Hot water cast-iron radiators work by heating the surrounding air through radiation and convection. Cast-iron radiators receive heat from the hot water in the boiler and store that heat in the cast iron. The cast iron radiates heat into the room and warms the air around it.
Cast-iron radiators are supplied with hot water from a central boiler that generally runs on gas or electricity. When the water in the boiler reaches a certain temperature, it is distributed through pipes to the radiators. Newer cast-iron radiators use circulating pumps to move the water to and from the boiler. Old-fashioned cast-iron radiators rely on gravity, with hot water rising due to convection and cold water falling back into the boiler. As the air surrounding a cast-iron radiator warms and rises, cool air is drawn into and underneath the radiator, which creates the vertical air currents needed to distribute heat through a room.
Hot water cast-iron radiators are designed with loops and coils to maximize the amount of surface area available to radiate heat. These radiators also need to be placed so that air can flow freely around them. If a decorative cover or cabinet is used on the radiator, it needs to have lots of venting on the top and bottom.
Convection accounts for the majority of heat delivered by cast-iron radiators, and it is important to keep furniture and heavy curtains away from the radiator to keep the air flow unrestricted.