5 minutes
Per radiator
£0
DIY cost
Top cold
Sign of air
Frequently Asked Questions
How often should I bleed my radiators?
Bleed your radiators once a year — ideally in autumn before the heating season starts. If you notice a radiator going cold at the top mid-winter, bleed it then. Some systems collect air faster than others, particularly if there's a small leak on the pipework or the system hasn't been properly vented.
Why do radiators need bleeding?
Air enters the heating system through the feed and expansion pipe, through small leaks, or when water is added to the system. Being lighter than water, the air rises to the highest points — typically the top of upstairs radiators — and collects there, preventing hot water from filling the radiator fully.
Can I bleed a radiator without a key?
Some modern radiators have a slotted bleed valve that can be opened with a flathead screwdriver. Traditional bleed valves need a square radiator key. If you can't find one, they're available from any DIY shop or hardware store for about £2.
What if water comes out instead of air?
If water comes out immediately when you open the bleed valve, the radiator doesn't have trapped air — it's full of water. Close the valve and look for other causes of the cold radiator, such as a stuck TRV or an imbalanced system.
My radiator is cold at the bottom — will bleeding help?
No. A radiator that's cold at the bottom but hot at the top has sludge build-up, not trapped air. Bleeding only removes air from the top. Sludge requires a power flush to remove. See our case study on power flushing in Woking for more information.
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