How to Bleed a Radiator — The Complete UK Guide
Cold spots at the top of a radiator mean trapped air. Bleeding takes five minutes, restores full heat output and can knock 10–15% off winter gas bills. Here is the trade-verified process — plus how to top up combi pressure afterwards.
Step-by-step method
1. Turn the heating off and let radiators cool
Hot water under pressure will spit and scald. Switch the boiler off at the programmer, wait 20–30 minutes until the radiators are cold to the touch. This also lets trapped air rise to the highest point of each rad.
2. Get the right bleed key
Most UK radiators use a standard square 5mm bleed key (clock-key). Modern designer rads may need a 6mm or a flat-blade screwdriver. A brass bleed key costs under £2 — keep one on the boiler.
3. Open the bleed valve a quarter turn
The bleed valve is the small square nipple at the top corner of the radiator. Hold a cloth or small jug under it. Turn the key anti-clockwise a quarter turn — you'll hear a hiss as air escapes.
4. Close the valve when water runs steady
As soon as a steady drip of water replaces the hiss, close the valve firmly (do not overtighten — the brass strips easily). Wipe the valve dry and move to the next radiator.
5. Top up the boiler pressure
Bleeding drops system pressure. Check the boiler gauge — it should read 1.0–1.5 bar cold. If lower, open the filling loop under the boiler until pressure hits 1.2 bar, then close it. Restart the heating.
Frequently asked questions
How often should I bleed radiators?+
Once a year at the start of the heating season, and any time a radiator is cold at the top but warm at the bottom.
What if no water comes out when I bleed?+
System pressure is too low. Top up via the filling loop to 1.2 bar and try again. If still nothing, the feed valve or expansion vessel may be faulty — call a Gas Safe engineer.
Do I bleed the boiler itself?+
No. Combi boilers have an automatic air vent. Bleeding is only for the radiators on the wet central-heating circuit.