Emergency heat, or EM heat, is a backup heating source used only when your heat pump is non-operational or is unable to achieve your set temperature. It is activated during extremely cold weather or when your heat pump malfunctions and cannot provide enough heat.
In most homes, the emergency heat source is either a gas furnace or electric resistance heating strips. Because emergency heat completely bypasses the heat pump and uses far more electricity, it should be used for short periods or during system failure.
Modern cold-climate heat pumps can operate efficiently in much lower temperatures, some even down to –22°F. However, for standard heat pumps, emergency heat provides essential support during harsh weather conditions.
How Does Emergency Heat Work?
To activate the emergency heat, look for settings like ‘E’ or ‘EM heat’ on your thermostat. In some thermostats, you may only find the ‘Aux heat’ option, which will also activate the secondary heating system. When emergency heat is engaged, your heat pump compressor turns off, and the backup heating system takes over.
EM heat can be turned on both manually and automatically. This depends on your system setup and your thermostat settings.
You can set the EM heat to turn on automatically when outdoor temperatures fall below a set point or if the heat pump can’t maintain the desired indoor temperature. This is especially useful when the temperature drops significantly overnight while you are sleeping. However, some homeowners may prefer manual activation because emergency heat is more expensive to run. Manual activation is also helpful when there is an issue with your heat pump. It’s best to use manual emergency heat only when necessary to ensure energy efficiency.
What Factors Cause the Emergency Heat To Activate?
Emergency heat mode should only operate when the heat pump is not working or temperatures are too low for it to operate effectively.
Defrost Mode
In extremely cold conditions, moisture in the air can freeze on the outdoor unit, forming frost on the coil. When this happens, the heat pump enters defrost mode to melt the ice. During this period, it cannot provide heat to your home, so the emergency heat automatically activates to maintain comfort.
Old HVAC System
Older or less efficient heat pumps may rely on secondary heating because they can’t extract enough energy from the outdoor air to adequately warm your home. Heat pumps typically last 10 to 15 years, though with proper maintenance, some can reach 25 years. If your unit is nearing the end of its lifespan, it may be time to consider a replacement.
Heat Pump Malfunctions
If your heat pump stops working due to a certain issue, the thermostat will switch to a backup heating system to maintain your preferred temperature. This ensures your home remains comfortable until the system is repaired.
