Wired Outdoor/Slab Temperature Sensor for T755S. The R250S can be used as an outdoor temperature sensor to set a Dual Fuel Balance Point. This feature chooses the most efficient heat source based on the outside temperature. At this predetermined point the system will automatically choose the best heating source to save energy. The R250S can also be used as a floor sensor in Radiant Heating applications. By sensing the temperature at the heat source this accessory greatly improves the comfort of the space and the efficiency of the heating system.