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. For Further Information And up to Date Literature Visit: Tekmar Documentation | Tekmar Controls | 521