A nine-floor brick building. Since around Feb. 2020 the entire stack (8 of the 60+ units) get very hot water in the toilets (when flushing). Additionally - when someone takes a shower, he/she gets extremely hot water for 5-6 minutes and only then the water cools down and it is possible to take a shower normally.

Note that once the water in the shower cools down (during the shower), flushing the toilets yields cold water as normally. Once the shower is over, if you flush the toilets then again - very hot water comes into it.

Could it be that someone at the stack installed some device? Like a water heater or a small laundry machine? Not all unit owners cooperate to allow a plumber to check that. What do you think could it be? Any ideas to know if someone installed a certain device that causing that?

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    Could be something as simple as a someone using a shower head shutoff valve top stop the shower and left the shower faucet on - when the hot and cold water taps are open and the shower head valve is closed, water can flow from the hot side through the shower mixer to the cold side. Backflow preventers on the hot/cold water inlets to the shower would prevent this, but would have to be installed on the offending unit's plumbing. – Johnny Aug 13 at 0:07
  • Thanks Johnny - could it be that this someone doesn't suffer from the issue? That person only causing the issue to others, but can take showers normally? – UK_Cambridge Aug 13 at 1:01
  • Yeah, if this is the case, they'll see the same thing as you -- the water will seem extra hot at first until the hot water in the cold water pipe works its way out. – Johnny Aug 13 at 2:24
  • Thank you Johnny. – UK_Cambridge Aug 13 at 16:24
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    @johnny your example is a good one I was thinking of a senior center I used to maintain on the side. Water and heat and trash was included and they always were on a shoestring budget. I had to call a boiler pro in I could figure out there was a problem and where it was but was not certified for that kind of work it turned out to be a mixing valve (or 3) in different areas getting stuck. I tracked it down with a non contact temp sensor you know the ones with the laser to point to the target and a digital display. The valves were in maintenance closets. This could end up as a liability issue. – Ed Beal Aug 13 at 20:50

Good news - the plumber found the issue: a problematic cartridge installed in one of the upper floors. It was replaced and everything is back to normal for all residents. It was extremely difficult to identify the source and required all residents in the stack to be available when the plumber visited. Thanks.

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