guide

How to bleed a radiator

If a radiator is warm at the bottom but cold at the top, trapped air is a common cause. Bleeding it is a simple job most people can do safely with a radiator key and a cloth. This guide explains how, and when it is better to call a professional.

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Signs a radiator needs bleeding

Cold at the top but warm at the bottom.
Takes much longer to heat up than other radiators.
Gurgling or rattling sounds when the heating runs.

Before you start

Turn the heating off and let the radiators cool before you begin. The water inside a hot radiator can scald. Never bleed a radiator while the system is hot.

What you will need

A radiator bleed key (or a flat-blade screwdriver for some modern valves).
A cloth or small container to catch drips.
An old towel to protect the floor.

How to bleed a radiator safely

With the heating off and the radiator cool, find the square bleed valve at the top corner.
Hold the cloth under the valve and turn the key slowly anti-clockwise, no more than a quarter to half a turn.
You should hear air hissing out. Do not open the valve fully.
When a steady trickle of water appears instead of air, close the valve firmly but gently. Do not overtighten.
Wipe away any drips and repeat on other affected radiators.

After bleeding: check your boiler pressure

Releasing air can lower the pressure in a sealed system. Once you have finished, check the pressure gauge on the boiler when it is cool. If it has dropped below the level marked in your boiler manual, the system may need topping up. If you are unsure how to do this, or the pressure keeps falling, ask a suitable heating professional.

When to call a professional

Radiators repeatedly fill with air after bleeding.
A radiator stays cold after bleeding and topping up.
The boiler pressure will not hold or keeps dropping.
You notice a leak from the valve, radiator or pipework.
The bleed valve is seized — do not force it.

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Questions

Frequently asked questions

How often should I bleed my radiators?

Many households bleed radiators once a year, often before winter. Bleed them sooner if you notice cold spots at the top or gurgling sounds.

Should the heating be on or off when bleeding a radiator?

Off, and cool. Bleeding a hot radiator risks scalding from hot water, and the system should not be running while you open the valve.

Why is my radiator still cold after bleeding it?

If a radiator stays cold at the bottom, the cause may be a build-up of sludge or a valve problem rather than air, which usually needs a professional to inspect.