Radiator cold at the top: how to bleed it

Cold at the top? That's usually just air.

The answer in 2 lines. A radiator that is warm at the bottom and cold at the top usually has air in it. With the heating off and the radiator cool, open the small bleed valve at the top a quarter turn with a radiator key until the hissing stops and water appears, then close it. Afterwards, check the boiler pressure: bleeding lowers it. If the radiator is cold at the bottom or cold all over, the cause is elsewhere (sludge, a stuck valve or the boiler).

The first 3 steps

With the heating on, feel the radiator: warm at the bottom and cold at the top means air. Cold at the bottom, or cold everywhere: see "Other causes" below.
Turn the heating off and wait until the radiators are cool.
Get a radiator key (from any DIY shop or hardware store; some valves take a flat screwdriver), an old cloth and a small container.

Stop: danger

Do not bleed a hot radiator: the water can scald. Heating off, radiators cool.
Electric radiators (no pipes) have nothing to bleed: never open them.
Smell of gas: do not use switches, flames or your phone indoors; open doors and windows, turn off the gas at the meter if you can, get out and call the free National Gas Emergency Service on 0800 111 999 (England, Scotland and Wales), 24/7.
Headaches, dizziness, nausea or tiredness, especially in several people at once or easing when you go outside: think carbon monoxide. Open windows, switch the appliance off if you can, get out into fresh air and call 999 if anyone feels unwell (or NHS 111 for advice).

Bleeding a radiator, step by step

Find the bleed valve: a small square-headed screw at one top corner of the radiator.
Hold the cloth and container underneath, fit the key and turn it anticlockwise a quarter to half a turn. You will hear air hiss out.
When water comes out steadily with no more bubbles, turn it clockwise to close. Snug, not forced.
Do the same on any other radiators that are cold at the top. In a house, people often start downstairs and work up.
Check the boiler pressure gauge. If it has dropped below the figure in your manual (often 1 to 1.5 bar on a combi or system boiler), top it up with the boiler cold: see our boiler pressure guide.
Turn the heating back on and feel the radiator after an hour.

Step-by-step guides with photos: British Gas and Which?.

What not to do

Unscrewing the bleed screw all the way: it can come out and be hard to get back in.
Bleeding with the heating on.
Forgetting the boiler pressure afterwards: the boiler may lock out the next morning.

Other causes

Cold at the bottom, warm at the top: usually sludge (rust and debris) settled in the radiator. It may need a flush or a filter fitted by a heating engineer (Screwfix explains).
One radiator cold all over: the thermostatic valve (TRV) may be turned down or its pin stuck, or a valve closed (Baxi's checklist).
All radiators cold: the boiler, the timer or the pressure: see our boiler checks guide.
Upstairs radiators poor: air, or pressure too low.
Bleeding every week: air is getting in, or water is escaping somewhere. Call a heating engineer.
A small tank in the loft and no pressure gauge: you have an open-vented system. Bleeding works the same way; there is no pressure to top up, as the tank refills it.

What you can do yourself, and where to stop

You can: bleed radiators once a year when the heating goes back on, top up the pressure as the manual shows, and check the TRVs are not on the lowest setting.
Stop there if the bleed screw is snapped or leaks, a valve will not turn, water drips under a radiator or the boiler, or the pressure keeps falling: call a heating engineer (Gas Safe registered for anything touching a gas boiler: how to check).
If you rent: bleeding is normally something tenants can do. Repairs to the heating system are the landlord's (Landlord and Tenant Act 1985, section 11): report them in writing. For a block with communal heating, tell the managing agent before touching anything.

Good to know

Bleed your radiators at the start of each heating season, and again if they gurgle.
Keep a radiator key somewhere you will find it: they cost very little and are sold in most hardware shops.

In Leeds, Hannah turns the heating back on in October: the living-room radiator is warm at the bottom and cold at the top. She switches the heating off, waits an hour, opens the bleed valve a quarter turn: air hisses, then water, and she closes it. The boiler gauge has dropped to 0.8 bar, so she tops it up to 1.2 bar following the manual. That evening the radiator is hot all over.

A made-up story, for illustration.

Bobby

Sources, verification, confidence

Verified on 1 November 2026, for England. Confidence: high for the bleeding method and the safety steps; medium for typical pressure values, which depend on your boiler (your manual comes first). We found no official statistics on radiator problems (checked 1 November 2026). This guide describes common cases, not a diagnosis.