Learn how to bleed a radiator safely in 6 steps. Fix cold spots, quiet gurgling and improve heating efficiency. Need help? Call 020 8050 9933.
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If your radiators feel cold at the top or you hear gurgling, trapped air is the usual cause. Bleeding a radiator takes about 5 minutes per unit and restores full heating efficiency. Follow the steps below, and if the problem returns quickly, our Gas Safe engineers can pressure-check your system across London and the South East.
A radiator bleed key (or flat-blade screwdriver on modern valves), an old cloth, and a small container to catch drips. Turn off the central heating and let radiators cool for at least 20 minutes so hot water doesn't spray out.
Switch the boiler off at the programmer. This stops the pump pushing more air around the system while you work.
It's the small square nut at the top corner of the radiator. Hold the cloth underneath to catch water.
Turn the key anti-clockwise a quarter turn. You'll hear a hiss as trapped air escapes. Keep the container ready.
As soon as a steady stream of water (no bubbles) appears, tighten the valve clockwise. Don't overtighten — snug is enough.
Start with the radiator furthest from the boiler and work back. Upstairs radiators usually need bleeding last.
Check the pressure gauge. Combi boilers should read 1.0–1.5 bar cold. Use the filling loop to top up if needed, then restart the heating.
Once a year, ideally at the start of the heating season. If cold spots return within weeks, book a heating engineer.
Yes, provided the boiler is off and the system is cool. Never open the valve fully — a quarter turn is enough.
Boiler pressure is likely too low. Top up via the filling loop to 1.2 bar, then try again.