A heating device includes a case having a plurality of openings and a plurality of slits are defined in bars of the case so that a plurality of heating boards are engaged with the slits of the case at an equal distance. A power control device is connected to the case and includes a power line having a plurality of male connectors. Each heating board includes a heating layer sandwiched between two isolation plates. Two wires are connected to the heating layer and each wire has female connectors, which are connected with the male connectors of the power line. The heating layers of the heating boards dispense heat evenly from the openings of the case.