After another check I realized something was wrong with the water in my apartment.
There was pressure fluctuations, and the boiler was giving barely warm water.
I contacted local specialists, but everyone gave different explanations — pipes.
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 the real cause.
It's a useful site for home issues to anyone who wants to save money on repairs.