Why your boiler keeps losing pressure

A sealed heating system should hold its pressure for months. If the gauge drops below one bar every couple of weeks, something is letting water out, and topping it up is treating the symptom.

Three things it usually is

A leak on a radiator valve or a pipe joint, often under a floor. A failed expansion vessel, which is the most common and the least obvious. Or a pressure relief valve passing water out through the outside pipe.

How to tell which

Look at the copper pipe coming through the outside wall near the boiler. If it drips after the heating has been on, the relief valve is passing and the expansion vessel has almost certainly lost its charge.

An expansion vessel can often be recharged rather than replaced. It takes about forty minutes and is a great deal cheaper than a new boiler, whatever anyone tells you.