When the top of your central heating radiator goes cold, it usually means trapped air is in there – after it got stuck in the central heating system.
Because air rises, it goes to the top of your central heating radiator and prevents hot water from getting there.
The problem can be solved by releasing air in the radiator (remember to switch the central heating system off first).
Use your radiator key to loosen valve, which should be on one end of your central heating radiator.
You should hear the air coming out in the form of hissing. When water starts to flow out close the vent again. It is best to bring a cloth while doing this so water doesn’t get everywhere.