What Is Auxiliary Heat?
Auxiliary heat (often labeled "AUX" on thermostats) is a secondary heating source built into a heat pump system. It automatically activates when outdoor temperatures drop below freezing or when the heat pump cannot efficiently reach your thermostat's set temperature alone. It helps ensure your home stays warm during periods of extreme cold or when the heat pump cannot meet the heating demand on its own. The temperature at which auxiliary heat activates varies by system and depends on factors such as the heat pump's capacity, outdoor temperature, indoor temperature, and thermostat controls.
How Auxiliary Heat Works
Auxiliary heat typically uses electric resistance heating strips—similar to the coils in a toaster—located inside the air handler. When the primary heat pump needs additional capacity, Carrier heat pump systems can use these supplemental heating elements to provide additional warmth.
There is no single outdoor temperature at which every Carrier heat pump activates auxiliary heat. The thermal balance point is the outdoor temperature at which a heat pump's heating capacity intersects with a home's heating demand. Below that point, supplemental heat may be needed to meet the home's heating load.
In dual-fuel heating systems, the auxiliary heat source might be a gas furnace or oil furnace rather than electric strips.
Carrier's variable-speed heat pumps are designed to adjust heating output as conditions change. For example, the Carrier Infinity® 23 Heat Pump (27VNA3) uses a variable-speed compressor and can operate in heating mode down to -13°F, while the Carrier Infinity® 21 Ultimate Cold Climate Heat Pump (27VNA1) is designed for heating operation down to -23°F. These published operating ranges demonstrate why auxiliary-heat activation should not be tied to one universal temperature threshold.
What Does Auxiliary Heat Mean for Your Comfort?
Carrier heat pump systems use auxiliary heat as supplemental heating when the primary heat pump needs additional capacity to maintain indoor comfort. While heat pumps are designed to efficiently move heat, their heating capacity and the home's heating demand change as outdoor temperatures fall. Auxiliary heat can help fill that gap.
Key Benefits
- Continuous Comfort: Prevents indoor temperatures from dropping during frigid weather.
- Rapid Heating: Helps raise the indoor temperature quickly if you increase your thermostat setting by more than 2-3 degrees at once.
- System Protection: Assists during the defrost cycle to prevent cold air from blowing out of your vents while the outdoor unit de-ices itself.
When Does Auxiliary Heat Turn On?
Carrier heat pumps are designed to use auxiliary heat when additional heating capacity is needed, but the specific activation point varies by system. It generally activates in three scenarios:
- Extreme Cold: When outdoor temperatures fall below the system's thermal balance point, the heat pump may need supplemental heat to help meet the home's heating load. This balance point is specific to the heat pump, home, and system design rather than a universal temperature such as 30°F or 35°F.
- Rapid Temperature Changes: If you manually raise your thermostat by several degrees, the thermostat and system controls may call for auxiliary heat to help the system reach the new temperature more quickly.
- Defrost Mode: When the outdoor unit develops frost or ice during heating operation, the system can enter a defrost cycle. Carrier heat pumps use advanced defrost technology, and supplemental heat can help maintain indoor comfort during this temporary cycle.
Dave Lappe, owner of Lappe Heating and Air in Evansville, Indiana, explains that the type and efficiency level of a heating system directly impacts how often auxiliary heat is needed during colder weather.
“High-end, variable-speed heat pumps can operate effectively at much lower outdoor temperatures without relying on an auxiliary heat source,” Lappe said. “Single-stage, lower-efficiency systems may require auxiliary heat to engage closer to around 32 degrees.”
He added that minimizing the use of auxiliary heat is generally the goal for homeowners looking to reduce energy costs.
“The objective is to use auxiliary heat as little as possible in order to maintain efficiency and keep operating costs down,” he said.
Carrier Auxiliary Heat Activation Guide
Carrier does not use one universal outdoor-temperature threshold for auxiliary heat across all heat pump systems. Instead, auxiliary-heat operation depends on the relationship between the heat pump's capacity and the home's heating demand. This is commonly described as the system's thermal balance point.
| System condition | What may happen |
|---|---|
| Mild outdoor temperatures | The Carrier heat pump may provide the home's heating demand without auxiliary heat. |
| Falling outdoor temperatures | The heat pump may operate at higher capacity and run longer to maintain the thermostat setting. |
| Below the system's thermal balance point | Auxiliary heat may operate to provide additional heating capacity. |
| Thermostat temperature raised significantly | Auxiliary heat may be called to help the system recover the desired indoor temperature. |
| Outdoor-unit defrost cycle | Supplemental heat may operate to help maintain indoor comfort while the heat pump defrosts. |
This Carrier framework is intended to explain why AUX operation varies from one home and heat pump system to another—not to establish a single AUX activation temperature. Carrier's variable-speed Infinity heat pumps illustrate this range: the Infinity 23 Heat Pump (27VNA3) is designed for heating operation down to -13°F, while the Infinity 21 Ultimate Cold Climate Heat Pump (27VNA1) is designed for heating operation down to -23°F.
Auxiliary Heat vs. Emergency Heat
Homeowners often confuse auxiliary heat with emergency heat, but they function differently.
- Auxiliary Heat (AUX): Automatic. It runs alongside your heat pump to boost heating power. This is normal operation in cold weather.
- Emergency Heat (EM Heat): Manual. You must physically switch your thermostat to this setting. It shuts off the heat pump entirely and uses only the backup heating source (usually electric resistance).
Important: You should only switch to Emergency Heat if your heat pump has physically broken down or malfunctioned. Carrier recommends contacting a qualified HVAC professional when your heat pump is not operating properly rather than using Emergency Heat as a substitute for normal heat-pump operation.
Managing Heating Costs with Auxiliary Heat
Because electric resistance heating typically costs more to operate than heat-pump heating, frequent use of auxiliary heat can increase your electric bill. Here is how to keep costs in check:
- Don't "Setback" Too Far: Avoid dropping your thermostat temperature significantly at night. Recovering that heat in the morning often forces the auxiliary heat to turn on.
- Use Programmable Thermostats: Smart thermostats can "ramp up" temperature slowly to avoid triggering the AUX setting.
- Maintain Your System: Dirty filters or low refrigerant can force the system to rely on backup heat even in mild weather.
Connect With A Carrier Dealer On Auxiliary Heat
Not sure if you need auxiliary heat? A Carrier dealer can evaluate your home’s heating needs and existing system to determine whether auxiliary heat is necessary. They’ll help you choose the right setup to ensure efficient and reliable comfort all winter long. Schedule an appointment today.
Frequently Asked Questions
Auxiliary heat is a backup heating mechanism that turns on automatically when the heat pump cannot pull enough warmth from the outside air to meet your thermostat setting.
It means your HVAC system is using a secondary heat source (usually electric strips) to supplement the primary heat pump because it is too cold outside for the pump to work alone efficiently.
If AUX heat runs in mild weather (above 40°F), it may indicate a dirty air filter, a refrigerant leak, a malfunctioning sensor, or that the thermostat was raised too quickly.
Yes, generally. Electric auxiliary heat is 100% efficient but much more expensive to operate than a heat pump, which can be 300% efficient. It should only run when necessary.
Yes, auxiliary heat and backup heat generally refer to the same concept: a supplemental heat source that supports or replaces the primary heat pump heat when needed.
About The Author: Travis Baugh is a Digital Brand Marketing Manager for Carrier, where he develops informative, straightforward content to help homeowners better understand heating, cooling, and indoor air quality. His writing is focused on empowering homeowners to make confident, well-informed choices about their home comfort systems.
About The Expert Reviewer: Zach Dickey is a Product Manager for Residential Heat Pumps at Carrier Corporation, where he helps develop innovative heating and cooling solutions that meet homeowners' evolving comfort needs while aligning with industry trends, energy efficiency standards, and regulatory requirements.