Abstract: An electric heater, and an apparatus, a heating and air conditioning system and a vehicle, each comprising the electric heater, are provided. The electric heater comprises an outer frame; a heating core configured to connect to a power source and disposed within the outer frame; and a sealing-waterproof glue member disposed within the outer frame and configured to encase at least one end of the heating core. The heating core further comprises: a plurality of heat dissipating components and heating components arranged alternately, and each of the heat dissipating component is coupled with a heating component via a thermal conductor. Each of the heating components further comprises a core tube and a positive temperature coefficient thermistor disposed in the core tube.