Abstract: An energy control system for selectively controlling permissible temperature ranges in a plurality of remote zones from a central control location where the zones each have a heat transfer unit for controlling temperature in the zone. The system is intended for use where the zones are interconnected by pairs of conductors, preferably telephone circuits, with the control location. The system includes a switching arrangement for selectively controlling voltages on the conductors, as by reversing the polarity of normal DC voltage across a conductor pair. Voltage responsive circuits at each zone are interconnected with such conductors. Heat transfer control switching circuits in each zone are associated with the voltages responsive circuits for altering normal operation of the heat transfer unit in the zone in response to a detected change in voltage on the conductor by the voltage responsive circuit.