Switch the heating off and let the radiator cool. Hold a cloth under the bleed valve, then turn the valve slowly anticlockwise with a radiator key. You’ll hear air hiss out. The second water starts to dribble, shut the valve. Finally, check your boiler pressure and top it up if it has dropped.
You know the morning. It’s the first properly cold one of the year, you flick the heating on, and ten minutes later you’re standing in the lounge with a hand on the radiator. Warm at the bottom. Cold at the top. Maybe a bit of gurgling too, like it’s got something to say.
That’s trapped air. It’s about the most common central heating niggle there is, and the fix costs a pound or two. No plumber, no callout fee. A small key, an old towel and ten minutes will do it.
Here’s how to do it properly, what to check afterwards, and what to think about if it doesn’t work.
Why do radiators need bleeding in the first place?
Your heating is a sealed loop of water that goes round the house, through the boiler, and back again. In a perfect world, it would only ever contain water. In real life, air creeps in. It can get in through tiny leaks, after a top-up, or from gas produced when metal inside the system slowly corrodes.
Air rises, so it ends up at the top of the radiator. Hot water can’t push past it. So that patch stays cold while you pay to heat the rest.
Most homes need to do this once or twice a year. Early autumn is the sweet spot. You’ll catch the problem before you’re relying on the heating every day.
How to tell if a radiator needs bleeding
Don’t worry about ticking every box here. One or two signs is usually enough.
- The top of the radiator is cold while the bottom is warm
- Gurgling, bubbling or trickling noises when the heating runs
- One radiator takes ages to warm up compared with the rest
- A room feels chilly even though the thermostat says otherwise
- The boiler seems to run for longer than it used to
About fifteen minutes after the heating comes on, run your hand over each radiator. Careful, some will be very hot. Make a mental note of the ones with cold patches.
What you’ll need
Not much, honestly.
- A radiator bleed key. Any DIY shop sells them. Some newer radiators take a flat-head screwdriver instead, so look at yours first.
- An old towel or cloth. The water that comes out is often grubby and dark, and it stains carpets.
- A small bowl or mug, if you want to catch more of it.
- Your boiler manual, in case you need to top up the pressure afterwards.
How to bleed a radiator, step by step
1. Turn the heating off
Switch it off at the thermostat or programmer, then give it time to cool. There are two reasons. One, you won’t scald yourself. Two, if the pump’s still running while the valve’s open, it can drag even more air into the system.
2. Start with the radiator furthest from the boiler
If you’ve got several to do, work from the furthest one back towards the boiler. In a two-storey house, do downstairs first and then go up. Air moves upwards, so this order helps you clear the lot.
3. Find the bleed valve
Look at the top corner of the radiator, usually at one end. It’s a small round fitting with a square or slotted centre. On some models it’s on the side or back.
4. Get the cloth in place
Hold it under the valve. Put some on the floor too, because water can spit sideways.
5. Open the valve slowly
Slot the key on and turn it anticlockwise, only a quarter to half a turn. That hiss is air escaping. Good.
Don’t unscrew it all the way. On some valves the entire screw can pop out, and then you’ve got water going everywhere.
6. Wait for the hiss to stop
Keep the key where it is and hold the cloth steady. The hissing fades, there’s a bit of spluttering, and then a steady trickle of water.
7. Close the valve
As soon as the water runs steadily, turn the key clockwise until it’s shut. Snug is enough. Cranking it down can damage the valve.
8. Wipe up and move on
Dry off the valve and the floor, then repeat for the next radiator.
Check your boiler pressure afterwards
This is the step people skip, and it causes more grief than the bleeding itself.
Whenever you let air or water out, the pressure in the system drops a little. If it drops too far, plenty of boilers simply stop working.
- Find the pressure gauge on the front of the boiler. Some models show a digital reading instead.
- Most systems should sit at roughly 1 to 1.5 bar when cold. Your manual will give the exact number for your model.
- If it’s low, you’ll need to re-pressurise it using the filling loop, usually a braided hose with a valve or two under the boiler. Follow the manual closely.
- Don’t overfill it. Too much pressure can trigger a fault, and then you’ll have to release some.
If you’re topping up the pressure every few days, something’s wrong. There may be a leak. It’s worth reading what can happen after a hidden plumbing leak, because water damage spreads faster than most people expect.
Once that’s done, turn the heating back on. Give it fifteen to twenty minutes, then check each radiator again.
What if bleeding doesn’t fix it?
Sometimes you do everything right and the radiator’s still cold in places. The pattern usually tells you what’s going on.
Cold at the top only. There’s probably still some air in there. Bleed it once more.
Cold at the bottom, warm at the top. This is a different problem. It’s usually sludge, a build-up of rust and debris that settles at the base. Bleeding won’t touch it. You’ll likely need a system clean, often called a power flush, from a heating engineer.
Cold in the middle, hot around the edges. Also a sign of sludge or a blockage forming inside.
Cold all over. The radiator might not be getting hot water at all. The usual suspects are a stuck thermostatic radiator valve (TRV), a closed lockshield valve or a circulation fault. Stuck TRV pins are common after the summer. A gentle tap sometimes frees them, but don’t force anything.
Several radiators cold, or the whole house. This could be low pressure, a failing pump or a boiler issue. Check the pressure first. If that’s fine, call someone.
Anything involving the boiler itself needs a Gas Safe registered engineer. Bleeding a radiator isn’t gas work. Opening up the boiler is.
The rest of your autumn checklist
Bleeding is one job on a longer list. If you’d rather not have a heating breakdown on the first frosty morning, it’s worth doing these too.
- Look at your condensate pipe. On a condensing boiler, a frozen outside pipe can shut the whole thing down. We’ve covered it in our guide to a frozen condensate pipe.
- Book your annual boiler service before the engineers fill up in November and December.
- Find your stopcock. If a pipe splits, you want to shut the water off quickly. Our guide to fixing a burst pipe in winter walks you through it.
- Listen for odd noises. Banging, whistling and rattling aren’t always air. Our list of warning signs your plumbing system is failing helps you tell them apart.
- Test your carbon monoxide alarm. It should be near the boiler, and it should actually work.
Thinking bigger, like new radiators, new pipework or a new boiler? Work out the cost before you commit. Our piece on whether a home renovation is worth the investment can help you decide where the money does the most good.
If you’re renting
Bleeding a radiator is normally fine for tenants. It’s basic upkeep, and plenty of landlords and housing associations encourage it. If you’re unsure, check your tenancy agreement or just ask.
What you shouldn’t do is remove radiators, change pipework or open the boiler. If a radiator stays cold after bleeding, or the pressure keeps falling, report it in writing and keep a copy. Landlords are generally responsible for keeping heating installations in good working order.
Mistakes people make
- Bleeding with the heating on. The pump can pull in more air, and the water’s hot.
- Opening the valve too far. A quarter to half a turn is plenty.
- Forgetting the cloth. That dirty water leaves marks.
- Skipping the pressure check. If the boiler cuts out after bleeding, it’s nearly always pressure.
- Bleeding a radiator that’s cold at the bottom. That points to sludge, not air.
- Ignoring a radiator that needs bleeding every week. Constant air means something’s wrong, maybe a leak or corrosion.
Frequently asked questions
How often should I bleed my radiators?
Once a year is a good habit, ideally at the start of autumn. If one develops cold spots or starts gurgling, bleed it when it happens.
Should the heating be on or off?
Off. Turn it off and let the radiators cool first. It’s safer, and it stops the pump pulling in more air.
Can I bleed a radiator on a combi boiler?
Yes. The only difference is that you’ll usually need to check the boiler pressure afterwards and top it up with the filling loop. Always follow the manual for your model.
Why is my radiator cold at the bottom but hot at the top?
That’s usually sludge or a blockage, not air. Bleeding won’t fix it. A heating engineer can clean the system or the radiator.
How long does it take?
Five to ten minutes per radiator. The whole house takes under an hour.
What if only water comes out and no air?
Then there’s probably no air in that radiator. The cause could be a stuck valve, a blockage or a circulation problem. Close the valve and look at the other causes above.
Will bleeding radiators save me money?
It can help. A radiator full of air heats the room less effectively, so the boiler works longer to reach the temperature you’ve set. Clear the air and the whole radiator does its job.
Final thoughts
Bleeding a radiator is one of the easiest ways to make your home warmer before winter. A cheap key, a cloth and a bit of patience. Heating off, valve open slowly, close it when water appears, check the boiler pressure. That’s really it.
If it doesn’t work, the cold spots tell you what to try next. Cold at the bottom points to sludge. Cold all over points to a stuck valve or a circulation fault. Anything involving the boiler is a job for a Gas Safe engineer.
Do it this week, before the first hard frost, and your heating will be ready when you need it.