What the pressure gauge is telling you
A sealed heating system runs at around 1 to 1.5 bar when cold. Below 1 bar, most boilers lock out rather than run, which is the sudden no-heating-no-hot-water moment that sends people to the manual. Above about 2.5 bar when cold, the system is overfilled and will vent the excess through the pressure relief pipe outside.
Pressure drifts down slowly in any system because tiny amounts of air escape through automatic vents. A top-up once or twice a year is normal. A top-up once a week is a symptom.
The five-minute repressurise
With the boiler off and cool, find the filling loop: usually a braided silver hose under the boiler with a valve at each end, sometimes a built-in lever on newer models. Open the valves slowly and watch the gauge climb. At 1.2 to 1.5 bar, close both valves firmly, then restart the boiler and clear the fault code if one is showing.
Two cautions. Do not overfill, because you will then be letting water out through a radiator bleed valve to bring it back down. And if the filling loop is missing or you are unsure, the boiler manual for your exact model will show the procedure; every manufacturer publishes them online.
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If pressure falls again within days of a top-up, water is going somewhere. Check the visible suspects first: damp under radiator valves, staining on ceilings below pipe runs, and drips from the pressure relief pipe where it exits the wall outside, which points at an internal fault rather than a radiator.
A leak inside the boiler itself, often the pressure relief valve or the expansion vessel, is an engineer job and usually a modest one. Repeated pressure loss on a boiler already past ten years old is also one of the classic end-of-life patterns covered in how long a boiler lasts, so if the callouts are stacking up, it is worth pricing a replacement alongside the repair. You can compare quotes from local Gas Safe installers with a few questions about the job.
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Up to three trusted, Gas Safe registered installers quote for the work. Free, no obligation.
Compare boiler quotes