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

When should you bleed your radiators?

If a radiator is hot at the bottom but cold at the top, it has trapped air inside. You might also hear gurgling or trickling noises from the radiator. Bleeding lets the air out so hot water can fill the radiator completely.

What you need

A radiator key (available from any hardware shop for about £1) and a cloth or small container to catch any drips. That is it.

Step-by-step

1. Turn the heating on and wait for the radiators to warm up so you can identify which ones have cold spots.
2. Turn the heating off and let the radiators cool for 10 minutes - this stops hot water spraying out.
3. Find the bleed valve - it is the small square fitting at the top corner of the radiator.
4. Place your cloth underneath it.
5. Insert the radiator key and turn anticlockwise about a quarter turn.
6. You will hear air hissing out.
7. As soon as water starts to come out steadily, close the valve by turning clockwise.

Check the boiler pressure

Bleeding radiators releases water from the system, which drops the pressure. After bleeding, check your boiler pressure gauge. If it is below 1 bar, repressurise the system using the filling loop underneath the boiler. Top it up to about 1.2 bar.

What if bleeding does not fix it?

If the radiator is still cold after bleeding, or if it is cold at the bottom rather than the top, the problem is sludge rather than air. A power flush is the most effective solution for this. Give us a call and we will advise you.