How to bleed a radiator
If a radiator is cold at the top but warm at the bottom, trapped air is stopping hot water filling it. Letting the air out takes a radiator key, a cloth and about 15 minutes.
- Time
- 15 minutes
- Difficulty
- Easy
- Cost
- Low cost
- Steps
- 8
- Checked
What you’ll need
- Radiator bleed key (some modern radiators take a flat-head screwdriver instead) Search Amazon for radiator bleed key
- Cloth or old towel
- Small container or bowl to catch drips
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Safety first
- Don't bleed radiators while the heating is on: the water can be very hot and the pump can push out more water than you expect.
- Don't open the valve more than about half a turn. If it comes out, hold your thumb over the hole and screw it back in straight away.
- Radiator water is often black and can stain carpets, so keep the towel underneath.
Step by step
-
Step 1: Check which radiators need bleeding
With the heating on, carefully feel each radiator. One that is cold at the top and warm at the bottom, or has cold patches, has air trapped inside. Make a note of which ones, then turn the heating off.
Next: step 2 -
Step 2: Turn the heating off and let the radiators cool
Switch the central heating off at the boiler or thermostat and wait until the radiators are cool. Water from a hot radiator can scald, and you get a cleaner result when the pump isn't running.
Next: step 3 -
Step 3: Find the bleed valve
The bleed valve is a small square-headed valve (or a slotted screw) at one of the top corners of the radiator. Put your towel on the floor under it.
Next: step 4 -
Step 4: Open the valve slowly
Fit the key on the valve, hold the cloth just under it and turn the key anticlockwise a quarter to half a turn. You should hear hissing as the air escapes. Don't unscrew it fully: the valve can come right out.
Next: step 5 -
Step 5: Close it when water appears
When the hissing stops and water starts to dribble out steadily, turn the key clockwise to close the valve. Snug it up, but don't force it. Wipe up any drips, because radiator water can be dirty and may stain.
Next: step 6 -
Step 6: Repeat on the other radiators
Bleed any other radiators that had cold spots. If your home has more than one floor, many installers suggest starting downstairs and working up, because air tends to rise.
Next: step 7 -
Step 7: Check the boiler pressure
Bleeding lets water out of the system, so the pressure can drop. Look at the gauge on the front of the boiler: on most combi and system boilers it should read about 1 to 1.5 bar when the heating is off. If it's below 1 bar, top it up using the filling loop. See our guide to repressurising a combi boiler.
Next: step 8 -
Step 8: Turn the heating back on and check
Switch the heating on and, once it has warmed up, check that each radiator is now hot all the way to the top.
Tips
- Bleeding radiators at the start of autumn, before you rely on the heating, saves an evening with a cold room.
- Lost the key? Radiator keys are cheap and sold at most hardware shops and supermarkets. Keep one taped near the boiler.
- If a radiator is cold at the bottom but warm at the top, the problem is usually sludge rather than air, and bleeding won't fix it.
When to call a professional
If radiators need bleeding every few weeks, the pressure keeps dropping, or radiators stay cold after bleeding, there may be a leak, sludge build-up or a fault with the pump or valves. Book a heating engineer. Anything inside a gas boiler must be done by a Gas Safe registered engineer.
Questions people ask
How often should I bleed my radiators?
Only when they need it, for example when there are cold spots at the top. Checking them once a year when you first turn the heating on is a sensible habit.
What if no air or water comes out?
The valve may be blocked with paint or debris. Close it and try again gently. If nothing still comes out and the radiator stays cold, ask a heating engineer to look.
Do I need to repressurise the boiler after bleeding?
Often, yes. Check the boiler gauge after bleeding. If it's below about 1 bar, top it up following your boiler's instructions.
Sources
Checked 9 October 2026. We wrote this guide in our own words, using:
- British Gas: how to bleed a radiator (britishgas.co.uk)
- Baxi: how to bleed a radiator (baxi.co.uk)
- British Gas: how to re-pressurise your boiler (britishgas.co.uk)
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Printed from howtoguides.uk/guides/how-to-bleed-a-radiator/ · Checked 9 October 2026 · General information only. For gas, electrics, structural work, medical, legal or money decisions, use a qualified professional.