Abstract: Thermostat control of separate heating and cooling apparatus is achieved with a system that requires only a two-conductor connection between the thermostat and the apparatus. A pair of oppositely-poled diodes is employed at each end of one of the conductors of the connection so that when the control system is established, by a selector switch, in the heating mode the half cycles of one polarity of an AC voltage effect the operation of the heating apparatus, whereas when the system is switched to the cooling mode the opposite polarity half cycles cause operation of the cooling apparatus. The invention is especially advantageous when a central air-conditioning system is added to a building where only a two-wire cable connects the thermostat to the existing heating system. Rewiring of the thermostat cable is unnecessary.