All radiators heat using a combination of radiation and convection heat. Radiation works by heating people directly; as the surface of the radiator heats up, it pushes heat out into the room. Radiated heat is effective no matter what the size of your room – however, radiated heat only makes up about 66% of a radiator’s heat output.
Convection heat, meanwhile, works by filling rooms with warm air. As cool air circulates around the room it is warmed by the hot elements enclosed within the body of the radiator. Because hot air rises, warmer air will rise to the ceiling, allowing the remaining cooler air to circulate through the radiator and warm up in turn. The problem with heating by convection is that warmed air can be lost easily through draughts, extractor fans and opened doors and windows. The poorer your insulation, the faster your room will lose heat, so you will need more powerful radiators to keep the warm air topped up. In larger rooms and rooms with high ceilings you are effectively trying to heat a larger volume of air than in standard sized rooms, so you will need to choose more powerful radiators to keep the entire space filled with warm air.