How to bleed a radiator, and when a cold radiator needs more than that
A radiator that is warm at the bottom and cold at the top usually has air trapped in it, and letting it out takes ten minutes and a cheap key. Other cold spots point to different problems.
The first cold evenings of autumn are when many households switch the heating back on and find that one radiator is not pulling its weight. Often the fix is a job anyone can do. The trick is knowing which kind of cold radiator you have.
Work out what the radiator is telling you
Once the heating has been on for a while, carefully feel the radiator at the top and the bottom.
- Cold at the top, warm at the bottom. Air is trapped inside. This is the classic sign a radiator needs bleeding.
- Warm at the top, cold at the bottom or in the middle. This often points to sludge, a build-up of rust and debris settling inside. Bleeding will not fix it.
- Cold all over, while others are hot. The valves may be closed or stuck, or the system may need balancing.
- Several radiators cold upstairs. On some systems this can be a sign of low pressure or a problem with the pump.
What you need.
- A radiator key, which costs a few pounds from a DIY shop or hardware store. Some modern valves take a flat-head screwdriver instead.
- An old cloth or towel.
- A small container to catch drips.
Bleeding a radiator, step by step
- Turn the heating off and let the radiators cool down. Bleeding a hot radiator can release hot water, and turning the pump off stops more air being drawn in.
- Find the bleed valve. It is a small square fitting at one of the top corners of the radiator.
- Hold the cloth under the valve, fit the key and turn it slowly anticlockwise, about a quarter of a turn. You should hear a hiss as air escapes.
- Wait for water. When the hissing stops and a little water starts to dribble out, turn the key clockwise to close the valve. Do not overtighten it.
- Wipe up any drips. The water can be dark and may stain carpets.
- Repeat on any other radiators that need it. If several need bleeding, start with those downstairs.
Check the boiler pressure
This is the step most people forget. Letting air out of a sealed central heating system, which includes most combination boilers, lowers the pressure. Look at the pressure gauge on the front of the boiler or underneath it. Your boiler manual will say what the pressure should be when the system is cold, and many gauges have a green zone marked on them.
If the pressure has dropped below the recommended level, the system will need topping up using the filling loop, a tap or lever usually found under the boiler. The manual explains how to do this for your model. If you are not confident, ask a heating engineer.
Older open-vented systems, which have a water tank in the loft, top themselves up and do not need repressurising.
Turn the heating back on. Switch the heating back on and check the radiators after half an hour or so. A radiator that heats evenly from top to bottom is now working properly. If it needs bleeding again within a few weeks, air is getting into the system somewhere, and it is worth having it looked at.
When to call someone in
Sludge, stuck valves, pressure that keeps dropping and noisy pipes usually need a heating engineer. Any work on the boiler itself, or on gas pipes and appliances, must by law be done by an engineer listed on the Gas Safe Register. You can check an engineer's registration on the Gas Safe Register website before they start.
A radiator that is working properly heats the room faster, which means the heating does not need to run as long to keep the house comfortable.
