If you’ve noticed “aux heat” or “auxiliary heat” flashing on your thermostat during cold weather, you might wonder whether your heating system is working correctly or if you’re about to see a spike in your energy bill. Understanding what this indicator means helps you use your heat pump efficiently and recognize when something needs attention.
This article explains how auxiliary heat works, when it activates, why it costs more to run, and how to keep your heating system running efficiently through winter. You’ll also learn the difference between auxiliary and emergency heat, and when to call a professional.
What Is Auxiliary Heat on a Thermostat?
Auxiliary heat is a backup heating system that automatically activates when your heat pump cannot keep up with your home’s heating demand. Most commonly, this supplemental heat comes from electric resistance heating strips installed in your air handler, which work alongside your heat pump to bring your home to the desired temperature faster during very cold weather or when you’ve set a large temperature increase on your thermostat.
How Heat Pumps Work in Cold Weather
Heat pumps move heat from outside air into your home rather than generating heat by burning fuel. This process works efficiently in moderate temperatures but becomes less effective as outdoor temperatures drop.
When outdoor temperatures fall below approximately 35 to 40 degrees Fahrenheit, most heat pumps struggle to extract enough heat from the cold air. The system must work harder and longer to maintain comfortable indoor temperatures, which is when auxiliary heat steps in to help.
The Balance Point
Every heat pump has a balance point, the outdoor temperature at which the heat pump alone can no longer keep up with your home’s heat loss. This temperature varies based on your heat pump’s efficiency, your home’s insulation, and your heating needs.
Below this balance point, your system needs supplemental heat to maintain the temperature you’ve set. The aux heat indicator appears when this backup system engages.
When Auxiliary Heat Activates
Your heating system triggers auxiliary heat in specific situations that require faster or more powerful heating than your heat pump can deliver alone. Recognizing these scenarios helps you understand whether the system is functioning normally.
Temperature Setback Recovery
If you raise your thermostat setting by more than 3 degrees, many systems automatically engage auxiliary heat to reach the new temperature quickly. This prevents your heat pump from running for hours trying to close a large temperature gap.
This feature works well for comfort but can increase energy costs if you frequently make large thermostat adjustments. Smaller, gradual temperature changes allow your heat pump to work alone more often.
Defrost Cycles
Heat pumps periodically reverse their operation to melt frost that builds up on the outdoor coil during cold, humid conditions. During this defrost cycle, which typically lasts 5 to 15 minutes, the outdoor unit cannot heat your home.
Auxiliary heat runs during defrost cycles to prevent cold air from blowing through your vents. You may notice the aux heat light appearing briefly every 30 to 90 minutes during freezing weather.
Extended Cold Periods
When outdoor temperatures remain well below freezing for hours or days, your heat pump may need continuous help from auxiliary heat. This is normal operation in very cold climates and ensures your home stays warm even when the heat pump’s efficiency drops.
If you live in a region with frequent subfreezing temperatures, you might see the aux heat indicator regularly throughout winter. The system is designed to work this way.
Why Auxiliary Heat Costs More to Run
Electric resistance heat strips generate warmth by passing electricity through high-resistance coils, similar to how a toaster works. This method converts electricity directly into heat with nearly 100% efficiency, but it uses significantly more electricity than a heat pump.
Heat pumps move heat rather than create it, which makes them two to three times more energy-efficient than resistance heating in moderate temperatures. When auxiliary heat runs, you’re using the less efficient backup system, which increases your electric bill.
Cost Comparison
The exact cost difference depends on your electricity rates and how long auxiliary heat runs. In many cases, running auxiliary heat can double or triple your heating costs compared to operating the heat pump alone.
Brief aux heat use during temperature recovery or defrost cycles adds moderate costs. Continuous auxiliary heat during extended cold snaps has a much larger impact on your monthly bill.
Auxiliary Heat vs. Emergency Heat
While auxiliary heat and emergency heat both use the same electric resistance heating elements, they function differently and serve separate purposes. Understanding this distinction prevents confusion and helps you use your system correctly.
How Emergency Heat Works
Emergency heat is a manual setting on your thermostat that shuts off the heat pump completely and runs only the backup electric heat strips. You activate this mode using a dedicated switch or setting on your thermostat.
Use emergency heat only when your heat pump has failed or is malfunctioning. Running in emergency mode for extended periods wastes energy and significantly increases heating costs because you’re not benefiting from the heat pump’s efficiency.
When to Use Emergency Heat
Switch to emergency heat if you notice these signs of heat pump failure:
- The outdoor unit is frozen solid and won’t defrost
- Strange grinding, screeching, or banging noises from the outdoor unit
- The outdoor fan isn’t running during a heating call
- Ice buildup on the indoor coil or refrigerant lines
- Insufficient heat even when auxiliary heat runs continuously
After switching to emergency heat, contact an HVAC professional immediately to diagnose and repair the problem. Do not leave your system in emergency mode once repairs are complete.
Troubleshooting Frequent Auxiliary Heat Use
Occasional aux heat activation is normal, but if the indicator stays on constantly or appears during mild weather, your system may have an efficiency problem. Several issues can cause excessive auxiliary heat use.
Thermostat Settings
Check your thermostat’s auxiliary heat settings if available. Some programmable thermostats let you adjust when backup heat engages, and incorrect programming can trigger aux heat too readily.
Also verify that your thermostat isn’t set to emergency heat mode by mistake. Look for a switch or setting labeled “EM heat” or “emergency” and make sure it’s in the normal or auto position.
Dirty Air Filters
Clogged air filters restrict airflow through your system, forcing the heat pump to work harder and run longer. This reduced efficiency can cause auxiliary heat to activate more frequently than necessary.
Replace or clean your air filter every 30 to 90 days during heating season. Homes with pets, allergies, or high dust levels need more frequent filter changes.
Low Refrigerant
Heat pumps depend on proper refrigerant charge to transfer heat efficiently. If your system is low on refrigerant due to a leak, heating capacity drops and auxiliary heat must compensate.
Only licensed HVAC technicians should check refrigerant levels or repair leaks. Refrigerant systems operate under high pressure and require specialized equipment and certification to service safely.
Undersized Heat Pump
If your heat pump is too small for your home’s square footage, insulation level, or climate zone, it will rely heavily on auxiliary heat to maintain comfort. This design mismatch causes high operating costs from the beginning.
Consult an HVAC professional to determine whether your heat pump is properly sized. Upgrading to a correctly sized unit or adding insulation may be more cost-effective than running auxiliary heat constantly.
Reducing Auxiliary Heat Runtime
While you can’t eliminate aux heat entirely in cold climates, several strategies minimize how often it runs and reduce the impact on your energy bills. Small changes to your habits and home can make a measurable difference.
Maintain Steady Temperatures
Avoid large thermostat adjustments that trigger auxiliary heat for temperature recovery. Instead of dropping the temperature at night and raising it sharply in the morning, maintain a consistent setting or make smaller changes of 2 degrees or less.
If you use a programmable thermostat, schedule temperature changes to occur gradually over an hour rather than all at once. This gentler approach lets your heat pump work without backup assistance.
Improve Home Insulation
Better insulation reduces heat loss, which lowers your heat pump’s balance point and delays the need for auxiliary heat. Focus on attics, exterior walls, basements, and crawl spaces where heat escapes most readily.
Sealing air leaks around windows, doors, outlets, and penetrations also helps. These improvements benefit all your heating and cooling systems, not just your heat pump.
Clear the Outdoor Unit
Keep snow, ice, leaves, and debris away from your heat pump’s outdoor unit. Blocked airflow reduces efficiency and can cause the unit to ice up, leading to longer defrost cycles and more auxiliary heat use.
Maintain at least two feet of clearance around all sides of the outdoor unit. Never cover the unit with tarps or enclosures, as heat pumps need continuous airflow even during winter operation.
Schedule Professional Maintenance
Annual heat pump maintenance keeps your system running efficiently and catches small problems before they force excessive auxiliary heat use. A qualified HVAC technician can clean coils, check refrigerant levels, test controls, and optimize performance.
Schedule maintenance in fall before heating season begins. The technician can identify worn parts, efficiency losses, or settings that might cause unnecessary backup heat activation.
When to Call a Professional
Some auxiliary heat situations require professional diagnosis and repair rather than homeowner troubleshooting. Recognize these warning signs that indicate a service call is necessary.
Contact an HVAC technician if auxiliary heat runs continuously for more than a few hours during moderately cold weather (above 30 degrees), if your energy bills spike suddenly without explanation, or if you notice ice forming on indoor components or refrigerant lines. Also call for service if the outdoor unit makes unusual noises, fails to start, or cycles on and off rapidly.
What to Expect During Service
A qualified technician will test your system’s heating capacity, measure refrigerant pressures and temperatures, check electrical components and controls, inspect the reversing valve, and verify proper defrost operation. This diagnostic process identifies whether mechanical failure, refrigerant issues, or control problems are causing excessive auxiliary heat use.
Repair costs vary widely depending on the problem. Simple fixes like replacing a defective sensor or adjusting controls cost significantly less than compressor replacement or refrigerant leak repairs.
Understanding Your Heating Bills
Knowing how much auxiliary heat you’re using helps you evaluate whether your system is performing efficiently and whether efficiency improvements would pay off. Most thermostats don’t track aux heat runtime separately, but your heating patterns offer clues.
Compare your electric bills from similar weather periods year to year. Significant increases may indicate that your heat pump’s efficiency has declined and auxiliary heat is compensating for the loss.
Smart Thermostat Benefits
Some smart thermostats track and report how many hours your system runs in each mode, including auxiliary heat. This data helps you see exactly when and why backup heat activates.
Advanced models can also reduce auxiliary heat use by learning your schedule and adjusting temperatures gradually. The upfront cost of a compatible smart thermostat may be offset by energy savings if you currently rely heavily on backup heat.
Heat Pumps in Extreme Cold Climates
If you live where winter temperatures regularly drop below 20 degrees Fahrenheit, standard heat pumps will use auxiliary heat extensively. Newer cold-climate heat pumps maintain better efficiency at lower temperatures and reduce backup heat dependency.
Cold-climate or “hyperheating” models use enhanced compressors and refrigerants designed for subfreezing performance. These systems can provide full heating capacity down to 5 degrees Fahrenheit or lower without assistance, though they cost more initially than conventional heat pumps.
Dual Fuel Systems
Another option for very cold regions is a dual fuel system that pairs a heat pump with a gas or oil furnace instead of electric resistance heat. The system automatically switches to the furnace when outdoor temperatures make heat pump operation inefficient.
This setup provides efficient heat pump operation in moderate weather and cost-effective furnace heating in extreme cold. Installation requires both heating systems and control equipment that manages the switchover, which increases upfront costs but can lower long-term operating expenses in cold climates.
Making Informed Heating Decisions
Auxiliary heat is a normal, necessary part of heat pump operation that ensures comfort when outdoor temperatures challenge your system’s capacity. Brief periods of aux heat use during cold snaps, temperature recovery, or defrost cycles don’t indicate a problem and are built into the system’s design.
Monitor your system’s performance, maintain steady indoor temperatures when possible, keep up with filter changes and outdoor unit clearance, and schedule annual professional maintenance. These practical steps minimize auxiliary heat runtime and keep your heating costs predictable.
If your aux heat indicator stays on constantly or your energy bills climb unexpectedly, contact a licensed HVAC professional to diagnose the issue. Addressing efficiency problems early prevents bigger repairs and reduces the strain on your budget during the coldest months of the year.
We hope this guide has helped you understand how auxiliary heat works and how to manage your heat pump system efficiently. Explore more helpful Home and Garden articles for additional tips on maintaining your home year-round.