How to Repressurise a Boiler Safely (UK)
Bring a UK combi boiler back to 1.2 bar cold. Filling loop use, safe limits, when NOT to repressurise.
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Target pressure
1.2 bar cold. On heating, expect a rise to 1.8–2.2 bar — normal. Above 2.8 bar the PRV discharges.
Filling loop procedure
Turn boiler off. Open both filling loop valves slowly. Watch gauge climb to 1.2 bar. Close both valves — top first, then bottom. Restart boiler.
- External filling loops must be disconnected after use (Water Regs G255).
Don't repressurise if…
Pressure drops within 24 h — find the leak first. Repeated top-ups mask a hidden fault and add oxygen to the system, corroding rads and HEX from the inside.
Frequently asked questions
- How often is it OK to top up?
- Once per year max. More often means investigate leaks or failed expansion vessel.
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