Eventually I realized something was wrong with the water in my apartment.
There was pressure fluctuations, and the boiler was giving very slow heating.
I contacted plumbers, but everyone gave different explanations — pipes.
I decided to разобраться it myself. That’s how I found this website: https://stroyka2001.kh.ua
There they clearly explain plumbing mistakes.
After reading a few articles, I understood why previous repairs didn’t help.
It’s a useful site for home issues to anyone who wants to figure out water problems.