The heating is on, the boiler is working, but one radiator is stone cold at the top while the bottom is warm. Or it gurgles every time the heating kicks in. Either way, the room never quite gets warm, and it is easy to assume something expensive has gone wrong.
Usually it has not. Air gets into central heating systems over time, rises to the top of the radiator and stops hot water filling it. Letting that air out is a quick job that needs nothing more than a cheap radiator key and an old towel.
Here is how to bleed a radiator properly, including the step that is easiest to forget: checking the boiler pressure afterwards.
Turn the heating off and let the radiators cool. Hold a cloth under the bleed valve at the top corner of the radiator, open it slightly with a radiator key, and let the air hiss out. Close it as soon as water starts to flow steadily.
Once every radiator is done, check the pressure gauge on your boiler. Bleeding lets pressure out of the system, and if it drops too low the heating will not work properly.
Heating off, radiators cool, open the valve a little, close it when water appears, then check the boiler pressure.
Does your radiator actually need bleeding?
Bleeding only fixes one thing: trapped air. According to Worcester Bosch, a radiator that is cold at the top usually points to trapped air, while one that is cold at the bottom is usually caused by a build-up of sludge. They look similar from across the room, but they need completely different fixes.
What is your radiator doing?
Put the heating on, give the radiators time to warm up, then carefully feel the top and bottom of each one.
- What you noticeCold at the top, warm at the bottomLikely causeTrapped airStart hereBleed the radiator
- What you noticeGurgling or bubbling noises when the heating is onLikely causeTrapped airStart hereBleed the radiator
- What you noticeSlow to warm up compared with the othersLikely causeOften trapped airStart hereBleed it, then check the boiler pressure
- What you noticeWarm at the top, cold at the bottomLikely causeSludge in the systemStart hereA heating engineer, not a radiator key
- What you noticeStill cold after bleeding, with normal boiler pressureLikely causeA wider system problemStart hereA heating engineer
- What you noticeCold all over while the other radiators are hotLikely causeValve turned down, or a faultStart hereCheck the valve on the side is turned up, then call an engineer
- What you noticeBoiler pressure keeps dropping after you top it upLikely causePossibly a leak in the systemStart hereA heating engineer
What you need
A radiator key, which fits the square bleed screw on most radiators and costs very little from any DIY shop or supermarket. British Gas notes that some more modern radiators use a flat-head screwdriver instead. The rest is in the list below.
Before you start
- Radiator key, or a flat-head screwdriver for some modern radiators
- Cloth or old towel
- Small container to catch the water
- Heating off and radiators cool
- A note of which radiators were cold at the top
- Your boiler manual, in case the pressure needs topping up
How to bleed a radiator
Do every radiator that needs it in one go, then check the pressure once at the end.
Find the radiators that need it
Put the heating on and wait for the radiators to warm up, then check each one. Any that are cold at the top need bleeding. Make a note of which ones, because you will not be able to tell once they have cooled down.
Turn the heating off and let it cool
British Gas says it is very important that the heating is off and the radiators are cool before you start. The water inside can be very hot.
Start with the radiator furthest from the boiler
Viessmann recommends starting with the radiator furthest away from the boiler, as it is likely to contain the most air, and working back towards it.
Open the valve slightly
The bleed valve is at one of the top corners of the radiator. Hold the cloth underneath it, fit the key and turn it anti-clockwise, only a little. You should hear a hiss as the air escapes.
Do not open it all the way. British Gas warns this can cause a big drop in boiler pressure, and the screw can be fiddly to get back in if it comes out.
Close it when water flows steadily
Once the hissing stops and water starts to come out in a steady flow rather than spitting, turn the key clockwise to close the valve. Do not overtighten it. Wipe up any drips, since the water can be dirty and may mark carpets.
Check the boiler pressure
Worcester Bosch says to top up the boiler pressure once you have finished, because pressure can be lost while bleeding. British Gas puts the right range at 1.0 to 1.5 bar with the heating off, rising to around 2 bar when it is on. If it has dropped below that, follow your boiler manual to top it up.
Turn the heating back on and check
Give the system time to warm up, then feel each radiator you bled. It should now be warm all the way to the top.
How often should you bleed radiators?
Smart Energy GB suggests about once a year, ideally before winter when the heating starts being used more regularly. Beyond that, bleed any radiator as soon as it shows the signs above: cold at the top, slow to warm up or gurgling. Checking them now, before the first proper cold spell, means you find the problem on a mild day rather than a freezing one.
Keep the heat in the room
Once your radiators are heating properly, it is worth making sure that heat stays in the room. The Energy Saving Trust says radiator reflector panels, fitted behind radiators on uninsulated external walls, could save around £18 a year in Great Britain by reflecting heat back into the room rather than into the wall.
When bleeding does not fix it
If a radiator is warm at the top but cold at the bottom, bleeding will not help. That pattern usually means sludge has built up inside the system, and clearing it is a job for a heating engineer.
The same goes for a radiator that stays cold after bleeding when the boiler pressure is normal, or boiler pressure that keeps dropping even after you have topped it up. Smart Energy GB advises calling in a professional at that point. Any work on the boiler itself must be done by a Gas Safe registered engineer.
If one room stays cold even with a working radiator, our guide to why your house might be so damp in winter is worth a look, since damp air makes a room feel colder than it is, and our best electric heaters UK guide covers options for rooms the central heating struggles to reach.
Bleeding radiators: your questions answered
Should the heating be on or off when you bleed a radiator?
Off. Put it on first to find which radiators are cold at the top, then turn it off and let the radiators cool before you open any valves. The water inside can be very hot, so it is not safe to open a valve while the system is running.
Which radiator should you bleed first?
Viessmann recommends starting with the radiator furthest away from the boiler, since it is likely to hold the most air, then working back towards the boiler.
What should my boiler pressure be after bleeding radiators?
British Gas puts the right range at 1.0 to 1.5 bar with the heating off, rising to around 2 bar when the heating is on. If it has dropped below that after bleeding, follow your boiler manual to top it up.
How often should you bleed your radiators?
About once a year, ideally before winter, plus any time a radiator is cold at the top, slow to warm up or gurgling. If the same radiator needs bleeding every few weeks, it is worth asking a heating engineer to find out why air keeps getting in.
Why is my radiator still cold after bleeding it?
If it is cold at the bottom rather than the top, the likely cause is sludge rather than air, which bleeding will not fix. If it is cold all over, check the valve on the side of the radiator is turned up and the boiler pressure is in range. If the pressure is normal and the radiator still will not heat up, call a heating engineer.
Can you bleed a radiator with a screwdriver?
On some more modern radiators, yes. British Gas notes that these use a flat-head screwdriver rather than a radiator key. Most radiators still have a square bleed screw that needs a proper key.
