guide
Boiler pressure low
How to read boiler pressure, when topping up is reasonable, and when low pressure means a leak or a professional visit.
What the gauge should show
Most sealed system boilers show pressure on a gauge marked in bar. Manufacturers often recommend around 1.0–1.5 bar when the system is cold — check your boiler’s handbook for the exact range. Pressure that repeatedly falls after topping up usually means water is leaving the system.
Safe steps for many combi boilers
- Switch the boiler off and allow it to cool if it has been firing.
- Confirm the gauge is below the handbook minimum when cold.
- Locate the filling loop (often a silver braided hose with one or two valves beneath the boiler).
- Open the valves slowly until the gauge rises into the normal cold range, then close them firmly.
- Check for drips at the loop and restart the boiler per the manufacturer’s instructions.
If you cannot find a filling loop, the valves are seized, or pressure will not hold, stop and call a professional.
Safety notes
- Gas boiler repairs and internal gas work must be done by a Gas Safe registered engineer — Super Plumbers uses Gas Safe registered partner engineers for gas work; you can check the licence number.
- Do not keep topping up daily; that masks a leak and can corrode the system.
- If you smell gas, open windows, avoid switches and flames, and call the National Gas Emergency Service on 0800 111 999.
When to call a professional
Book boiler repair if pressure falls again within days, you see damp at radiators or pipework, the boiler shows a pressure-related fault code, or you are not confident using the filling loop. Related issues include radiator repair and central heating pipework leaks. Call 01332 314146 for prioritised help when you have no heating or hot water.