I have had 2 new radiators installed and niether one of them heat up as much as the others. I have noticed that one of the pipes that feed the radiator is very hot whereas the other pipe is not so hot and neither is the radiator itself. After a long time the bottom of the radiator is not as hot as the top. I have made sure there is no air in the radiator and both of the valves are fully open. I am not a plumber so don't really understand what the problem could be - they are both brand new radiators and the boiler itself was replaced a year ago.