The Emerson 1F82-0261 digital thermostat is a 5 1 1 day programmable thermostat that works with heat pump systems (2 Heat / 1 Cool). This thermostat is compatible with Oil/gas/electric and millivolt applications. Note:The display backlight improves display contrast in low lighting conditions. When the “C” terminal is connected, selecting backlight CdL On will keep the light on continuously. Select backlight OFF will turn the light on momentarily when any key is pressed. When the “C” terminal is not connected, regardless of the backlight selection, the light will be on momentarily when any key is pressed.