Boiler keeps losing pressure
A sealed heating system does not use water, so a gauge that keeps falling means water is escaping somewhere. Here is how to work out where.
What it usually means
Your boiler is a sealed system. The same water goes round and round, and nothing consumes it. So when the pressure gauge falls, the water has physically left the system — through a joint, a valve, the safety valve, or a failed expansion vessel.
That distinction matters because topping the system up treats the symptom. It also does harm: every refill introduces fresh oxygenated water, and dissolved oxygen is what produces the black magnetite sludge that eventually kills pumps, blocks heat exchangers and leaves radiators cold at the bottom.
The right question is not "how do I top it up" but "how fast is it falling". Losing pressure once a year is normal. Once a month is a leak. Once a day is a leak that is probably doing damage somewhere you have not looked.
How to narrow it down
Work through these in order.
How often are you having to top it up?
Once or twice a year is unremarkable and needs no action beyond refilling. More than three times a year means there is a leak worth finding. Daily or weekly means water is going somewhere at a rate that will eventually show itself as damage.
Is there a copper pipe outside the wall near the boiler that drips?
That is the pressure relief valve discharge. If it drips, or if there is limescale staining below it, the safety valve is passing — either because debris is holding it open or because the expansion vessel has failed and pressure is spiking every time the system heats.
Does the pressure rise a long way when the heating runs?
A rise of half a bar or so is normal. A rise from 1.2 to 2.8 bar means the expansion vessel is not absorbing expansion, which forces water out of the safety valve on every cycle. Re-charging or replacing the vessel fixes it.
Have you bled radiators recently?
Then you have taken water out of the system yourself, and the drop is expected rather than a fault. Top back up to 1.2 bar and see whether it holds.
Any damp patches, staining or a musty smell?
Check under radiators, around every valve, in the airing cupboard, and along the ceiling below the pipe runs. A slow leak under a floor often shows first as a cold, slightly damp patch of carpet or a stain on a ceiling below.
Codes commonly shown with this
- CE 207Worcester BoschSystem water pressure has dropped too low for the boiler to run safely. Topping the system back up to 1.2 bar usually clears it in five minutes.Get it checked
- F1IdealSystem pressure has dropped below the minimum the boiler will run at. Topping it back up to 1.2 bar usually clears it straight away.Get it checked
- E119BaxiSystem pressure has fallen below the level the boiler will operate at. Refilling to 1.2 bar and resetting usually clears it in a few minutes.Get it checked
- F.22VaillantThe boiler has shut down because there is not enough water in the system. Pressure has dropped below roughly 0.5 bar and it will not fire until you top it up.Get it checked
When it is urgent
Take care
What it costs
Typical repair cost
£0–£450
Typical UK prices including parts and labour, updated August 2026. Your quote may differ.
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