The T021-M18 thermostat has been specifically designed for stable temperature control of discharge or mixed air dampers used in all types of packaged and custom air handlers. It is a perfect replacement for older type 135 ohm electromechanical thermostats since it provides improved accuracy at a more competitive price. The M18 can also be used as an economizer thermostat with the addition of one relay and a changeover thermostat. Order 022-0003 temperature sensor separately.