Eventually I realized something was wrong with the water in my apartment.
There was low pressure, and the boiler was giving almost no hot water.
I contacted local specialists, but everyone gave different explanations — old system.
Then I started looking for answers. That’s how I found this website: https://stroyka2001.kh.ua
There they clearly explain causes of low pressure.
After reading a few articles, I understood what had been overlooked.
Now I recommend this website to anyone who wants to save money on repairs.