Radiator Bleeding: A Simple DIY Guide

If some of your radiators are warm while others remain stubbornly cold, trapped air is likely the culprit. The good news is that bleeding radiators is one of the easiest DIY maintenance tasks you can do, requiring only a radiator key and a few minutes of your time. Bleeding removes trapped air pockets, allowing hot water to circulate properly and restoring even heat throughout your home.

Why Air Gets Trapped

Air naturally enters heating systems during maintenance work, after repairs, or simply through small leaks. Unlike water, air doesn't conduct heat efficiently, so even small pockets reduce radiator efficiency. You'll notice this as a cold section at the top of a radiator while the bottom remains warm.

What You'll Need

• A radiator key (costs £2-5 from any DIY shop)
• A small container or cloth to catch water
• Your boiler set to normal operating temperature

The Bleeding Process

First, switch off your boiler and let the system cool slightly—you don't want to burn yourself on hot water. Locate the small square or hexagonal valve at the top corner of each radiator. This is the bleed valve.

Place your cloth or container under the valve to catch drips. Insert the radiator key and turn it anticlockwise about a quarter turn. You'll hear a hissing sound as air escapes. Keep turning until water starts to trickle out steadily with no air bubbles. This usually takes 10-30 seconds per radiator.

Once water flows smoothly, turn the key clockwise to close the valve tightly. Don't over-tighten, as this can damage the valve. Repeat this process for every radiator in your home, starting with the ones furthest from your boiler and working towards it.

After Bleeding

Once you've finished, check your boiler's pressure gauge. The bleeding process may have reduced system pressure slightly. If pressure has dropped below 1 bar, you'll need to re-pressurise using your filling loop.

Turn your boiler back on and let the system heat up. You should notice improved heat distribution throughout your home. If a radiator remains cold after bleeding, the problem may be a stuck valve or a more serious blockage requiring professional attention.

Prevention

Bleed radiators annually, ideally in autumn before the heating season. This keeps your system running efficiently and prevents air accumulation. If you're frequently bleeding the same radiator, you have a leak that needs professional investigation.