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

Why Do Radiators Need Bleeding?

Over time, air can become trapped in your central heating system, causing radiators to feel warm at the bottom but cold at the top. This trapped air reduces the efficiency of your heating and costs you more to run. Bleeding your radiators releases this trapped air and allows hot water to fill the whole radiator.

What You'll Need

Step-by-Step Guide

1. Turn On Your Heating

Switch on your central heating and wait for the system to fully warm up — usually about 15–20 minutes. This will help you identify which radiators need bleeding.

2. Check Each Radiator

Once warm, feel each radiator from top to bottom. If a radiator is cold at the top and warm at the bottom, it needs bleeding. Cold all over can indicate a different problem, such as a closed valve or sludge buildup.

3. Turn Off the Heating

Before bleeding, switch off the central heating and allow the system to cool slightly. This makes the process safer and prevents scalding.

4. Locate the Bleed Valve

The bleed valve is at the top of the radiator, usually at one end. It looks like a small square or slot fitting.

5. Open the Bleed Valve

Place your cloth under the valve to catch any drips. Insert the bleed key and turn anticlockwise slowly — you should hear a hissing sound as air escapes. Hold the cloth ready.

6. Close When Water Appears

As soon as water starts to trickle out (it may be discoloured — that's normal), close the valve by turning clockwise. Don't overtighten.

7. Check the Boiler Pressure

After bleeding, check your boiler's pressure gauge. Bleeding can cause the pressure to drop. Refer to your boiler manual to repressurise if needed — typically to around 1–1.5 bar.

When to Call a Professional

If your radiators keep getting air in them, it could be a sign of a larger problem — such as a corroded system or a faulty auto air vent. If in doubt, call Halo Heating on 01483 676108 and we'll investigate.