did a little test. I shut off the valve coming out of #1 and the hot water still flowed in the unit in question (in fact it was hotter than normal). Then I shut off #2 and the SAME THING HAPPENED--they still got hot water.

Yesterday I had a plumber come out and look at the situation. Ninety-three dollars later, the only thing he could say is that it looked like I may have a cross connect between the hot and cold somewhere inside the walls--it is impossible to trace the lines once they leave the basement. But he couldn't explain the situation where when one or the other of the hot water heater valves is shut off I get water that is hotter than when they are both on.

BTW there is no third water heater (in case you were going to ask, as several people have). We've looked all over.

Any other ideas? We're trying to avoid having to tear up the walls trying to get at this problem.

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Two water heaters for three units? What's wrong with this picture? Each unit should have its own water heater. What happens when the cold water feed is shut off to both warer heaters? Some where the cold water feed is getting into the hot side. If this were a single family unit I would tell you to shut down each single handled faucet at the angle stops and then check the siutuation, that's where I locate most cross connections However a multi unit building with three units sharing two water heaters is quite another story. The first thing you have to ascertain is your piping layout. What feeds what. Then, and only then, you can plot a course of action and repair.
As you've found out, to the tune of ninety three bucks, a plain old garden variety plumber ain't going to get it. I could have told you had a crossconnection just setting here and it wouldn't cost you a cent. What you need to do is get a hold of the service manager of a large company, explain your problem, and ask them to send out the shop trouble shooter. Every large plumbimg shop has one. This is a experienced man that they send out on screw ups, customer complaints and jobs like yours. I was our shops trouble shooter for nine years before I retired so I know that most shops have a man that they use in a "iffy" situation or a job where the plumber in charge runs into a situation he needs help getting out of. You need someone to "trouble shoot" your piping lay out so you'll know where to start. This has to be done "on site". As much as we'd like to help, you simply have to be there to check it out. Good luck, Tom

I just talked to the previous owner of the building and he did in fact inform me that there is a water heater hidden behind the wall in the kitchen. The back of the cupboard apparently comes out and conceals a 30-gallon tank.

Idiot?? Whoever hides water heaters like that is the idiot.