The air duct heater is mainly used for air heating in the air duct. The specifications are divided into three types: low temperature, medium temperature and high temperature. The common feature in the structure is that the electric heating tube is supported by the steel plate to reduce the vibration of the electric heating tube when the fan is stopped. An over temperature control unit is installed in the junction box. The low temperature type can be directly installed on the air duct, and the medium temperature type and high temperature type have 100 mm thick heat insulating material on the outer wall of the passage to the heater junction box due to the difference in structure, on the one hand, the external heat dissipation of the entire passage is reduced, and on the other hand, The temperature inside the wiring chamber is reduced.