devices may also be employed any number of mosfets 156 can be mounted in any configuration on the printed circuit board 150 a plurality of bores 158 are optionally formed through the printed circuit board 150 the bores 158 improve heat flow between the switching devices on the printed circuit board pcb 150 and the underlying first plate 73 a temperature sensor 159 such as a ptd is mounted on the printed circuit board 150 typically over or adjacent to the bores 158 the temperature sensor 159 measures the temperature of the printed circuit board 150 and provides a temperature proportional signal to the controller 28 which is used by the controller 28 to control the on off cycle of the heating elements 104 106 and 107 to further enhance transfer of the heat generated by the mosfets 156 to the first plate 140 a highly conductive pad or plate 160 hereafter referred to as a sill pad 160 is interposed in contact between the printed circuit board 150 and the first plate 73 as shown in fig3 8 and 9 the sill pad 160 typically has a planar shape and dimensions to extend over at least a portion of the first plate 73 the pad 160 isolates stray electrical currents to negative ground through the screws 75 provides a positive contact between the mosfets and the thermal mass 40 and stabilizes heat loss through the adjacent cover by maintaining the temperature of the plate 73 at a higher temperature to thereby create a lower temperature differential or gradient with respect to the thermal mass 40.