Air in a radiator will rise to the top forming a pocket stopping the hot water from getting to that part.This can be released, but remember to turn the heating off first.
When air is released from an open vented system, the water in the system will be topped up by the feed and expansion tank.
If a radiator in a sealed system needs bleeding the pressure in the system will be reduced. The system will, therefore, needed to be toped up. However, sealed systems are different from open vented ones and have no feed and expansion tank. The instruction manual for your system may have details of how to top up the system. If not, or if you have any doubts at all, contact a suitably qualified heating engineer.
You will need a radiator key and an old rag. Radiator keys are readily available from DIY stores and ironmongers. Armed with the rag beneath, use the radiator key to slacken the air bleed valve which is at one end towards the top of the radiator. There will be a hissing sound as the air comes out. As soon as water begins to flow, close the vent again and wipe away any water. Take care not to get scalded – the water may be pretty hot. The heating can then be switched back on.
Some systems have an automatic air release valve fitted. This usually has a small red top which should be slack to enable the air to escape.
Radiators should not need frequent ‘bleeding’. If they do, air is getting in and this should be sorted out by a professional heating engineer.
