How to Bleed a Radiator

If your radiator is cold at the top but warm at the bottom, there's air trapped in the system — preventing the hot water from fully circulating. Bleeding removes the air and restores full heating performance.

What you need

• Radiator bleed key (available from any DIY store for under £2)
• Old cloth or small bowl

Step by step

1. Turn your heating on and let it reach full temperature
2. Identify which radiators are cold at the top
3. Turn the heating off and wait 10 minutes for the system to cool slightly (you don't want boiling water spraying out)
4. Find the bleed valve — a small square nut at the top of the radiator, usually at one end
5. Place your cloth under the valve and insert the bleed key
6. Turn the key slowly anti-clockwise (about a quarter turn) — you'll hear air hissing out
7. When water starts dripping out, close the valve immediately
8. Repeat for all cold radiators
9. Check your boiler pressure — bleeding reduces system pressure, so you may need to re-pressurise

When to call an engineer

If radiators repeatedly need bleeding, or if the water coming out is black or very dirty, you likely have a sludge problem. A power flush and inhibitor treatment will fix this — contact us for a quote.