EM heat on a thermostat stands for "emergency heat" and is a setting that controls your homes backup heating system. If you have an emergency heat thermostat setting, you likely have a heat pump, as well as a gas, oil, electric, or hot-water backup system. The EM heat setting is activated when the primary heating system for the home fails, and it should only be used in emergencies. When temperatures drop too low, the heat pump isnt able to pull enough warmth from the outdoor air to heat your home to the desired temperature, and a secondary heating source, like emergency heat, can be used to keep you warm. However, when the EM heat mode is activated, the heat pump is essentially turned off, and only the secondary heating system can provide heat to the home, resulting in higher bills and less efficient heating because you are now relying solely on the secondary system. It is important to note that manually activating your EM heat sets up a different chain of events, and the system will bypass the main heat pump and be forced into auxiliary mode, relying solely on the backup heat source to heat your home.