I live in a condo that has 13 units in two buildings and am the president of the board. A few months ago we had a new water pump installed in our boiler room. Shortly after that all three units in my tier of the building experienced hot water fluctuations in the shower. The water would go from being hot to cold and back. This would happen several times while using the shower.

We had the plumber out who replaced and installed the new pump to check and see if it was the cause of the issue but he couldn't find anything wrong. We replaced all the cartridges in the tub faucets and that didn't help. The plumber checked again for a possible cause in our boiler room but again couldn't find anything. So as a test he turned off the hot water return line which stopped the water from fluctuating. It wasn't an issue to do it because my tier is directly above the boiler room so we get hot water rather quickly.

Now owners in the adjacent building which is further away from the boiler room are having the same issue with the hot water fluctuating in the shower. A plumber was recently out again to check for the cause but couldn't find anything. His theory is that someone remodeled their bathroom around the time the new water pump was installed and crossed the lines there. He advised us to check any units that had major work done in their bathrooms recently.

The plumber says a large volume of cold water is getting in the hot water line. He showed me the hot water line in the boiler room and we both felt it get cold and go back to being hot a couple of times. Since I know very little about plumbing would this be the cause of the issue? Is there something else that may be causing this? Could a bad cartridge in someone's tub faucet be the cause (which seems unlikely to me)? If anyone could point me in the right direction I'd appreciate it.

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    Me thinks you should get a new plumber. This isn't a cartridge issue and I doubt it was caused by anyone doing a renovation at the same time (although possible, unlikely). – DMoore Jan 11 '16 at 15:40
  • Thanks a lot for the reply. Yeah. I'm thinking the same too. We're going to try getting a different plumber out this week. But your saying it is possible a bathroom renovation that swapped lines in a unit could be the cause and would effect the whole building? – QR600 Jan 11 '16 at 16:44
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    It's possible but unlikely. To fix a problem like this there is a ton of mapping/troubleshooting involved. – DMoore Jan 11 '16 at 16:48
  • Questions to ask. Why did the problem change buildings in the test. Sounds like a pressure drop when some one flushed the toilet in a neighboring apt,however, the return line going cold that throws a wrench in my posable cause. If it is a pressure drop a small pressure tank,like for a well but 1-3 gal size, plumbed into the hot circuit has fixed this problem is multi bathroom houses. If it already has said pressure tank,then check the air pressure setting. The bladder may have failed. – David Droescher Mar 29 '16 at 18:11

Your plumber is right... or his test was successful in identifying the problem. Since the hot water is installed in a loop system, and cutting off the return seems to solve the problem (or at least reduced the issue), then obviously there is a bad cartrige or crossed line somewhere.

Cold water usually has a slightly higher pressure than the hot water because it has a shorter path to travel (it doesn't travel through a boiler). So, with a bad cartrige (maybe missing an o-ring or something) or a crossed pipe, the cold water would flow into the hot (instead of hot into cold).

Regarding the test, since the hot water is installed in a loop system, the pressure is actually even lower than a non-return type system. The lower pressure of the hot water loop system allows more cold water to enter the hot water line. When he shut off the return (no loop), the problem was lessened to a noticeable extent. This strongly suggest a crossed pipe or faulty cartrige.

First, since your problems started when the water pump got changed, you should start there. Ask your plumber if there is a one way check valve on the return line. this valve prevents cold water to be drawn back though the return line. If not have him install one. Second ask all condo owners if any of them installed a recirculating pump in their units. This to will cause the same symptoms you are having. Hope this solves your problem.

Definitely sounds like cold water crossing over into the hot water loop. I'd be inclined to schedule a maintenance window during which the hot water supply is shut completely off (including the return line / recirc pump), and you can get access to the units that share that loop.

Hypothesis: Water will keep flowing from any of the hot water taps; eventually cold water since you're no longer supplying hot water to the system. (Start with the top floor; otherwise this test is confounded by all the water in the pipes above you).

If water does continue to flow, you've got a leak or crossover from the cold water system into the hot; your ears, a stethoscope, and/or selectively shutting off cold water valves could isolate this. I've seen bad faucets cause this problem in apartment buildings, but appliances (particularly washing machines) can do it too. Problems are most likely to crop up on lower floors first where the pressure is higher.

My guess is you'll find that hot/cold lines are crossed (or improperly connected) at one point in the building. When a resident runs hot water, the line is getting refilled with cold water from this crossover point, rather than the water heater. Your recirculating hot water system eventually pulls that cold water back to the heater, but it means intermittent cold water from the hot water pipes!

If the water does stop, if you can drain the entire hot water system and no water flows at all -- then you've established that there's no leak or crossover, at least while the connected faucets are closed. Before turning the hot water back on I'd be inclined to test those faucets. All faucets will allow some cold water back into a drained hot water system, but only when the faucet is open-- and it shouldn't be too much.

Last resort, after turning everything back on, would be to monitor the temperatures of both the "hot" and return lines at the water heater for a while. The outgoing hot water should have a nearly constant temperature. The return line should be just slightly cooler, but still constant; If the "hot" (outlet side) temperature drops quickly when someone uses hot water, the heater itself could have a problem such as a broken dip tube (or other 'short circuit' between hot and cold).

Do you have a pressure tank installed on the hot water line (output of the boiler)? Could it be that momentarily the pressure drops and colder water is drawn out of the pressure tank into the hot line?

Pressure tanks are usually installed to regulate the line pressure to minimize fluctuations in water flow and change the resonant frequency of the system so that you don't get water hammer.

It could be that the old pump was sufficient to deliver hot water despite the fluctuations and the new pump is just a bit less powerful and now it's dependent on the pressure tank, which cools off and delivers cold water intermittently.

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