How to bleed a radiator
Cold at the top but warm at the bottom usually means trapped air. It's a two-minute job with the right key — here's how to do it properly.
You'll need
A radiator bleed key (a few pounds from any hardware shop), an old towel or cloth, a small container to catch drips, and a pair of gloves.
Before you start
Always turn the heating off and let radiators cool before bleeding them. Water inside a hot system can come out scalding hot and under pressure — wear gloves and protect flooring nearby. Bleeding a radiator while the heating is running is also counterproductive: the pump keeps drawing more air into the system from elsewhere, so you undo the job as you go.
Why radiators need bleeding
Air gets trapped in central heating systems over time — it collects at the highest point it can reach, which is usually the top of a radiator. That pocket of air stops hot water from filling the whole radiator, so you get a cold patch at the top even though the bottom heats up fine. Bleeding the radiator lets that trapped air escape so water can fill the space instead.
Step by step
- Turn off your heating and give the radiators time to cool down fully before you touch the valve.
- Find the bleed valve. It's the small square-shaped fitting, usually at the top corner of the radiator at one end.
- Get ready to catch water. Hold a cloth or small container underneath, then fit the radiator key onto the valve.
- Open it slowly. Turn the key gently anti-clockwise — about a quarter turn is usually enough. You'll hear a hissing sound as air escapes.
- Close it as soon as water appears. When the hissing stops and water starts to trickle out steadily, turn the key clockwise to close the valve. Snug is enough — don't force it.
- Check your boiler pressure afterwards. Bleeding radiators lets a small amount of water out of the system, which can drop the pressure slightly. If the gauge has dropped below 1 bar, see our guide to topping up boiler pressure.
Work round each radiator that has a cold top, one at a time. If you've got several to do, it's worth starting downstairs and working up, since air tends to migrate upward through the system. It's also worth doing a quick check of all your radiators every couple of months, and especially just before winter, rather than waiting until one's noticeably cold.
If it doesn't fix it
If a radiator is still cold at the top after bleeding, stays cold all over even after bleeding, or the valve won't turn or keeps weeping water after you've closed it, that points to something else going on — a stuck valve, a balancing issue, or sludge build-up inside the radiator. If no air or water comes out at all when you open the valve, it may be blocked or seized rather than genuinely empty of air — don't force it. Either way, it's worth getting looked at rather than forcing anything.
Radiator still not heating properly?
If bleeding hasn't sorted it, message Ash with which radiators are affected and what you've tried.