The Thermostat 518 accurately controls the room temperature for a hydronic heating zone using Pulse Width Modulation (PWM) technology. Simple up and down buttons and a display with large type make this thermostat easy to read and use. The 518 automatically detects a single auxiliary sensor to measure either outdoor, floor or remote room temperature. This easy to install thermostat is a direct replacement for tekmar Thermostats 507 and 508. An optional Adaptor Plate 012 is available which allows the thermostat to be installed onto a single gang electrical box.