The ESRTM Mechanical Room Thermostat is an economical and simple to use room thermostat. To control your room temperature, simply turn the mechanical control dial until your desired temperature is showing at the temperature mark. When you turn the dial past the current temperature, you will hear an audible “click”. This indicates that the system is being turned ON (if calling for heat when turning the temperature up) or OFF (if not calling for heat when turning the temperature down). The temperature can be set between 10°C to 30°C. The mechanical two wire thermostat is easy and simple to install. It can be either surface or pattress box mounted, with an industry standard back plate, making it an ideal replacement for any 2 or 3 wire thermostat or new build. The ESRTM is double insulated and also has volt free contacts, making it suitable for systems with either a combination or normal boiler. If you have a three wire installation, simply cut back and make safe the neutral, and this two wire thermostat will give you years of faithful service.http://www.esicontrols.co.uk/esrtm.html