If a radiator is hot at the bottom and cold at the top, there is air trapped in it. Bleeding releases that air, and it is a genuinely simple job most people can do themselves in about ten minutes.
If it is cold at the bottom and hot at the top, that is a different problem entirely. That is sludge, and bleeding will not touch it.
Why air gets into a heating system
Air accumulates over time and rises to the highest points, which is the top of your radiators. It gets in through:
- Fresh water, which contains dissolved air released as the water heats
- Small leaks, which let air in as water escapes
- Pump cavitation, which creates bubbles
- Corrosion inside the system, which produces hydrogen gas
- Any system work, since new installations and repairs introduce air
Signs a radiator needs bleeding
- Cold patches at the top of the radiator
- Hot at the bottom, cold at the top
- Gurgling or trickling noises
- Some radiators not heating properly
- Longer warm-up times than usual
It is also worth bleeding the radiators at these points, whether or not you have noticed anything:
- Once a year before winter, as part of getting the heating ready
- After the system has been drained down
- After any boiler or pipework repair
- After topping up the boiler pressure, since fresh water brings dissolved air in with it
What you need
- A radiator key, which costs a pound or two from any hardware shop
- An old towel
- A small container or jug
- Kitchen roll for drips
Most modern radiators take a square 5mm key. Some older ones need a flat screwdriver instead.
The steps
1. Turn the heating on and let it run for ten to fifteen minutes. This pushes the air up to the radiators.
2. Find which radiators need it. Feel each one. Cold at the top and hot at the bottom means it needs bleeding.
3. Turn the heating off and wait. Let the radiators cool for thirty to sixty minutes. This is the step people skip and it is the one that causes scalds.
4. Find the bleed valve. Top corner of the radiator, a small square valve with a slot in the middle.
5. Put your towel down and hold the container under the valve.
6. Open the valve slowly. Insert the key and turn anticlockwise about half a turn. You will hear hissing as air escapes. Leave it open while it hisses. As soon as steady water appears, close it by turning clockwise. Do not overtighten.
7. Wipe the valve and check it is not dripping.
8. Repeat for the other radiators. In a house with more than one floor, always start downstairs and work up.
9. Check the boiler pressure. Bleeding releases water, which drops system pressure. The gauge should read between 1 and 1.5 bar cold. If it is below 1, top it up using the filling loop.
10. Turn the heating back on and check everything heats evenly.
Topping up the pressure afterwards
If the gauge has dropped below 1 bar, this is the usual way to top it up on a combi or system boiler:
- Turn the boiler off
- Find the filling loop, usually a flexible hose with a valve at each end, under the boiler
- Open both valves slowly and watch the gauge rise
- Close both valves firmly once it reads 1 to 1.5 bar
- Turn the boiler back on
Some boilers have a built-in filling lever or key instead of a hose, and the boiler manual shows where it is. If you are unsure, ask rather than guess. If you overshoot, letting a little water out through a bleed valve brings the pressure back down.
Needing to top up regularly is not normal. The system is losing water somewhere, and our guide to a boiler losing pressure explains how to track it down.
When it does not work
No hissing at all. Either there is no air in that radiator, or the valve is stuck. Do not force a stuck valve.
Water comes out immediately. There was no air. Close it and check the others.
Still cold after bleeding. Usually sludge blocking the radiator, a stuck thermostatic valve, a valve that is not fully open, or low boiler pressure. Sludge needs a powerflush rather than a bleed key. If the radiator is cold at the bottom and warm at the top, our guide to radiators cold at the bottom explains what is going on.
You need to bleed every few weeks. This is not normal. Air is continuously entering through a leak or corrosion, and that needs diagnosing properly.
The valve will not close or is dripping. Catch the drips, turn the heating off, and call someone. This one is urgent because you cannot stop it yourself.
When to call a heating engineer
Bleeding is a DIY job. Most of what goes wrong around it is not. Get the system looked at if:
- Bleeding has not cleared the cold spots
- You are bleeding the same radiators every few weeks
- The boiler pressure keeps dropping
- Some radiators never heat up at all
- A valve is stuck, damaged or will not close
- The water that comes out is black or rusty brown
- The system is old and struggles to heat the house
- You would simply rather not do it yourself
Dark water deserves a special mention. It is sludge, created by corrosion inside the radiators, and it is the same material that settles at the bottom of a radiator and blocks it. Bleeding lets the air out but does nothing about the sludge.
Preventing air building up
- Annual boiler service
- Corrosion inhibitor in the system, which reduces gas formation
- Fix small leaks promptly
- Keep boiler pressure correct
- Powerflush every five to six years in hard water areas
- Bleed before winter as part of getting the heating ready
Inhibitor is the item on that list most people have never heard of. It is a chemical added to the water in the heating system to slow corrosion inside the radiators. Less corrosion means less gas collecting as air, and less sludge blocking the radiators. Inhibitor levels are checked and topped up as part of an annual boiler service.
Frequently asked
How often should I bleed my radiators?
Most homes need it once a year, usually before winter. If you are bleeding more often than that, air is getting in from somewhere and there is likely a leak or a corrosion problem that needs looking at properly.
Do I need to bleed radiators if they are working fine?
No. If they heat evenly with no cold spots and no gurgling, leave them alone. An annual check before winter is still sensible as part of getting the heating ready.
Why is only one radiator not heating up?
A single cold radiator usually points to that radiator rather than the whole system. Bleed it first, then check both valves are fully open. If it is still cold, it is probably blocked with sludge and needs professional cleaning.
Can I bleed radiators with the heating on?
No. Turn the heating off and let the radiators cool for thirty to sixty minutes first. Bleeding a hot radiator releases water under pressure and you will scald yourself.
What does it mean if black water comes out when I bleed a radiator?
It means there is sludge in the system, caused by corrosion inside the radiators. Close the valve once the air is out, as normal. Bleeding will not shift sludge, so if the water is dark or the radiators are cold at the bottom, the system needs cleaning, usually with a powerflush, and inhibitor adding afterwards.