Why is my boiler losing pressure?

Every combi and system boiler runs at a set pressure, usually between 1 and 1.5 bar when the heating is off, shown on a gauge on the front of the boiler. It drifts down slowly over months as a matter of course, and topping it up once or twice a year with the filling loop is entirely normal. Needing to do it every few weeks is not, and it means one of a short list of things.

A slow, general drop over months

This is usually nothing more than air being released from the system over time through the automatic air vent, and is the one cause that does not need an engineer — just top the pressure back up.

A drop that happens quickly, over days rather than months

This points to an actual leak somewhere in the system, and it does not have to be dramatic or visible. A weeping pipe joint under a floor, a radiator valve that has started to seep, or a fault inside the boiler itself can all lose enough water to show up on the gauge without ever leaving a visible puddle. Our leak detection page covers how we trace one that is not obvious.

Pressure drops specifically when the heating is on, then recovers

This is a different pattern and points at the expansion vessel — a component that absorbs the pressure increase as water heats up and expands. If it has lost its charge of air, the system pressure spikes when hot and can trip a safety valve, discharging water and dropping the pressure once it cools. This is a boiler-specific fault rather than a leak in the pipework, and is covered under boiler repair.

What to check yourself before calling

Look underneath the boiler and around any radiator valves for damp patches, and check whether the pressure relief valve (a small pipe usually running outside through an external wall) is dripping. Photograph the gauge reading and note whether the drop is fast or slow — both of those help whoever comes out get to the actual cause faster rather than needing to start from nothing.

If the pressure has dropped to zero, or the boiler has locked out with a fault code, that is worth treating as urgent rather than waiting — see emergency plumbing if it cannot wait for a normal appointment.