The word "EMER" on a thermostat indicates that the thermostat is in Emergency Heat mode and that the Emergency Heat is currently running. Emergency Heat is a secondary heating system that acts as a backup to keep your house warm when your primary heat fails. If you have an emergency heat thermostat setting, you likely have a heat pump, as well as a gas, oil, electric, or hot-water back-up system. The heat pump collects frost on the coils, and when it turns off temporarily to defrost the unit, the secondary source kicks in automatically. Therefore, it is not necessary to switch to the EM heat setting yourself. However, manually activating your EM heat sets up a different chain of events. The system will bypass the main heat pump and be forced into auxiliary mode, relying solely on the backup heat source. It is not recommended to switch your thermostat to emergency heat just because its cold outside, as it will raise your energy bill and decrease efficiency.