Why does my boiler keep losing pressure?

Why Does My Boiler Keep Losing Pressure?

Boiler pressure loss is one of the most common calls we receive from London homeowners. The normal operating pressure is between 1–1.5 bar when cold. Here's what causes it:

Most Common Causes

1. Water leak in the system — check radiator valves, pipework joints, and the boiler itself for any drips or damp patches.
2. Pressure relief valve (PRV) discharging — if the PRV is releasing water outside, the system pressure is reaching its limit. This needs urgent investigation.
3. Recently bled radiators — bleeding releases air but also water, dropping pressure. Re-pressurise via the filling loop.
4. Faulty filling loop — if the filling loop is left slightly open or is defective, pressure fluctuates.
5. Expansion vessel failure — the expansion vessel absorbs pressure changes. When it fails, pressure swings wildly.

How to Re-Pressurise Your Boiler

Locate the filling loop (usually a silver or blue flexible hose under the boiler), open both valves slowly until the gauge reads 1–1.5 bar, then close both valves firmly. If pressure drops again within days, there is a leak — call a Gas Safe engineer.

When to Call an Engineer

If your boiler loses pressure more than once a month, there is an underlying fault. Our Gas Safe engineers serve all of W1, SW1, NW3 and surrounding areas — call 020 7458 4949.