What Is Auxiliary Heat on a Heat Pump: How It Works, Usage, and Energy Savings

Many American homes rely on heat pumps for energy-efficient heating and cooling. Auxiliary heat plays a crucial role in heat pump systems, especially when outdoor temperatures drop. This article explores what auxiliary heat is, how it works, common scenarios for its use, and ways to maximize energy savings while keeping your home comfortable.

Auxiliary Heat Explained

Auxiliary heat (aux heat) is a supplemental heating feature included in most heat pump systems. It provides additional warmth when your primary heat pump struggles to extract enough heat from the outside air, typically in very cold weather.

Heat pumps move heat from outside to inside, even in winter. However, when temperatures drop below 35°F to 40°F, their efficiency decreases. At this point, the system activates auxiliary heat for extra support.

How Does Auxiliary Heat Work?

Auxiliary heat is usually generated by electric resistance heating elements, similar to those in an electric oven or furnace. When your thermostat detects the need for more heat than the heat pump can provide alone, auxiliary heating elements turn on automatically.

This process is managed by your system’s controls, ensuring smooth transitions between regular and auxiliary heating.

Types Of Auxiliary Heat Sources

• Electric Resistance Heat: The most common; heats air directly using electric coils.
• Gas Or Oil Furnace: Used in “dual fuel” or “hybrid” heat pump setups, switching to fossil-fuel heat in very cold weather.
• Hydronic Systems: Provide supplemental heating using hot water from a boiler.

The Difference Between Auxiliary Heat And Emergency Heat

Although often confused, auxiliary heat and emergency heat are different settings.

• Auxiliary Heat: Kicks in automatically when the heat pump can’t keep up.
• Emergency Heat: Must be manually activated on the thermostat; turns off the heat pump compressor and uses only supplemental heat. It’s for use in emergencies, like system failure or icing.

Auxiliary heat operates automatically during cold spells, while emergency heat is reserved for special situations.

When Does A Heat Pump Use Auxiliary Heat?

Auxiliary heat activates in several scenarios:

1. Very Cold Weather: When outdoor temperatures drop below the heat pump’s optimal operating range.
2. Large Temperature Setbacks: When your thermostat is raised several degrees at once, demanding more heat quickly.
3. Defrost Cycles: During the defrost mode, the system temporarily uses auxiliary heat to maintain comfort indoors.

This feature is designed for short periods and to ensure comfort without manual intervention.

How To Tell If Auxiliary Heat Is Running

• Thermostat Display: Most modern thermostats display “AUX HEAT” or a similar message when it’s active.
• Higher Heating Bills: Because electric resistance heat is less efficient, bills may rise during prolonged cold spells.
• Warm Air Output: Air from vents may feel hotter when auxiliary heat operates.

Monitoring the “AUX” indicator can help track system performance and energy use.

Energy Efficiency: Auxiliary Heat Vs. Heat Pump

A key consideration for many homeowners is energy use. Electric resistance auxiliary heat is much less efficient than standard heat pump operation. Heat pumps move heat, consuming less energy, while auxiliary heat generates heat using electric coils, which is more costly.

System Mode	Energy Efficiency	Relative Operating Cost
Heat Pump Only	High (COP 2-4+)	Low
Auxiliary Heat	Low (COP ~1)	High

It’s best to minimize auxiliary heat use when possible to control energy costs.

Tips To Reduce Auxiliary Heat Usage

1. Set Thermostat Gradually: Increase temperatures slowly to avoid large setpoint jumps, which trigger auxiliary heat.
2. Use A Programmable Thermostat: Schedule temperature changes during non-peak hours and avoid drastic shifts.
3. Seal And Insulate: Improve home insulation, weatherstripping, and sealing to maintain warmth and reduce load on the heat pump.
4. Maintain Your System: Have regular professional maintenance and change filters for peak efficiency.
5. Consider Dual-Fuel Options: Dual-fuel systems use a more efficient gas furnace below certain temperatures.

Smart thermostat management can make a notable impact on overall energy savings.

Common Myths About Auxiliary Heat

• “Auxiliary heat should run all the time in winter.” False. It should only run when additional heating is truly required.
• “Switching to emergency heat will save money in cold weather.” Incorrect. Emergency heat is only for system failures, and running it continuously is expensive.
• “You can disable auxiliary heat without problems.” Not advised, as extreme cold could leave your home under-heated or freeze pipes.

Understanding auxiliary heat operation helps avoid costly mistakes and ensures system reliability.

When To Worry About Auxiliary Heat Usage

Short auxiliary heat cycles are normal in extreme cold. However, if “AUX HEAT” shows hourly or daily for extended periods:

• Possible System Issues: Undersized heat pump, refrigerant problems, or failing components.
• Poor Insulation: Your home may be losing heat faster than the system can provide it.
• Thermostat Issues: Incorrect settings or malfunction may cause unnecessary auxiliary heat use.

Persistent auxiliary heat usage should be diagnosed by a licensed HVAC technician.

Maintenance Tips For Heat Pumps With Auxiliary Heat

1. Change Filters Regularly: Blocked filters reduce efficiency and put more strain on the system.
2. Clean Outdoor Units: Remove debris, snow, and ice from coils and fans.
3. Check Thermostat Settings: Make sure your thermostat is correctly programmed and set to “HEAT” or “AUTO.”
4. Schedule Annual Service: Professional inspection ensures components work optimally, lowers operating costs, and extends equipment life.

Routine maintenance maximizes heat pump efficiency and comfort throughout the winter.

Upgrading Your System: Smart Thermostats And Modern Heat Pumps

Advances in heat pump technology and thermostats make it easier to manage auxiliary heat:

• Smart thermostats can optimize temperature changes, avoiding unnecessary auxiliary heating.
• High-efficiency heat pumps operate at lower temperatures before needing support from auxiliary heat.
• Dual-fuel systems switch intelligently between electricity and natural gas for economical heating in all conditions.

Modern controls and technology can substantially decrease reliance on costly auxiliary heat sources.

Choosing The Right System Size And Features

Proper sizing and selection of heat pump equipment are critical. Oversized or undersized units rely more on auxiliary heat and degrade comfort. When upgrading or installing a new system, consider:

• Home Size And Insulation
• Local Climate
• Efficiency Ratings (SEER, HSPF)
• Auxiliary Heat Type And Control Options

Consulting with a reputable HVAC contractor ensures your system is tailored to your home and minimizes auxiliary heat usage.

Heat Pump Auxiliary Heat: Frequently Asked Questions

Question	Answer
Will my energy bill go up with auxiliary heat?	Yes, during periods when aux heat is running, expect higher energy costs compared to normal heat pump operation.
Should I avoid setting the thermostat too low?	Yes. Drastic temp increases force auxiliary heat to catch up. Moderate your thermostat scheduling to save energy.
How do I know if my heat pump relies too much on auxiliary heat?	If “AUX” displays often or energy bills spike, have a technician inspect your system’s size, operation, and home insulation.

Auxiliary Heat In Different Regions Of The U.S.

Homes in southern climates use auxiliary heat sparingly. Northern and mountain regions experience more frequent aux heat operation, especially during severe cold snaps. Proper system design, insulation, and smart controls make the biggest difference—in any region.

Regional climate directly affects how much auxiliary heat your system uses, so understanding your climate zone can inform your heating strategy.

Signs It’s Time To Call A Professional

• Extended Auxiliary Heat Cycles: AUX stays on despite moderate temperatures.
• Inefficient Heating: Slow temperature recovery, cold spots, or drafts.
• Abnormal Noises Or Smells: Unusual odors or sounds during operation.
• Visible Frost Or Ice On Outdoor Unit: May signal defrost issues or airflow blockages.

Prompt attention can prevent costly repairs and ensure system safety and efficiency.

Auxiliary Heat And Home Comfort Strategies

• Use Ceiling Fans In Reverse: Gently push warm air down for better comfort.
• Close Curtains At Night: Retain heat inside and block drafts.
• Zone Heating: Heat occupied spaces only for improved control.

Smart comfort strategies keep your home feeling warmer with less auxiliary heat.

Conclusion: The Role Of Auxiliary Heat In Modern Heating

Auxiliary heat on a heat pump is a vital backup during the coldest days, but it comes with higher energy costs. Optimal system setup, preventative maintenance, good insulation, and smart thermostat use keep auxiliary heat use—and your bills—under control, ensuring both efficiency and comfort in your American home.