The EM heat setting on a Honeywell thermostat refers to the emergency backup heating system that can be turned on if the primary heating system for the home fails. The EM HT (Emergency Heat) feature allows the user to use a backup heat source when the heat pump is not working. This setting controls your homes backup heating system, which could be a backup gas furnace, an electric heat strip, oil, or a hot-water 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 back-up system. When the weather gets too cold, your heat pump isn’t able to pull enough warmth from the outdoor air to heat your home to the desired temperature. A secondary heating source, like emergency heat, can be used to keep you warm. The EM heat setting should only be used in an emergency when the primary heating system fails, and it is not meant to be used as a primary heating source.