EM heat on a thermostat stands for "emergency heat" and is a setting that controls your homes backup heating system. If you have this setting, you likely have a heat pump and a secondary heating source, such as gas, oil, electric, or hot-water. The emergency heat setting is used when the temperature drops too low for the heat pump to be able to remove heat from outside, typically below 35 degrees Fahrenheit. At this point, the secondary heating source is activated to provide heat to the home. When you manually set the emergency heat on your thermostat, your system bypasses the heat pump and directly accesses either the electric heat strip or the gas or oil furnace. However, its important to note that using emergency heat can lead to high energy bills and put a great strain on the backup element that is only meant to be used in extreme circumstances for short periods of time. Therefore, its recommended to only use emergency heat when necessary and to understand how to properly use it before switching it on.