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 Control Systems Thermostats 507 and 508.