Abstract: The room thermostat for the room whose temperature is to be set-back is enclosed in an enclosure having an air inlet at the bottom and an air outlet at the top. Air from the room flows into the enclosure through the inlet and out through the outlet. During the interval when the temperature is to be set-back, the inlet air is preheated by a lamp in a second enclosure in communication with the first enclosure through the inlet. The lamp remains energized throughout the whole set-back interval. To set the magnitude of the set-back the lamp is pivotal between a position where substantially all of its heat is maintained within the second enclosure and a position where a large portion of its heat is dissipated outside of the second enclosure. The room thermostat responds to the increased temperature in the first enclosure to set-back the ambient temperature in the room.