Abstract: A thermostat based on indoor effective-temperature with cooling time a function of the difference between the inside temperature and temperature-set while off time is the remainder of a time cycle. This permits indoor temperature to vary with heat-load to provide comfort and conserve power under all heat-load conditions. During high heat-load conditions it can provide comfort and save a significant amount of power. It provides comfort down to temperature-set, when the heat load decreases in the evenings and at night. For low heat-load conditions, it eliminates cooling under-shoot and controlled off time reduces or eliminates stuffiness between cooling cycles.