How to bleed a radiator
If there’s air trapped inside of your radiators, you may need to bleed it. Trapped air will cause your radiator to have cold spots, reducing the efficiency of it. You can bleed your radiators yourself, and it can vastly improve the efficiency of your entire heating system.
Start by turning your central heating system off. If you leave the central heating pump running this may cause more air to be drawn in to the system and you may find hot water spraying out of the radiator when it comes to bleeding it.
Gather your tools: all you will need is a dry cloth like a tea towel, and a bowl or tray to catch water. The only specialist bit of kit you’ll need is a radiator key, which you can get cheaply at most DIY shops.
Find the square bleed screw at the top corner of your radiator, and place the bowl or tray directly underneath it at the base of the radiator.
Next, place the radiator key over the bleed screw (it should fit snugly), cover it with another cloth and slowly turn the key anti-clockwise for about half a turn (do not remove the bleed screw). As the air releases, you'll hear a hissing sound. At this point, hold the cloth close to prevent water dripping or spitting.
Once the hissing stops and water begins to trickle out steadily, tighten the screw again, being careful not to over-tighten and damage the valve.
Switch the heating system on again, and if necessary, check the boiler pressure. If it's below the optimal level as stated in your manufacturer’s instructions, you may need to top it up by using the filling loop on your boiler. This is usually a tap or lever on the main water supply to your boiler.
Finally, check that all the radiators are heating evenly and that none of the bleed valves are leaking. You may need to bleed some radiators a second time, but if the problem still isn't resolved, contact a heating engineer.