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 ensures your home stays warm even during extreme cold snaps.
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 struggles to extract enough heat from the outside air (usually below 35-40°F), the system triggers these coils to provide supplemental warmth.
In dual-fuel systems, the auxiliary heat source might be a gas or oil furnace rather than electric strips.
What Does Auxiliary Heat Mean for Your Comfort?
Auxiliary heat is the bridge between efficiency and comfort. While heat pumps are incredibly efficient at moving heat, they can lose capacity as temperatures plummet. Auxiliary heat fills 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?
Your system is designed to use auxiliary heat sparingly because it uses more energy than standard heat pump operation. It generally activates in three scenarios:
- Extreme Cold: When outdoor temperatures fall below the system's "balance point" (often around 30-35°F), the heat pump needs help maintaining the set temperature.
- Rapid Temperature Changes: If you manually raise your thermostat by 3 degrees or more, the system assumes you want heat fast and engages the AUX mode to speed up the process.
- Defrost Mode: When the outdoor unit freezes over, the system reverses to melt the ice. The auxiliary heat turns on to keep the air blowing into your home warm during this short 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.
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 (e.g., covered in heavy snow or ice that won't melt). Using Emergency Heat when the heat pump is working acts as an efficiency killer, leading to significantly higher energy bills.
Managing Heating Costs with Auxiliary Heat
Because electric resistance heating is less efficient than heat pump technology, frequent use of auxiliary heat can spike 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.