How to bleed a radiator (a 5-minute job you can do yourself)

A radiator that is warm at the bottom but cold at the top is one of the most common heating niggles, and one of the few you can safely fix yourself in about five minutes. Here is how.

First, check it is actually air

Feel the radiator with the heating on. Cold at the top, warm at the bottom means trapped air, which bleeding fixes. Cold at the bottom usually means sludge, which needs a power flush, not bleeding. Our guide on why boilers get noisy covers sludge in more detail.

What you need

• A radiator bleed key (cheap from any DIY shop) or a flat-head screwdriver, depending on your valve
• A cloth and a small bowl to catch drips

Step by step

1. Turn the heating off and let the radiators cool.
2. Find the bleed valve, a small square nut at the top corner of the radiator.
3. Hold the cloth under it and turn the key slowly anticlockwise, a quarter to half a turn. You will hear a hiss as air escapes.
4. When water starts to trickle out steadily, close the valve.
5. Repeat on any other cold radiators, then check your boiler pressure.

Check the pressure afterwards

Bleeding releases pressure, so your boiler gauge may drop below 1.0 bar. If it does, top it back up, our guide on boiler pressure walks through how. The Energy Saving Trust has more on getting your heating running efficiently.

When to call an engineer

If bleeding does not help, the radiator is cold at the bottom, or air keeps coming back, it is worth a professional look. Only a Gas Safe registered engineer should touch the boiler itself. If your system is tired and it is one of several issues, our signs you need a new boiler guide will help you weigh up repair versus replace, or just get a fixed price.