Apologies if I am teaching my grannie.....If a radiator is cold at the top it is probably air, hence bleed the rad,by slightly undoing the bleed valve until you can hear the air escaping. Be ready to redo the valve as soon as hot water replaces the air. A well designed sustem will have one rad slightly higher than the rest so all the air collects in it, so you only have to bleed the one rad. If a radiator is cold at the bottom, it may have a build of of sludge and the system needs cleaning/power washing. Do not use British gas for this, they have outrageous charges...... As suggested, if you have a 'sealed' system you will need to top it up with water. If it is sealed, you will have a small pressure dial and a valve turning the top up water supply on and off to the system. I do hope you warm up soon!
Iain Muir ● 4984d