Air duct heater is mainly used to heat the air within the air duct. The specification is divided into three types, i.e. low, medium, high temperature. The structure in common is to use steel plate to support electric heating pipe to decrease the pipe vibration when the blower is stopped. The over-temperature control unit is installed in every junction box. The low temperature type can be directly installed on the air duct. However, as to medium & high temperature type, insulation material of thickness 100mm is inserted between passage outer wall and junction box to decrease external heat dissipation and reduce wiring chamber temperature.