Abstract: An autocycling control circuit unit is provided for controlling heating and/or airconditioning systems to provide controlled comfort and fuel savings. The unit contains a night thermostat which is continually coupled to the system for maintaining a minimum temperature level. A programmable timer enables and disables a day thermostat during settable, predetermined time intervals. The timer includes a feedback circuit for varying the on-off cycling of the day thermostat, after initial warm-up, in accordance with the rate at which the day thermostat calls for heat. The unit contains a backup battery and charger to insure the proper timing sequence in the event of system power outages.