Trapped air in a radiator stops hot water reaching the top of the panel. The bottom feels warm, the top feels cold, and your boiler works harder to heat a room that's losing efficiency. Bleeding takes five minutes per radiator and uses tools you can buy for a pound.
What you need
- A radiator bleed key (£1-2 at any DIY shop)
- A small towel or cloth
- A cup or small container to catch water
Step by step
- 1. Switch the heating OFF and let the system cool for 30 minutes (so you don't scald yourself)
- 2. Find the bleed valve: a small square nut on the top corner of the radiator
- 3. Hold the cloth and cup under the valve
- 4. Turn the bleed key anti-clockwise no more than a quarter turn: you should hear a hiss as air escapes
- 5. When water starts to dribble out (not hiss), the air is out: close the valve immediately
- 6. Wipe up any drips
- 7. Repeat for every radiator in the house
The order matters
Start with the radiator furthest from the boiler and work your way back. If you have an upstairs and downstairs, do all of downstairs first (downstairs has more pressure pushing air up), then upstairs.
Top up the boiler pressure afterwards
Bleeding releases water from the system, which drops boiler pressure. Once you've done every radiator, check the boiler pressure gauge. If it's below 1 bar, top up using the filling loop (see my separate guide on this).
If a radiator still has cold spots after bleeding, the problem is probably sludge build-up in the bottom of the panel. That needs a power flush, not a bleed. Call me to take a look.