How to re-pressurise a combi boiler, and when not to
Last reviewed · owner Mike Fellows · general information, not legal advice
A combi boiler needs its filling loop opened until the pressure gauge sits between 1 and 1.5 bar cold. Turn the boiler off, open both filling-loop valves slowly, watch the gauge, close the valves at 1.2 bar, then restart. Topping up is user-serviceable; a boiler that loses pressure repeatedly has a leak or a failing part, and anything beyond the filling loop is Gas Safe engineer work.
What pressure should a combi boiler be at?
Between 1 and 1.5 bar when the system is cold, on the gauge on the front of the boiler. Below about 0.8 bar most boilers lock out and refuse to fire, which is the usual reason the heating stops on a cold morning. Pressure rises when the system heats, so a reading taken mid-afternoon with the heating on will sit higher. Judge it cold.
How do I top the pressure up?
Switch the boiler off and let it cool. Find the filling loop, usually a braided silver hose beneath the boiler with a small valve at each end. Open both valves slowly and you will hear water enter the system; watch the gauge climb. Close both valves when it reaches about 1.2 bar. Restart the boiler. If your filling loop is detachable, it should be disconnected once you are done, which is also the requirement in most manufacturer instructions.
When is low pressure a job for an engineer?
When it keeps happening. A system that needs topping up more than a couple of times a year is losing water somewhere: a weeping radiator valve, a failing expansion vessel, or a leak under the floor. Repeated topping up masks the fault and loads the system with fresh, oxygenated water that accelerates corrosion. Anything involving the sealed gas side of the boiler is not user-serviceable at all. By law, gas work must be done by a Gas Safe registered engineer.
Sources
- HSE, Gas health and safety in the home (accessed 2026-09-05)
- GOV.UK, Gas Safe Register requirement (accessed 2026-09-05)
This page is general information for property professionals, not legal advice. For a decision that matters, take advice on your specific facts.
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