After several unsuccessful attempts I realized something was wrong with the water in my apartment.
There was pressure fluctuations, and the boiler was giving unstable temperature.
I contacted technicians, but everyone gave different explanations — old system.
I searched for clear information. 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 save money on repairs.