How to Bleed a Radiator Step by Step
Release trapped air from a radiator with a bleed key, check boiler pressure afterward and know when a cold spot at the top means something else.
If your radiator is warm at the bottom but cold at the top, the likeliest culprit is trapped air. Bleeding the radiator releases it, restoring even heat, and it is one of the easiest and most satisfying jobs in the house. It takes about ten minutes per radiator and needs no special skill.
Why air gets in
Air can enter a sealed heating system when it is topped up, after repairs, or through tiny leaks over time. It rises, collects at the top of the radiator and stops hot water from filling the panel. Gurgling or a noisy system can be a sign.
What you need
- A radiator bleed key, which is cheap from a DIY shop, or a flat screwdriver for some valves
- An old cloth or towel
- A small container or a dish to catch drips
- Your boiler manual, for the pressure range
Before you start
- Switch the heating on and let radiators warm up so you can see which ones have cold spots.
- Switch the heating off before bleeding, and let the pump stop. Bleeding with the pump running pulls in air.
- Check boiler pressure on the gauge. Note the reading so you can compare afterward. The pressure basics are in boiler pressure basics.
Bleeding the radiator
- Hold the cloth under the bleed valve, which is a small square or slotted fitting at the top corner.
- Fit the key and turn it anticlockwise a quarter turn. Hold it steady.
- Listen for a hiss as air escapes.
- When water begins to dribble out, close the valve by turning clockwise. Do not over-tighten.
- Wipe the area dry.
- Repeat on other radiators, starting with the one nearest the boiler, working to the farthest. Start downstairs, then upstairs.
After bleeding
- Check the boiler pressure. It may have dropped. If it is below the recommended range, top it up following your manual's guidance, or see our boiler pressure basics article. If you are not sure, a Gas Safe registered engineer can advise.
- Switch the heating back on and check that radiators warm evenly.
- Look for any drips from the valve, and wipe the area clean.
Mistakes to avoid
- Opening the valve fully. A quarter turn is enough, otherwise water sprays out.
- Bleeding with the system hot and running. Hot water can scald.
- Ignoring pressure afterwards. Too low and the boiler may shut down.
- Forgetting a tank-fed system. If you have a feed-and-expansion tank in the loft, check its level.
If it keeps happening
Frequent air in radiators points to something else. Possible causes include a leak, a pump or valve issue, or corrosion producing gas in the system. Black sludge can build up and block parts of the radiator. A cold area at the bottom is a different problem from air, covered in radiator not heating. Recurring air and falling pressure should be checked by a qualified heating engineer, and anything involving the boiler itself is a Gas Safe job.
A worked example: a three-bed semi
Imagine a 1930s semi with eight radiators and a sealed system. Two upstairs radiators are cold at the top and the house takes ages to warm up. Switching the heating off first and letting it cool for twenty minutes or so avoids scalding. Starting with the downstairs radiator nearest the boiler, you open each valve a quarter turn, wait for the hiss to stop and the first dribble to appear, then close it. Eight radiators at roughly five minutes each is about forty minutes including the wiping up.
On the boiler gauge you might see the reading fall from about 1.3 bar to under 1 bar by the end. That is normal, because the air has been replaced by water drawn from the circuit. Topping up following your manual, then running the heating, is the last step. If the radiators are now evenly hot, the job is done.
Quick diagnosis: cold top, cold bottom or cold middle
| What you feel | Likely cause | First step |
|---|---|---|
| Cold at the top, hot at the bottom | Trapped air | Bleed the radiator |
| Cold at the bottom, hot at the top | Sludge or debris settling | Not a bleeding job; see below |
| Cold in the middle, hot around the edges | Sludge in the centre | Needs a system flush by an engineer |
| Whole radiator cold | Stuck valve, closed valve or no flow | Check the valve and thermostatic head |
Older systems and special cases
- Towel rails and ladder radiators often have the bleed valve on the side or top corner.
- Radiators in flats may share pipework with others. If a communal system is involved, report persistent air to your managing agent rather than draining anything down yourself.
- Old brass valves can be stiff or round-edged. If the key slips, stop. Forcing it risks a leak you cannot stop without draining the system.
- Open-vented systems with a header tank in the loft behave differently from sealed ones, so the pressure gauge advice does not apply.
When to stop and call a professional
If a valve will not close properly, keeps weeping, or the water that comes out is black and thick, stop and ask a qualified heating engineer. Anything touching the boiler itself is a job for a Gas Safe registered engineer. The wider picture of costs is in how much a plumber costs.
Frequently asked questions
How often should I bleed radiators?
Only when needed, such as at the start of the heating season or when cold spots appear. Frequent bleeding suggests a system issue.
Do I need to turn the heating off?
Yes. Turn off the heating and wait for the system to settle before bleeding, as the pump can otherwise draw in air.
Why does my radiator have water but is still cold?
It may be a stuck valve, a blocked pipe or sludge. See radiator not heating for how to tell.