The Programmable Thermostat 521 is designed for three different modes of operation: single stage heating, two stages of heating with a fan, or one stage of heating and one stage of cooling with a fan. Two 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.

• Three different modes of operation: single stage heating, two stages of heating with a fan, or one stage of heating and one stage of cooling with a fan
• Two auxiliary sensors may be added
• Slab Sensor 079 is included to measure floor temperature
• Features: 7 day programmable schedule, backlight, cooling, fan, 4 hour battery-free clock backup, and optimum start
• Features: Outdoor and floor temperature display, permanent temperature hold, radiant floor heating, time clock, and 2-stage heating