Why your boiler keeps losing pressure

Cause 1: a leak on the heating circuit

The most common cause, and the easiest to confirm. Check radiator valves for green or white crusting, look under the boiler for drip marks, and feel along skirting boards for damp patches. Pinhole leaks under floors show as a warm strip on the floor covering.

A visible joint repair is usually a short visit. A buried pipe needs tracing first, which is why we quote that separately once we know what we are dealing with.

Cause 2: a failed expansion vessel

The vessel absorbs the extra volume as water heats up. When its internal diaphragm fails, pressure spikes when the heating runs and drops back once it cools, often below one bar overnight. That swinging gauge is the giveaway.

Re-charging the vessel is often enough. If the diaphragm has gone, it needs replacing, or an external vessel fitting alongside the boiler.

Cause 3: a passing pressure relief valve

If the vessel has failed and pressure keeps climbing, the relief valve dumps water out through a copper pipe on the outside wall. Look for a dripping pipe or a limescale stain on the brickwork below it. Once the valve has passed debris it rarely seals again and should be changed.

What to do in the meantime

Top up to between one and one and a half bar cold, and note how long it holds. A drop within a day points to a leak; a slow drift over weeks points at the vessel. Bring that timing to the call and we can usually arrive with the right part on the van.