The B737NG has door area heaters on all 4 cabin (pax/service) doors, as well as all over wing emergency exit hatches. The heaters are electrically powered thru the cab/util switch in the flight deck and the aircon system provides the motive flow of air across the heaters which is directed at the door areas to be warmed. The system is powered when the aircraft is in the air and the cab/utility switch is on.
Framer,
The heaters are for pax/crew comfort only. They have nothing to do with external icing.
Last edited by Gas Bags; 27th Aug 2013 at 03:17.