When I decided to figure it out myself I realized something was wrong with the water in my apartment.
There was unstable water supply, and the boiler was giving barely warm water.
I contacted plumbers, but everyone gave different explanations — pipes.
I began reading technical materials. 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.
I advise paying attention to it to anyone who wants to avoid unnecessary repairs.