Venstar ACC0400 Outdoor Sensor for Platinum Slimline Thermostats Allows you to see the outdoor temp from the convenience of your thermostat display Compatible with T1700, T1800, T1900, T2700, T2800, and T2900 thermostats Compatible model thermostats will track the high and low temps Clean white finish Dimensions: 2.75W x .5D x 4.5H in.