Short answer
Use emergency heat only when your heat pump is not working or is damaged, such as when the outdoor unit has failed, is badly iced over, or has been hit by debris, and you need temporary warmth until a technician can look at it. It is not meant for ordinary cold weather.
What emergency heat does
In emergency heat mode (often labeled EM HEAT), the thermostat tells the system to skip the outdoor heat pump and heat using the backup source only. In many homes this is electric resistance heating strips in the air handler, which work like a large hair dryer. In some setups the backup is a gas furnace. Check your system type if you are not sure.
Auxiliary heat is different. It is the same backup source, but it turns on automatically, together with the heat pump, when the heat pump alone cannot keep up. You do not have to select it. Emergency heat is a manual choice that turns the heat pump off.
When it makes sense to use it
- The outdoor unit has stopped running and the house is getting cold.
- The outdoor unit is visibly damaged by a fallen branch, ice storm or debris.
- The outdoor coil is encased in thick ice and does not clear through normal defrost.
In each case, emergency heat is a stopgap. Call a heating and cooling technician soon and, while you wait, do not keep operating a visibly damaged unit.
When not to use it
- Because it is cold outside. Heat pumps are designed to run in cold weather, and auxiliary heat will come on by itself if needed.
- During a normal defrost cycle. The outdoor unit may steam or look frosty briefly, and backup heat may run for a short time. That is normal; see is it normal for a heat pump to steam.
- Because the air from the vents feels cooler than a furnace’s. Heat pump air is often warmer than room temperature but not as hot as furnace air.
Some systems in milder climates do not have an emergency heat setting at all.
What to do before switching
- Check the thermostat is set to heat and the setpoint is above room temperature.
- Check the breaker and the filter.
- Look at the outdoor unit from a safe distance for ice, damage or blockage. Do not stand on or lean over a damaged unit.
- If it is safe, clear loose debris from around it.
If the unit is wrapped in ice, do not chip at it with tools, as the coil and refrigerant lines are easily damaged. Turning to emergency heat and calling a professional is safer.
Drawbacks of emergency heat
The main drawback is cost. Electric resistance heat generally uses considerably more electricity than a heat pump for the same amount of heat, so bills can rise noticeably if it runs for long. It also puts all the load on the backup heaters, which are not designed to be your main source of heat for long periods. Finally, it can hide a problem: the house stays warm while the heat pump fault goes unfixed.
I accidentally had emergency heat on. What should I do?
Do not panic. Switch the thermostat back to the normal heat mode. A running EM HEAT indicator, often a light or message, should go off. Then check that the outdoor unit starts up and the house heats normally. A brief period on emergency heat is typically just a higher bill for that time. If the heat pump does not resume or the indicator stays on, call a technician. You may also want to read about heat pump recovery mode, since a recovery message can look similar at first glance.
Cold weather and the lowest temperature to run a heat pump
There is no single cutoff that applies to every model. Efficiency falls as the outdoor temperature drops, and many modern cold-climate heat pumps are rated to operate at quite low temperatures, while older units rely more on backup heat. Check the manufacturer’s specifications for your model. If you see long run times in the cold, see our guide on whether a heat pump should run all the time.
Is 74 a good winter setting?
It depends on your preferences and your budget. Many guidance sources suggest lower settings to save energy, and some heat pump owners prefer holding a steady temperature. A higher setpoint means more run time, and a large jump from a lower setting can trigger backup heat. Choose a comfortable temperature and keep changes small.
When to call a professional
- The outdoor unit is not running, is damaged, or stays iced over.
- You smell burning, see smoke or sparks, or breakers trip. Switch the system off and call a technician; do not keep resetting breakers.
- You are using emergency heat for more than a short time.
- Electric heat strips or wiring need inspection. Do not open panels yourself, as line-voltage components can cause serious injury.