If your radiator is warm at the bottom but cold at the top, there's trapped air in the system. Bleeding it releases that air and lets hot water fill the full radiator again.
Here's how to do it, and what to watch for when you're done.
What you'll need
- A radiator bleed key (sold at any DIY or hardware shop for under £2)
- A cloth or small bowl
- That's it
Step-by-step
1. Turn the heating on first
Let the system heat up fully so you can identify which radiators need bleeding — the cold-top ones. Then turn the heating off and wait 20–30 minutes. You want the water cooler before you open the valve.
2. Locate the bleed valve
It's a small square peg in a round plug, usually at the top corner of the radiator. Your bleed key fits over it.
3. Open the valve slowly
Turn the key anti-clockwise — just a quarter to half a turn. You'll hear a hissing sound. That's air escaping. Hold the cloth underneath to catch any water that spits out.
4. Close it when water flows steadily
When the hissing stops and water comes out in a steady stream without sputtering, close the valve clockwise. Don't overtighten — just firm enough to stop the drip.
5. Check your boiler pressure
Bleeding a radiator releases water along with the air, which drops system pressure. Check your boiler's pressure gauge after bleeding. It should read between 1 and 1.5 bar cold. If it's dropped below 1, you'll need to repressurise — see our guide on boiler pressure.
How often should you bleed radiators?
Once a year is a good habit — typically at the start of the heating season. If a radiator keeps going cold at the top within weeks of bleeding, there may be a deeper issue like a corroded system producing hydrogen gas, which is worth having looked at.
When bleeding doesn't fix it
If the radiator is cold all the way through — not just at the top — the problem is likely a stuck or closed valve, or a system that needs balancing rather than bleeding. Cold in patches across multiple radiators can also point to sludge buildup.
These are worth getting properly diagnosed rather than guessing at.
If your radiators aren't heating properly and you're in Hemel Hempstead, Ealing, or NW London — get in touch. We'll find the cause and fix it properly.