The Programmable Thermostat 521 is designed for 3 different modes of operation: single stage heating, 2 stages of heating with a fan, or 1 stage of heating and 1 stage of cooling with a fan. 2 auxiliary sensors may be added to measure the floor, outdoor or room temperature. A Slab Sensor 079 is included to measure floor temperature to protect the floor from overheating and enhance comfort. The programmable schedule supports either a 7-day or 24 hour schedule with 2 or 4 events per day. A permanent temperature hold button overrides the programmable schedule. An optional Adaptor Plate 012 is available to allow the thermostat to mount onto a single gang electrical box.