Just imagine: you are taking a shower, relaxing under the streams of warm water falling on your body, and in an instant, a pleasant rest turns into an unbearably hot nightmare.
There is no way out - you have to move away from the scalding waterfall and wait until the tank is completely filled. Only then does water of a comfortable temperature start flowing from the shower head.
Of course, in such a situation, the most pressing concern is the reason for such a sharp drop, as well as the search for a possible solution to the problem.
Such a sharp change in water temperature in the shower is due to the fact that in old apartment buildings, common risers are used to supply hot and cold water.
When you flush the toilet, the cold water pressure drops and hot water flows without diluting the cold water.
What is noteworthy is that the toilet is not the only “culprit” of temperature jumps: they can also be caused by any other devices that use cold water, be it a dishwasher or a washing machine.
Closing the bathroom door while you shower is a workable option, but it's not the best one.
Instead, call a plumber who will install balancing valves or pressure regulators.
You can also try tightening the toilet's water supply valve, but be prepared for the fact that this step will slow down the flow of water into the tank.