It is a temperature-sensitive device that controls and regulates the temperature in a space for systems such as a furnace, air conditioner, or both. When the indoor temperature drops below or rises above the thermostat setting, the device initiates or terminates either the heat or cooling modes, depending upon the thermostat setting and temperature set point. When initiated your furnace or air conditioner runs to warm or cool the house air to the setting you selected for your family’s comfort.