Glossary · Central heating, radiators and hot water
Why is my radiator not heating up?
A radiator that is not heating up is getting too little hot water. Trapped air is the usual reason when it is cold at the top. A valve that is set wrongly or stuck, sludge, low boiler pressure or a boiler fault are the other causes. Which one applies depends on which radiators are cold and where.
Key points
- Cold at the top usually means trapped air, and bleeding the radiator is the fix, according to Baxi and Worcester Bosch.
- If every radiator is cold, look at the boiler first: its settings, any error code and the pressure.
- If one radiator is completely cold, check its valve setting. Vaillant says a valve can also be blocked by sludge or limescale.
- Cold at the bottom and hot at the top points to sludge rather than air, and bleeding will not cure it.
- Bleeding can lower boiler pressure, so Worcester Bosch says to top it up afterwards.
What causes it
A radiator heats when hot water flows through it. If something restricts that flow, part or all of it stays cold. Vaillant says there are two kinds of cold radiator: ones that are cold in parts and warm in others, and ones that are completely cold.[2]Vaillant, Why is my radiator not working?
What you can check yourself
Work from the pattern of cold to the likely cause.
| What you see | Likely cause | What the manufacturers say |
|---|---|---|
| Cold at the top, warm below | Trapped air | Baxi and Vaillant say to bleed the radiator. |
| Hot at the top, cold at the bottom | Sludge | Baxi says residue builds up at the bottom. Worcester Bosch says it means you may need your system cleaned. See radiator cold at the bottom. |
| One radiator completely cold | Valve setting or blocked valve | Vaillant says to check the valve setting. A valve can be blocked by sludge or limescale. |
| All radiators cold | Boiler or pressure | Vaillant says the most common reason is the boiler itself, which may be set to heat hot water only. |
Air. Worcester Bosch describes bleeding like this. Turn the heating on to find which radiators have cold areas, then turn it off and let them cool. Put a cloth under the bleed valve at the top and turn the key anticlockwise. You will hear a hiss as air escapes. When water flows steadily, close the valve by turning clockwise. Then turn the heating back on and check the radiator is warm all over.[3]Worcester Bosch, How to bleed a radiator Baxi suggests starting with the radiator closest to the boiler when there is a lot of air.[1]Baxi, Why are my radiators cold?
Pressure. Bleeding lets pressure fall, so Worcester Bosch says to top up the boiler pressure afterwards.[3]Worcester Bosch, How to bleed a radiator Vaillant says pressure below 1 bar means the boiler has lost pressure, and that repeated loss can point to a leak, most commonly at radiator valves or other system parts.[4]Vaillant, Why is my boiler losing pressure? See low water pressure.
Valves. If a radiator has a thermostatic radiator valve, check it is not set too low. Baxi says the pin can stick, especially on a radiator that has not been used for a while, and that if you are not confident you should call a heating engineer.[1]Baxi, Why are my radiators cold? Glow-worm describes the valve at the other end as a lockshield valve, which an installer normally sets and which rarely needs changing.[5]Glow-worm, Why is my radiator cold at the bottom?
When to call an engineer
Call a heating engineer if bleeding does not help, if the radiator is cold at the bottom, if several radiators are cold, or if the boiler shows an error code. Vaillant says that in most cases a boiler fault must be put right by a Gas Safe registered engineer.[2]Vaillant, Why is my radiator not working? If the valve is blocked, Vaillant recommends a professional, who may also find the radiator itself is failing.[2]Vaillant, Why is my radiator not working?
Example
Mohammed in Leeds has a radiator in the back bedroom that is warm at the bottom and cold at the top. He turns the heating off, lets it cool, bleeds it with a radiator key, and checks the boiler pressure afterwards. The radiator now warms evenly. His hall radiator, which is hot at the top and cold at the bottom, is a different problem, and he asks an engineer to look at it.
What people get wrong
Many people bleed a radiator every time one is cold. Bleeding only helps when air is the problem. Worcester Bosch says that a radiator cold at the bottom and hot at the top could mean sludge, and bleeding will not shift that.
Needing to bleed often is a sign in its own right. Baxi lists radiators that need bleeding more frequently among the signs of sludge in the system.[1]Baxi, Why are my radiators cold?
The bottom line
A radiator that will not heat is usually short of hot water. Cold at the top means air, which you can bleed out. Cold at the bottom means sludge, and a fully cold radiator points to its valve, the boiler or the pressure. Check the pattern first, top up the pressure after bleeding, and call an engineer when the simple fixes do not work.
Common questions
Why is my radiator cold at the top?
Baxi says the most likely explanation is bubbles of air trapped inside the radiator. Air rises to the top and stops the hot water filling it, and bleeding the radiator releases it.
Why is only one radiator cold?
Baxi says one cold radiator is likely an air bubble or a stuck valve. Vaillant adds that if the whole radiator stays cold, check the valve setting, as a valve can also be blocked by sludge or limescale.
Do I need to bleed all my radiators?
Only those with cold spots. Worcester Bosch says to run the heating, check which radiators have cold areas, then switch the heating off and let them cool before bleeding those ones.
Why did my boiler pressure drop after bleeding?
Letting air out of a radiator lets the system draw in water, so pressure falls. Worcester Bosch says to top up the boiler pressure after bleeding. Vaillant lists bleeding as a common reason for lost pressure.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Baxi. Why are my radiators cold?. Checked 11 November 2026. ↩
- Vaillant. Why is my radiator not working?. Checked 11 November 2026. ↩
- Worcester Bosch. How to bleed a radiator. Checked 11 November 2026. ↩
- Vaillant. Why is my boiler losing pressure?. Checked 11 November 2026. ↩
- Glow-worm. Why is my radiator cold at the bottom?. Checked 11 November 2026. ↩