Boiler losing pressure? Here’s what’s going on

What boiler pressure should be

Most domestic boilers should sit between 1 and 1.5 bar when the heating is off (check the gauge on the front). Below about 1 bar and the boiler may cut out or fail to fire. A slow drop over weeks is common; a fast drop over hours or days usually means a leak.

Common causes of pressure loss

• A small leak somewhere in the system — radiators, pipework, or fittings. Often the most likely cause of repeated drops.
• A faulty pressure relief valve letting water escape outside (look for dripping from an outside pipe).
• A failing expansion vessel, which can no longer absorb the pressure changes as water heats and cools.
• Recently bled radiators — bleeding releases pressure, so a one-off drop after bleeding is normal and easily topped back up.

What you can safely do yourself

If the pressure has dropped just below 1 bar, you can usually re-pressurise the boiler using the filling loop — most manufacturers explain how in the manual or via a label near the boiler. Top it back up slowly to around 1.2 bar. If it drops again within a day or two, that’s a sign of an underlying problem rather than a one-off.

Never attempt to open up or repair the boiler itself — gas appliances must only be worked on by a Gas Safe registered engineer.

When to call a Gas Safe engineer

• The pressure keeps dropping even after you top it up — likely a leak or failed part.
• You can see water leaking from the boiler, pipework, or an outside overflow pipe.
• The boiler is cutting out, showing a fault code, or not heating properly.
• You’re not confident topping it up yourself.

Any repair involving the boiler’s internal parts must be done by a Gas Safe registered engineer — you can check their registration number on the Gas Safe Register. Once you’ve got a quote for the repair, it’s worth checking it’s in the fair range before you go ahead.

Frequently asked questions

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