Boiler losing pressure? What it means and what to do

Updated 1 July 2026

A boiler that keeps losing pressure is one of the most common heating problems in the UK — and often one of the cheapest to fix. Here's how to tell the difference between a five-minute top-up and a sign your boiler is on its way out.

What the pressure gauge should show

Most modern boilers have a pressure gauge on the front, and most systems are happy between 1 and 1.5 bar when the heating is off. Below about 0.8 bar, many boilers lock out and refuse to heat until pressure is restored.

Pressure rising to 2–2.5 bar while the heating runs is normal. Pressure that climbs above 3 bar, or drops back down within days of topping up, is not.

Topping up is usually safe to do yourself

Under the boiler you'll find a filling loop — usually a braided silver hose with one or two small valves. With the heating off and cool, open the valve(s) slowly until the gauge reaches about 1.2 bar, then close them firmly.

If you can't find the filling loop or the gauge doesn't respond, stop and check your boiler's manual — some models fill differently, and a wrong guess can make things worse.

If it keeps dropping, something is leaking

Water doesn't vanish. A system that needs topping up weekly is losing water somewhere: a weeping radiator valve, a pinhole in pipework, a failed expansion vessel, or — more seriously — a crack in the boiler's heat exchanger.

Check visible pipework and radiator valves for damp or staining first. If nothing is visible, the leak may be under floors or inside the boiler itself, which needs a Gas Safe engineer.

When pressure loss means replacement

On a boiler under 10 years old, a failed expansion vessel or pressure-relief valve is usually worth repairing. On an older boiler, a leaking heat exchanger often costs £500+ to replace — money most engineers will tell you is better put towards a new boiler with a 10–12 year warranty.

A useful rule of thumb: if the repair quote is more than a third of the price of a new boiler, and your boiler is over 10 years old, replacement usually wins on cost within a couple of years through efficiency savings alone.

This guide is general information, not a survey of your home. Anything involving gas must be done by a Gas Safe registered engineer.