A nine-floor brick building. Since around Feb. 2020 the entire stack 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?