Boiler losing pressure — why it happens and how to fix it
A sealed heating system should hold roughly 1 to 1.5 bar cold. If yours keeps sliding towards zero, water is escaping somewhere — pressure does not simply evaporate.
The good news: most pressure loss is a cheap fix. The bad news: if you are topping up more than once a month you have an active leak or a failed expansion vessel, and ignoring it will eventually cook the heat exchanger.
Try this first — free
Checks you can safely do yourself
Re-pressurise with the filling loop
Find the braided silver hose or internal filling key under the boiler. Open both valves slowly until the gauge reads 1.2 bar with the heating off, then close them fully. If it drops again within days, stop topping up and book an engineer.
Check every radiator valve and joint
Run your hand along the bottom of each radiator, around the valve nuts and under towel rails. A pinhole weep dries as fast as it leaks, so look for rust staining or a chalky white crust rather than a puddle.
Look at the outside wall under the boiler
A copper pipe dripping outside — the pressure relief discharge — means the boiler is dumping water. That is a failed expansion vessel or PRV, not something to fix yourself.
Check the pressure after bleeding radiators
Bleeding radiators releases water as well as air. If you bled the system recently, one top-up is normal and nothing is wrong.
Never remove the boiler casing or touch gas components yourself — that's illegal as well as dangerous. Anything behind the front cover is Gas Safe work.
If that didn't fix it
What it usually turns out to be
Leaking radiator valve or joint
Most common cause in older systems. Usually a valve gland or a compression joint that has relaxed.
£120 – £180
Failed expansion vessel
The vessel loses its air charge, pressure spikes when hot and dumps through the relief valve. Re-charging works sometimes; replacement is the reliable fix.
£150 – £320
Faulty pressure relief valve
Debris on the seat means it never seals again. Recognisable by constant dripping outside.
£150 – £240
Leaking heat exchanger
Internal corrosion. On a boiler over ten years old this is normally the end of the road.
£450+ or replace
Auto air vent passing
A cheap part inside the boiler that weeps and dries, so you never see the water.
£120 – £180
Prices inc VAT and include our £99 diagnostic. We confirm the price before any work starts — no surprises on the invoice.
Straight talk
When we'd tell you to replace instead
- The heat exchanger itself is leaking and the boiler is over 8 years old
- You are topping up weekly and two separate repairs have not held
- The boiler is out of warranty and parts are on extended lead times
- Repair quotes are creeping past a third of the cost of a new fitted boiler
Pressure at zero with no heating in winter is a same-day job for us — we carry vessels, PRVs and filling loops on the van.
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Frequently asked questions
What pressure should my boiler be at?
1 to 1.5 bar cold for almost every domestic combi. It will rise to around 2 bar when the heating is hot, which is normal.
Is it safe to keep topping up my boiler?
Occasionally, yes. Weekly, no — you are drawing fresh oxygenated water into the system every time, which accelerates corrosion and sludge, and you are masking a leak.
How much does it cost to fix a boiler losing pressure?
Our diagnostic is £99 inc VAT and includes the first 30 minutes of labour. Most pressure faults land between £120 and £320 inc VAT all in.
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