How to bleed a radiator
If your radiator is cold at the top, making gurgling noises or not heating evenly, there may be trapped air inside. Bleeding the radiator releases that air and helps your heating system work properly again.
Common sign
A radiator that is warm at the bottom but cold at the top often has trapped air and may need bleeding.
Safety first
Turn your heating off and let the radiators cool before bleeding them. Hot water or steam can escape from the bleed valve, so never bleed a radiator while the system is hot.
Watch how to bleed a radiator
This B&Q video gives a clear visual guide to bleeding a radiator. Radiator valves can vary, so take your time and stop if you are unsure.
Turn heating off
Switch the heating off and wait for the radiators to cool before starting.
Release trapped air
Use a radiator key to slowly open the bleed valve until you hear air escaping.
Close when water appears
When the hissing stops and water starts to drip out, close the valve carefully.
When does a radiator need bleeding?
Radiators can collect trapped air over time. When this happens, hot water cannot fully circulate through the radiator, which can leave the top cold while the bottom is still warm.
You may need to bleed a radiator if it is cold at the top, making gurgling noises, taking longer than usual to heat up, or heating unevenly compared with other radiators in the property.
What tools do you need?
You do not need many tools to bleed a radiator, but it is worth preparing everything first so you can catch any drips and avoid marking walls or flooring.
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1
Radiator key
A small radiator key is used to open the bleed valve. Some modern radiators may use a flat-head screwdriver instead.
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2
Old towel or cloth
Place this underneath the valve to protect the floor and wall from water marks.
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3
Small container
Use a container or bowl to catch any water that comes out once the air has escaped.
How to bleed a radiator step by step
Work carefully and do not force the valve. A small quarter turn is usually enough to start releasing air.
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1
Turn off your heating
Switch your central heating off and allow all radiators to cool down fully before starting.
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2
Find the bleed valve
The bleed valve is usually on one of the top corners of the radiator. It may have a small square or slotted insert.
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3
Protect the area
Hold a towel or cloth under the valve and keep a small container nearby to catch water.
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4
Open the valve slowly
Fit the radiator key onto the valve and turn it slowly anti-clockwise. You should hear a hissing sound as air escapes.
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5
Close the valve when water appears
As soon as the hissing stops and water starts to drip out, turn the key clockwise to close the valve.
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6
Do not overtighten
Close the valve firmly but gently. Overtightening can damage the valve.
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7
Check the radiator
Turn the heating back on and check the radiator heats evenly from top to bottom.
Check your boiler pressure afterwards
Bleeding radiators can cause boiler pressure to drop. After bleeding one or more radiators, check the boiler pressure gauge.
As a general guide, many boilers should be around 1 to 1.5 bar when cold. If the pressure is too low, you may need to repressurise the boiler.
When should you call an engineer?
You should get help if the radiator still does not heat properly after bleeding, the bleed valve is stuck or leaking, the boiler pressure keeps dropping, or several radiators are not working correctly.
Why is my radiator cold at the top?
This is usually caused by trapped air inside the radiator. Bleeding the radiator releases the air so hot water can circulate properly.
What should come out when I bleed a radiator?
At first, you may hear air hissing out. Once the air has escaped, water should begin to drip from the valve. Close the valve as soon as water appears.
Do I bleed radiators with the heating on or off?
Turn the heating off and allow the radiators to cool before bleeding them. This helps prevent hot water or steam escaping.
Do I need to check boiler pressure after bleeding radiators?
Yes. Bleeding radiators can reduce system pressure, so check the boiler pressure gauge afterwards and top it up if needed.
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How To bleed a radiator
If you've noticed your radiators are cold at the top or making gurgling noises, it's likely due to trapped air. To help you fix this common issue, we've found a very useful video guide from B&Q. The tutorial provides a clear, step-by-step demonstration on how to safely and effectively bleed your radiator, ensuring your heating system works at its best. Following their expert advice can help you improve your heating efficiency and get your radiators working properly again.
Here are some step by step instructions if you would prefer these.
Bleeding a radiator is a simple process that can significantly improve your heating system's efficiency by releasing trapped air. If a radiator is cold at the top but warm at the bottom, it's a clear sign that it needs bleeding.
Turn off your central heating. It's crucial to switch off your boiler and let all radiators cool down completely. This prevents hot water or steam from escaping, which could cause a burn.
Gather your tools. You'll need a radiator key (a small, square-headed metal tool available at most DIY stores), an old towel or cloth to catch any drips, and a container or bucket. Some modern radiators may not require a specific key and can be bled with a flat-head screwdriver.
Find the bleed valve. This small valve is usually located on the top corner of the radiator, either on the left or right side. It's a small round hole with a square or slotted insert inside.
Prepare the area. Place the towel or cloth directly under the bleed valve to protect your floor and wall. Have your container ready to catch any water that may come out.
Bleed the radiator. Fit the radiator key onto the valve and turn it slowly, a quarter to half a turn, counter-clockwise. You should hear a hissing sound as the trapped air escapes.
Close the valve. As soon as the hissing stops and water begins to drip out, turn the key clockwise to close the valve. Do not overtighten it, as this can damage the valve.
Check the pressure. After bleeding one or more radiators, you may notice that the pressure on your boiler's gauge has dropped. It's important to re-pressurise your boiler to its correct operating level (typically between 1.0 and 1.5 bar when cold). Check your boiler's manual for specific instructions on how to do this.
Turn the heating back on. Once the pressure is correct, switch your heating system back on and check that the radiators are heating up evenly.