If the top of a radiator is cool or much colder than the bottom, it needs bleeding to let out trapped air. Switch the central heating off and let the radiators cool down before you start.
The bleed valve sits at the top of the radiator and opens with a radiator key. Hold a cloth under it to catch drips. Turn the key slowly anticlockwise and you should hear air hiss out. Once the hissing stops and water appears, turn it clockwise to close the valve. Bleeding several radiators can drop the heating pressure, which is normal, so check the gauge and raise it as the maker's instructions say if it reads low.
If one radiator stays cold, take the cover off its thermostatic valve and check the pin still moves up and down. They can stick on a radiator that has sat unused. If some radiators get warmer than others on the same setting, the system may need balancing. A magnetic filter catches sludge, but a heavily blocked system needs a power flush first, because a filter alone will not clean it.
Balancing means adjusting the lockshield valves, which can cause leaks, so it is usually a job for an engineer. If bleeding has not warmed a radiator, or some radiators run cooler than others, ring me before turning valves yourself. I look after central heating across Hastings at Optimum HVAC, and balancing it properly now saves you mopping up a leak later.
