Why Is My Boiler Losing Pressure?

"My boiler keeps losing pressure" is one of the calls we get most often in York — and it's usually one of the easier things to sort out. Here's what's actually going on, in plain English.

What "losing pressure" actually means

Your boiler runs on a sealed system of water. The pressure gauge on the front (usually a dial with a green zone between 1 and 1.5 bar) tells you if there's enough water in that system to heat your home properly. If it keeps dropping below the green zone, water is escaping somewhere — or, less often, the gauge itself is playing up.

The most common causes

A small leak somewhere in the system — often at a radiator valve, a pipe joint, or inside the boiler itself. Not always visible; sometimes it's a slow drip that evaporates before you spot it.
Bleeding a radiator — every time you bleed a radiator to remove air, you also let a small amount of pressure out with it.
A faulty pressure relief valve — this is meant to release pressure if it gets too high, but a worn one can leak even at normal pressure.
A failing expansion vessel — the part that absorbs pressure changes as water heats and cools. When it fails, pressure swings become more noticeable.

What you can safely check yourself

Look at the pressure gauge — is it below 1 bar? That confirms you do have a genuine drop, not just a one-off reading.
Check around visible pipework and radiator valves for any damp patches, staining, or drips.
If you've recently bled a radiator, that alone might explain a small, one-time drop — topping up the pressure (using your boiler's filling loop, if you're comfortable doing so) may be all that's needed.

Watch: how to top up your boiler pressure

When to call us

If you're topping up the pressure every few days, or it drops fast rather than gradually, that's worth a proper look — it usually points to a leak that needs finding, not just topping up again and again. Same goes if you can't find an obvious cause, or you're not confident using the filling loop yourself.

Give us a call and tell us what the gauge is doing and how often — that alone often helps us work out roughly what's going on before we even arrive.

Last updated August 2026 by The Boiler Doctor, York.