Abstract: A compact, hermetically sealed, heat dissipating electric power unit comprises a P.C. board having a hole therethrough. A semiconductor electric power control device fits loosely in the hole. A backer of heat and electric current conductive material is bonded in heat and electric current conductive contact (1) with the P.C. board around the perimeter of the hole to hermetically seal the corresponding end of the hole and (2) with an opposed first electrode of the device, the backer acting as an electric current path and at least part of a heat sink for the device, the backer and P.C. board having similar thermal expansion coefficients so as to maintain the hermetic seal. An electrically conductive ground ring is electrically conductively fixed on the other face of the P.C. board and is electrically connected to the second electrode of the device and acts as a further electric current path. A cover of ceramic material has a thermal expansion coefficient similar to that of the core.

Abstract: A closed loop, electronic temperature control system for a tankless water heater. The control system incorporates error range comparison circuitry for comparing the difference between a water temperature selected from a temperature selection device and an actual water temperature sensed by a temperature sensing device. An error range comparison circuit operates to select a high gain when the sensed water temperature is within about +/-3.degree. Fahrenheit of the selected temperature, and a low gain when the sensed water temperature is more than +/-3.degree. Fahrenheit of the selected water temperature. A slow start network is included for applying current gradually to the heating element of the water heater when power is initially applied to the system. A master power down circuit quickly interrupts current flow to the heating element after the system has been powered down.