In our experience, most homeowners don’t think much about the Emergency Heat setting on their thermostat until temperatures drop sharply and the house doesn’t seem to warm as quickly as usual.
You may hear that Emergency Heat should be turned on whenever temperatures fall below freezing, but cold weather alone is not a reason to manually switch your thermostat to Emergency Heat.
When your heat pump is operating normally, it can continue heating your home in cold weather and use auxiliary heat automatically when it needs additional help. Emergency Heat is intended for situations where the heat pump itself has failed, has been damaged, or otherwise should not continue operating while you wait for repairs.
In this guide, we’ll explain when to use Emergency Heat on a heat pump, when not to, and how it differs from auxiliary heat.
When Should You Use Emergency Heat on a Heat Pump?
You should use Emergency Heat when the heat pump can’t operate normally or should be kept off, but you still need heat in the home.
In most systems, switching to Emergency Heat stops the thermostat from relying on the outdoor heat pump and uses the backup heating source instead.
That makes it a temporary option while you arrange for repairs.
Your Heat Pump Has Stopped Working
If the outdoor unit is not running, the compressor has failed, or the system is no longer producing useful heat, Emergency Heat can keep the home warm while you wait for service.
The Outdoor Unit Has Been Damaged
If a falling branch, storm debris, ice, or another impact has damaged the outdoor unit, you may not want the heat pump to continue operating until a technician has inspected it.
Emergency Heat allows the home to rely on the backup heating system without continuing to run the damaged outdoor unit.
The Heat Pump Is Heavily Iced Over and Cannot Defrost
A light layer of frost on the outdoor coil can be normal in cold weather, and heat pumps are designed to periodically run a defrost cycle to clear it.
If the system cannot clear the ice on its own, Emergency Heat may be used temporarily while the heat pump remains off and a technician diagnoses the cause of the icing.
What Temperature Should You Turn On Emergency Heat?
You may see recommendations to turn it on below 30°F, 35°F, or whenever temperatures fall below freezing, but those are not universal rules.
A properly operating heat pump can continue heating your home in cold weather, and many systems are designed to bring on auxiliary heat automatically when the heat pump needs additional help.
The temperature at which a heat pump begins relying more heavily on backup heat varies depending on the equipment, the size and insulation of the home, and how much heat the house is losing as outdoor temperatures fall.
Winters in the Upstate are usually mild enough that a properly sized heat pump can handle most cold days on its own. If your home in Greenville or the surrounding area seems to need Emergency Heat on an ordinary cold night, that usually points to a problem with the system rather than the weather.
Even if your thermostat displays AUX Heat during very cold weather, that does not mean you need to switch to Emergency Heat.
As long as the heat pump is operating normally, you should generally leave the thermostat in its regular Heat setting and allow the system to manage the backup heat automatically.
Emergency Heat vs. Auxiliary Heat
Emergency Heat and auxiliary heat both use your system’s backup heat source, but they are not the same thing.
Auxiliary heat turns on automatically when the heat pump needs extra help heating the home. You may see AUX Heat on the thermostat during very cold weather, when the system is recovering after you raise the temperature several degrees, or while the heat pump is going through a defrost cycle.
Emergency Heat is selected manually. It tells the system to rely on the backup heating source instead of normal heat pump operation.
If AUX Heat appears on your thermostat, you generally do not need to do anything.
As long as the heat pump is operating normally and the home is maintaining temperature, the system is using the backup heat as designed.
When You Should Not Use Emergency Heat
Emergency Heat is not meant to be a stronger or faster heating mode.
If the heat pump is operating normally, switching to Emergency Heat can take the primary heat pump out of the heating cycle and rely entirely on the backup system instead.
You generally should not switch to Emergency Heat:
- Just because the temperature drops below freezing. Heat pumps are designed to operate in cold weather, and many can continue providing heat well below 32°F.
- Because the heat pump is running for long periods. Longer run times are common as outdoor temperatures fall, especially if the system is still maintaining the thermostat setting.
- Because AUX Heat appears on the thermostat. Auxiliary heat is designed to come on automatically when the heat pump needs additional help.
- During a normal defrost cycle. Frost can develop on the outdoor coil in cold, damp weather, and the system should periodically defrost itself without you switching to Emergency Heat. You may notice steam rising from the outdoor unit during a defrost cycle, which is normal and is not smoke.
If the system is running continuously and the indoor temperature continues to fall, however, that can point to a problem with the heat pump, backup heat, airflow, or another part of the system.
Does Emergency Heat Cost More to Run?
In many heat pump systems, Emergency Heat costs more to run because it relies on the backup heating source instead of getting most of its heat from the heat pump.
If your system uses electric resistance heat as the backup, operating on Emergency Heat can use significantly more electricity than normal heat pump operation.
Most homes in the Upstate are served by Duke Energy, and electric heat strips can draw several times the electricity of a heat pump for the same amount of heat. Duke Energy’s standard residential rate in South Carolina is around 13 cents per kilowatt-hour as of this year, and customers on a time-of-use plan pay closer to 20 cents during the winter morning peak. That morning window is also when backup heat tends to run the longest, since the system is recovering from an overnight setback while outdoor temperatures are at their lowest.
Systems that use a gas, propane, or oil furnace as backup have different operating costs, so no single cost difference applies to every home. Some homes in the area with Piedmont Natural Gas service use a gas furnace as the backup, and the cost difference in those homes depends on gas and electric rates at the time.
This is another reason Emergency Heat is best used as a temporary setting when the heat pump cannot operate normally, rather than as an everyday heating mode during cold weather.
How Long Can You Run Emergency Heat?
You can run Emergency Heat while you are waiting for the heat pump to be repaired, as long as the backup heating system is operating normally.
There is no set number of hours that applies to every system, but Emergency Heat is not intended to replace normal heat pump operation for an extended period.
If the backup heat is electric, leaving it on can also increase your energy use considerably.
Once a technician has repaired the heat pump and it is safe to operate again, switch the thermostat back to its normal Heat setting so the system can return to using the heat pump as the primary heat source. Some homeowners forget to switch the thermostat back after a repair and do not notice until the next electric bill arrives.
If You Need Emergency Heat, Your Heat Pump May Need Service
If you switched to Emergency Heat because the outdoor unit isn’t running, the system is heavily iced over, or your home keeps getting colder despite the heat running, Emergency Heat is only a temporary way to keep the house warm.
A technician still needs to diagnose and repair the underlying problem before the heat pump can return to normal operation.
If your auxiliary heat is running most of the day in mild weather, it is another sign worth attention, since it can point to low refrigerant, a clogged filter, a stuck reversing valve, or a defrost control that is no longer working.
If your heat pump is not heating properly, Air Today can inspect the system, identify what is preventing it from operating normally, and make the necessary repairs. We provide heat pump repair throughout Greenville and the Upstate.
Call Air Today Heating and Cooling at (864) 295-0905 or contact us online to schedule service.