Boiler Keeps Losing Pressure — Why and How to Fix

Leak in the system

Any slow leak in radiators, pipework, or fittings will cause pressure to drop over time. Even a small weep is enough to drop the pressure below 1 bar over a few days.

Faulty expansion vessel

The expansion vessel absorbs pressure changes as water heats and cools. If the diaphragm fails, pressure can spike when hot and drop when cold.

Faulty pressure relief valve (PRV)

The PRV is a safety device that releases pressure if it gets too high. If it's weeping, pressure gradually bleeds away. Look for dripping from a pipe exiting through an outside wall.

Recently bled radiators

Bleeding radiators releases water from the system, causing a pressure drop. A one-off drop after bleeding is normal — just repressurise.

Practical checks and next steps

You can repressurise the boiler yourself as a temporary fix. But if it keeps dropping — especially more than once a week — there's a leak that needs finding and fixing before it causes water damage.

• Check the pressure gauge — repressurise to 1–1.5 bar using the filling loop.
• Note the date and how long before it drops again.
• Check all radiator valves and pipework for visible drips.
• Look outside for a dripping pipe from the boiler (the PRV discharge pipe).
• Check the boiler casing for internal drips.
• If it drops more than once a week without you bleeding anything, call a plumber.