A heat emitting unit is a device which transmits heat to the interior of a room or space. The two types of heat emitting units used in steam heating systems are radiators and convectors. Simply defined, a radiator is a heat emitting unit which transmits heat from a direct heating surface principally by means of radiation. A convector, on the other hand, may be defined as a heat emitting unit which transmits heat from a heating surface principally by means of convection. The heating surface of a convector is usually of the extended finned tube construction.