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.