How to Bleed a Radiator — Step-by-Step Guide

Why Do Radiators Need Bleeding?

Over time, air builds up inside radiators and prevents them from filling with hot water properly. The result is a cold top section while the bottom stays warm — meaning your boiler is working harder than it needs to. Bleeding releases the trapped air.

What You'll Need

• A radiator bleed key (available from any hardware shop for under £2, or sometimes stored near your boiler)
• A cloth or small container to catch water drips

Step-by-Step: How to Bleed a Radiator

1. Turn the heating on and let the system reach full temperature. Feel which radiators have cold spots near the top.
2. Turn the heating off and let it cool for 20–30 minutes before starting. This prevents scalding.
3. Locate the bleed valve — it's the small square fitting at one end of the radiator, usually near the top.
4. Place a cloth or container below the valve to catch any water that drips out.
5. Insert the bleed key and turn anticlockwise (about a quarter turn). You'll hear a hissing sound — that's the air escaping.
6. Wait until water appears — a steady drip or trickle of water means all the air has escaped. Close the valve by turning clockwise. Don't overtighten.
7. Check your boiler pressure. Bleeding reduces system pressure. If the pressure gauge drops below 1 bar, top up the system via the filling loop.

How Often Should You Bleed Radiators?

Once a year — ideally before the heating season starts (October/November) — is usually sufficient. If radiators are consistently cold at the top, or your system is noisy, bleed them more frequently and consider a system inhibitor treatment.

When to Call a Plumber

If you bleed the radiators but cold spots return quickly, or if your boiler pressure keeps dropping, there's likely a more underlying issue — such as a system leak or a failing seal. Give us a call and we can diagnose the problem properly.