- Signs your radiator has trapped air
- What you need to bleed a radiator
- Turn the heating off and let the radiators cool first
- How to bleed a radiator step by step
- Checking boiler pressure after bleeding radiators
- When bleeding the radiator doesn't fix the cold spots
- How often to bleed your radiators
- More home how-tos and sharing what you know
- Frequently asked questions
To bleed a radiator, turn the heating off and let it cool, hold a cloth under the bleed valve at the top, then turn a radiator key slowly anticlockwise until the hissing stops and water appears. Close the valve, repeat on other cold radiators, then check the boiler pressure gauge and top up if it's low.
A radiator that's warm at the bottom and cold at the top is one of the most common heating niggles in any home. Air gets trapped inside the radiator, sits at the top and stops hot water filling the whole panel, so the room never feels as warm as it should. Letting that air out is called bleeding, and it's a small job most people can do with a radiator key and an old towel.
I've pulled the steps below together from the official help pages of British Gas and boiler maker Baxi, so you're following the same order the heating firms give their own customers. Read the safety notes first, because the water inside a working radiator can be scalding.
Signs your radiator has trapped air
Both British Gas and Baxi start the same way: turn the heating on, let it warm up, then feel each radiator. Here's what to look and listen for:
- Cold at the top, hot at the bottom. This is the classic sign. Air rises, so it collects in the top of the panel.
- Cold patches along the top edge. British Gas lists these as another clue.
- Gurgling noises. Bubbling or gurgling sounds can mean air's moving around the system.
- A radiator that's hot all over. British Gas says to leave that one alone. It doesn't need bleeding.
If a radiator's cold at the bottom but warm at the top, that's a different problem. Baxi says remaining cold spots after bleeding can point to sludge or debris in the system, which is a job for a Gas Safe registered installer.
What you need to bleed a radiator
- A radiator bleed key. Baxi says most valves need one and they're widely sold in DIY shops.
- A flat-head screwdriver or Allen key instead, if your radiator's valve is made for one. British Gas notes that many modern radiators take a screwdriver.
- A cloth for grip and to catch drips.
- An old towel to protect the floor.
- A small container to catch any water that comes out.
Turn the heating off and let the radiators cool first
This is the step people skip, and it's the one that matters most. British Gas warns that the water and air inside can be scalding while the heating's running, so switch the central heating off and give the radiators time to cool before you touch a valve. Baxi gives the same advice. Don't rush it: a cool radiator is a safe radiator.
A quick word on gas safety while we're here. Bleeding a radiator means working on the radiator only. Never open up, adjust or take apart the boiler or any gas appliance yourself. If you think something's wrong with the boiler, call a Gas Safe registered engineer.
How to bleed a radiator step by step
Baxi suggests starting with the radiator highest up in the system, which is often a heated towel rail in the bathroom and easy to forget. Then work through every radiator that felt cold at the top.
- Find the bleed valve. It's a small square or slotted fitting at one top corner of the radiator.
- Hold your cloth under the valve and put the towel and container in place.
- Fit the key or screwdriver and turn it slowly anticlockwise, just a little at first. Use the cloth for grip if it's stiff.
- Listen for a hiss. That's the trapped air escaping.
- When the hissing stops and water starts to come out, turn the key clockwise to close the valve again.
- Wipe up any drips and move on to the next radiator that needs doing.
Don't open the valve too far. British Gas says a large drop in boiler pressure afterwards can be a sign the valve was opened more than it needed to be.
Checking boiler pressure after bleeding radiators
Letting air and a little water out of the system can lower the pressure, so this step matters, particularly if you have a combi boiler with a pressure gauge on the front. Here's what the two guides say:
| Boiler state | Pressure to look for | Source |
|---|---|---|
| Heating off | Between 1.0 and 1.5 bar | British Gas |
| Heating on | Around 2 bar | British Gas |
| General guide | About 1.5 bar on the front gauge | Baxi |
A small drop is normal. If the pressure's low, top it up following your boiler maker's instructions, which Baxi links to from its guide, then turn the heating back on and check every radiator warms evenly. If you're not sure how to top up your model, check its manual or ask a Gas Safe registered engineer.
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When bleeding the radiator doesn't fix the cold spots
- You can't find a bleed valve. British Gas says it may be hidden under paint or on the back of the radiator. If you still can't find one, ask a plumber.
- The radiator's still cold after bleeding. British Gas says this could point to a boiler fault, so get the system checked by a professional.
- Cold patches stay at the bottom or middle. Baxi says this can mean sludge or debris, which a Gas Safe registered installer can flush out and treat with an inhibitor. British Gas also says don't try to flush radiators yourself.
- You keep having to bleed the same radiator. Baxi says frequent bleeding can mean a leak. British Gas lists a leak, installation issues or the need for maintenance as possible causes of recurring air.
- You see rust or water around fittings. Baxi says to contact a Gas Safe registered engineer.
How often to bleed your radiators
There's no need to do it every week. Baxi suggests checking at least once a year, ideally when you switch the heating back on after summer. British Gas suggests every few months and again after long periods when the heating hasn't been used. If you find yourself bleeding the same radiator again and again, treat that as a sign to get the system looked at.
More home how-tos and sharing what you know
Heating jobs often come in clusters: once you've sorted the radiators, the next household chore usually isn't far away. If a slow bathroom is next on your list, I've also explained how to unblock a toilet using guidance from UK DIY retailers and safety bodies.
Heating engineers who blog can share their know-how through plumbing blogs that accept articles from tradespeople.
Frequently asked questions
Should the heating be on or off when you bleed a radiator?
Off. Turn the heating on first to find the cold radiators, then switch it off and let them cool before you open any valve. British Gas warns the water inside can be scalding while the heating runs.
Can I bleed a radiator without a radiator key?
Often, yes. British Gas says many modern radiators take a flat-head screwdriver and some take an Allen key. If neither fits, Baxi notes radiator keys are widely sold in DIY shops.
Which radiator should I bleed first?
Baxi suggests starting with the radiator highest up in the system, which is often a bathroom towel rail, then working through every radiator that felt cold at the top.
Why has my boiler pressure dropped after bleeding?
Bleeding lets out a little water as well as air, so a small drop is normal. British Gas says a large drop can mean the valve was opened too far. Top it up following your boiler's instructions or ask a Gas Safe registered engineer.
Why is my radiator still cold after bleeding?
If it's still cold, British Gas says there may be a boiler fault. Baxi says cold spots can also be sludge or debris, which needs a Gas Safe registered installer to clean the system.
