In the months after a system has been filled and periodically during the life of a system air bubbles can form in the water in the radiators. This will collect in the highest part of the radiators. Large amounts of air collecting in one radiator will cause the top to remain cold even when the rest of it is hot.
The aim of this task is to open the bleed valve at the top of the radiator that allows the air to escape. It is best to do this when the system is hot and at higher pressure. This may result in the system pressure becoming too low and needing to be topped up.
WARNING: Do not open the valve too far as there is a risk the centre pin will come out. An excessive amount of water would be lost from your system causing water damage to the building around the radiator.