Boiler pressure keeps dropping: what it means

What normal looks like

Most boilers show the system pressure on a gauge or the display. Your boiler’s manual gives the right range, cold and hot; it is normal for the pressure to rise a little as the system heats up and fall back as it cools.

A slow drop over many months can be normal. A drop every few days or weeks is not.

Small leaks

Look for drips, damp patches, green or white crust, or rust marks at radiator valves, bleed screws, manifold connections and under the boiler. A leak too small to drip can still evaporate from a warm pipe. Leaks hidden in walls or floors show as damp patches or stains.

A leak inside the boiler needs a technician; do not open the boiler casing yourself.

The expansion vessel and relief valve

Water expands when it heats. The expansion vessel, a small tank with an air cushion inside, absorbs that expansion. When it loses its air charge, the pressure shoots up as the system heats, the relief valve opens to protect the system and lets water out, usually through a pipe to the outside, and when everything cools the pressure is low.

A telltale sign: pressure very high when hot and low when cold, or a pipe outside that drips when the heating runs. Re-charging or replacing the vessel is a technician’s job.

After bleeding radiators

Bleeding lets out air and a little water, so a drop after bleeding is expected. Top up once to the right level using the filling loop as your manual describes, and close it fully afterwards. If pressure keeps falling after that, it is not the bleeding.

Gas safety first

Pressure problems are about water, not gas. But if you ever smell gas, turn the gas off at the meter, open the windows, do not touch electrical switches, leave the home and call the gas company’s emergency number.