Bleed a radiator
If your gas central heating isn't warming your home properly, or your radiators feel colder at the top than at the bottom, it could be because there are air bubbles trapped in the system. This means you need to bleed your radiators.
1. Turn off your central heating and wait for the radiator to cool down.
2. Find a radiator key. You can buy one cheaply from most hardware stores.
3. Look for a small valve at the top of one end of your radiator. Insert the key and turn it counter-clockwise to open the valve. You should hear a hissing sound as air escapes
4. Hold a kitchen towel or cloth under the key to catch any drips of water.
5. When a steady stream of water squirts through the valve, retighten it.