What Is Emergency or Auxiliary Heat?

By Laure Justice

October 26, 2025

When temperatures drop, you might notice "AUX" or "EM HEAT" flashing on your thermostat. But what do these terms mean? Understanding what emergency heat and auxiliary heat are can help you stay comfortable while keeping your energy bills in check.

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What Is Emergency Heat?

Emergency heat is a backup heating system that activates when your primary heat pump can't adequately warm your home. This secondary heating method typically uses electric resistance heating strips installed in your heating, ventilation and air conditioning (HVAC) system. If your outdoor unit malfunctions due to mechanical failure or ice buildup, flipping to emergency heat helps keep your home warm.

What Is Auxiliary Heat?

Auxiliary heat, often called aux heat, is essentially the same as emergency heat, but it activates automatically rather than manually. When outdoor temperatures drop and your heat pump can't warm the air, your thermostat triggers auxiliary heating to warm things up.

The key difference between emergency heat vs. auxiliary heat is how they're activated: Aux heat works with your heat pump, while emergency heat replaces the heat pump as your heating method.

What Is the Purpose of Emergency or Auxiliary Heat?

Heat pumps work efficiently in moderate temperatures, but their performance declines when the mercury drops. Most heat pumps struggle when outdoor temperatures fall below freezing. That's where backup heat becomes essential.

Auxiliary heat maintains comfortable indoor temperatures when your heat pump needs a boost. It prevents your system from running continuously without reaching your desired temperature, which wastes energy and strains the equipment.

Emergency heat provides a safety net when your heat pump fails. Rather than leaving your home without heat in freezing conditions, you can manually activate emergency heat.

How Does 'EM Heat' Work?

Understanding how the systems operate helps you use these heating methods effectively. Here's how it works:

Heat Pump Operation

Your heat pump normally transfers heat from outdoor air into your home, even when it's cold outside. The system circulates refrigerant that absorbs warmth from the exterior air and releases it indoors. This method proves highly energy-efficient compared to generating heat through combustion or electrical resistance.

Automatic Aux Heat Activation

When outdoor temperatures drop significantly or your thermostat detects the indoor temperature falling too far below your set point, the system automatically engages auxiliary heat. The electric heating strips turn on, while the heat pump continues running. Together, they provide enough heating power to reach your desired temperature quickly.

Manual Emergency Heat Activation

If your heat pump stops functioning, you can manually switch your thermostat to emergency heat mode. This shuts down the heat pump and runs only the electric resistance heating. You should only use this setting when your heat pump isn't working properly, as it's less efficient than normal operation.

Thermostat Settings and Control

Most modern thermostats clearly display when aux or emergency heat is running. Pay attention to these indicators, especially if auxiliary heat runs frequently or for extended periods. Constant aux heat usage might indicate a heat pump issue that needs professional attention.

Energy Efficiency Considerations

Electric resistance heating used in auxiliary and emergency heat systems consumes significantly more electricity than heat pump operation. While your heat pump might achieve 300% efficiency by moving existing heat, auxiliary electric heating strips convert electricity to heat at roughly 100% efficiency.

This difference impacts your utility bills substantially. Running auxiliary heat occasionally during extreme cold is common. However, if your system relies on aux heat regularly during moderate weather, you're spending far more on home heating than necessary.

How Much Do They Cost to Install?

According to HomeGuide, it typically costs between $10,000 and $25,000 (CAD 14,045 and CAD 35,113) to have a heat pump installed. Factors affecting the cost include:

System size and heating capacity
Energy efficiency ratings
Ductwork modifications or installation
Electrical system upgrades
Labor and permit costs

Some homeowners choose a dual-fuel system that combines a heat pump with a furnace as backup heat instead of electric resistance heating. These systems offer better energy efficiency when backup heating runs frequently in very cold climates.

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When to Use Emergency Heat

Only activate emergency heat mode manually when your heat pump isn't functioning. Signs you need emergency heat include:

Strange noises from your outdoor unit
Ice accumulation on the outdoor unit that doesn't melt
System running constantly without warming your home
Refrigerant leaks or unusual odors

Contact an HVAC professional promptly when using emergency heat. This mode should be temporary until repairs restore normal heat pump operation.

Never use emergency heat as your primary heating method during winter, even if it seems to warm your home effectively. The excessive electricity consumption will dramatically increase your energy bills and put unnecessary strain on your electrical system.

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