Abstract: A novel control circuit for heating and heating/cooling systems wherein in a heating mode the circuit controls a valve means comprising a main valve and a modulating valve through which gas is supplied to a burner. The main valve is a normally closed, solenoid-actuated type which opens when its coil is energized, and energization of its solenoid is controlled by a pair of normally open relay contacts connected in series. One of the contacts closes when the temperature in the space being heated drops below a preset temperature. The other contact remains closed so long as the temperature in the space being heated remains below a maximum limit. In a heating/cooling system the circuit is selectively operatively coupled with the valve means and a refrigeration unit. When heating is selected, the circuit operates as mentioned above. In cooling, a cooling solenoid of the refrigeration unit is energized when the temperature in the space being cooled rises above a preset temperature.