If you’ve noticed “aux heat” or “auxiliary heat” flashing on your thermostat during cold weather, you might wonder whether something is wrong with your heating system or if your energy bill is about to spike. Understanding what this indicator means helps you use your heat pump efficiently and know when to call for service.
This article explains how auxiliary heat works, why it activates, what it costs to run, and how to prevent it from running more than necessary. You’ll also learn when aux heat signals a problem that needs professional attention.
What Does Aux Heat Mean on My Thermostat?
Auxiliary heat refers to a backup heating system, typically electric resistance heating strips, that assists your heat pump when outdoor temperatures drop too low for the pump to heat your home efficiently on its own. This supplemental heat activates automatically to maintain your desired indoor temperature when the heat pump cannot keep up with demand.
How Heat Pumps Work and Why They Need Backup
Heat pumps don’t generate heat by burning fuel. Instead, they extract heat from outdoor air and transfer it inside, even when the air feels cold to you.
This process works efficiently in moderate temperatures but becomes less effective as outdoor temperatures fall below 35 to 40 degrees Fahrenheit. The colder the air outside, the less heat energy is available to extract, and the harder your heat pump must work.
When the Heat Pump Cannot Keep Up
When outdoor temperatures drop significantly or when you raise the thermostat setting by several degrees at once, your heat pump may struggle to warm your home quickly enough. Auxiliary heat kicks in to bridge the gap between what the heat pump can deliver and what your home needs.
This backup system ensures you stay comfortable even during the coldest days of winter. Without it, your home would gradually lose heat faster than the pump could replace it.
What Triggers Auxiliary Heat to Turn On
Several common situations cause the aux heat indicator to appear on your thermostat. Recognizing these scenarios helps you distinguish between normal operation and potential problems.
Extreme Cold Weather
Most heat pumps automatically engage auxiliary heat when outdoor temperatures fall below a certain threshold, typically between 25 and 35 degrees Fahrenheit depending on your system. This is normal and expected behavior in cold climates.
The heat pump continues to run but relies on the electric heating strips to supplement its output. Once outdoor temperatures rise above the threshold, the system should return to heat pump operation only.
Large Thermostat Adjustments
When you raise your thermostat setting by three or more degrees at once, auxiliary heat often activates to reach your target temperature faster. The heat pump alone would take too long to make up the difference.
After your home reaches the desired temperature, the aux heat should turn off and let the heat pump maintain comfort on its own. This temporary boost prevents you from waiting hours for your home to warm up.
Heat Pump Defrost Cycle
Heat pumps periodically reverse their operation to melt frost and ice that accumulates on the outdoor coil during cold, humid conditions. During this defrost cycle, the system temporarily stops heating your home and actually cools it slightly.
Auxiliary heat runs during defrost to prevent indoor temperatures from dropping. This typically lasts five to fifteen minutes and is a normal part of heat pump operation in winter.
The Cost Difference Between Heat Pump and Auxiliary Heat
Auxiliary heat uses electric resistance heating, which is two to three times more expensive to operate than the heat pump alone. This happens because resistance heating converts electricity directly into heat with no efficiency advantage, while heat pumps move existing heat from one place to another.
A heat pump typically provides three units of heat for every unit of electricity it consumes when outdoor temperatures are moderate. Electric resistance heating provides only one unit of heat per unit of electricity, making it far less efficient.
Impact on Your Energy Bill
Occasional auxiliary heat use during extreme cold or thermostat adjustments will not dramatically affect your monthly costs. However, if aux heat runs constantly or activates when outdoor temperatures are moderate, you may see your heating bills double or triple.
Monitoring how often the aux heat indicator appears helps you catch problems early. If it runs more than a few hours per day during moderately cold weather, investigate the cause.
Normal Versus Problem Operation
Knowing when auxiliary heat operation is appropriate versus when it signals a malfunction helps you avoid unnecessary service calls and catch real problems before they worsen. Pay attention to outdoor temperatures and how long the aux heat runs.
Signs of Normal Operation
- Aux heat activates only when outdoor temperatures drop below freezing
- The indicator appears briefly during defrost cycles
- Aux heat runs for 30 minutes to an hour after you raise the thermostat significantly
- Your home reaches and maintains the desired temperature
- The aux heat turns off once conditions return to normal
Warning Signs That Indicate a Problem
- Aux heat runs constantly even in moderate temperatures above 40 degrees
- Your home never reaches the thermostat setting despite aux heat running
- Heating bills increase dramatically without explanation
- The outdoor unit does not run at all while aux heat is active
- Strange noises, odors, or system cycling accompany the aux heat indicator
Any of these symptoms suggests your heat pump may not be functioning properly. The system may be relying entirely on expensive backup heat because the pump itself has failed or is severely compromised.
Common Heat Pump Problems That Cause Excessive Aux Heat Use
When auxiliary heat runs more than it should, several underlying issues may be responsible. Some require simple fixes you can handle yourself, while others need professional diagnosis and repair.
Dirty Air Filters
Clogged filters restrict airflow through your system, forcing the heat pump to work harder and produce less heat. Change your air filter every one to three months during heating season.
Reduced airflow can trick the thermostat into thinking the heat pump cannot keep up, triggering auxiliary heat unnecessarily. This simple maintenance task often resolves frequent aux heat activation.
Low Refrigerant Levels
Heat pumps rely on refrigerant to transfer heat from outdoors to indoors. If the system develops a leak and refrigerant levels drop, heating capacity decreases significantly.
Low refrigerant is not a normal maintenance issue and always indicates a leak that must be found and repaired by a licensed HVAC technician. Simply adding refrigerant without fixing the leak wastes money and harms the environment.
Frozen Outdoor Unit
Heavy ice buildup on the outdoor coil prevents proper heat exchange. While some frost is normal and clears during defrost cycles, thick ice that does not melt indicates a problem with the defrost system, airflow, or refrigerant charge.
Never try to chip ice off the outdoor unit yourself. Contact a professional to diagnose why the defrost cycle is not working properly.
Faulty Reversing Valve
The reversing valve switches your heat pump between heating and cooling modes. If this valve sticks or fails, the system may not be able to operate in heating mode at all, leaving only the auxiliary heat strips to warm your home.
This component requires professional replacement. Running on aux heat alone will result in extremely high energy bills until the pump is repaired.
Incorrect Thermostat Settings
Some thermostats allow you to set the temperature at which auxiliary heat engages. If this threshold is set too high, aux heat may activate when the heat pump could still handle the load efficiently.
Check your thermostat manual to see if this setting is adjustable. Setting the aux heat activation point at 35 degrees or below typically provides good balance between comfort and efficiency.
How to Minimize Auxiliary Heat Use and Lower Costs
While you cannot eliminate aux heat entirely in cold climates, you can reduce how often it runs and limit its impact on your energy bills. Smart habits and proper maintenance make a significant difference.
Avoid Large Thermostat Adjustments
Instead of raising the temperature by five degrees when you feel cold, increase it by one or two degrees at a time. This allows the heat pump to meet demand without triggering auxiliary heat.
Use programmable or smart thermostats to adjust temperatures gradually before you wake up or arrive home rather than making sudden changes. This strategy maintains comfort while minimizing backup heat use.
Keep Air Filters Clean
Replace or clean filters on schedule to maintain proper airflow. Mark your calendar or set phone reminders so this task does not slip through the cracks during busy months.
High-efficiency filters that capture more particles may need more frequent changes because they restrict airflow as they load with dust. Balance air quality goals with system performance.
Maintain the Outdoor Unit
Keep the area around your outdoor unit clear of leaves, snow, ice, and debris. Ensure at least two feet of clearance on all sides for proper airflow.
Gently remove light snow accumulation with a broom, but do not chip ice or force anything. Schedule annual professional maintenance to clean coils, check refrigerant levels, and verify all components work correctly.
Improve Home Insulation and Air Sealing
The less heat your home loses, the less your heating system must work. Add insulation to attics, seal air leaks around windows and doors, and use weatherstripping where needed.
These improvements reduce how often temperatures drop enough to trigger auxiliary heat. They also lower your overall heating costs regardless of which system is running.
Use Emergency Heat Only When Necessary
Many thermostats have an “emergency heat” or “EM heat” setting separate from auxiliary heat. This setting turns off the heat pump completely and runs only the electric resistance strips.
Use emergency heat only when the heat pump is broken and you are waiting for repairs. Never use it as a normal operating mode because it costs far more than necessary.
When to Call a Professional
Some auxiliary heat issues require immediate professional attention to prevent system damage or unsafe conditions. Do not delay contacting a licensed HVAC technician if you notice any of these situations.
Continuous Aux Heat in Moderate Weather
If auxiliary heat runs constantly when outdoor temperatures are above 40 degrees Fahrenheit, your heat pump likely has a serious problem. Continuing to operate this way wastes energy and may cause additional damage.
A technician can diagnose whether the issue involves refrigerant, electrical components, the compressor, or other critical parts. Prompt repair often prevents more expensive failures.
Strange Smells or Sounds
Burning smells when aux heat first activates for the season may be dust burning off the heating elements and typically clear within minutes. However, persistent burning odors, electrical smells, or unusual noises warrant immediate professional inspection.
Turn off your heating system if you smell anything that concerns you until a technician can evaluate the situation safely.
Tripped Circuit Breakers
Auxiliary heating strips draw significant electrical current. If your breaker trips repeatedly when aux heat runs, this indicates an electrical problem, overload, or component failure that a qualified technician must address.
Never replace a circuit breaker with a higher-rated one to prevent tripping. This creates a fire hazard by allowing wiring to carry more current than it was designed to handle safely.
System Age and Efficiency Concerns
Heat pumps that are fifteen to twenty years old may run auxiliary heat more often simply because their efficiency has declined. If your system requires frequent repairs and relies heavily on backup heat, replacement may be more cost-effective than ongoing repairs.
A qualified HVAC contractor can perform an energy analysis and help you compare the costs of continued operation versus upgrading to a newer, more efficient model. Modern heat pumps often perform better in cold weather than older systems.
Understanding Emergency Heat Versus Auxiliary Heat
These two terms often confuse homeowners because both involve backup heating. The key difference lies in whether the heat pump continues to operate.
Auxiliary heat runs automatically alongside your heat pump when extra heating is needed. The heat pump keeps working, and the electric strips supplement its output.
Emergency heat is a manual setting that shuts off the heat pump entirely and operates only the backup strips. Use this mode only when the heat pump itself is broken and needs repair.
If you accidentally switch to emergency heat and leave it on, your energy bills will soar because you are using the least efficient heating method available. Always return to normal heating mode once repairs are complete.
Regional Considerations for Heat Pump Operation
How often auxiliary heat runs depends heavily on your climate zone. Heat pumps work differently in mild versus harsh winter regions.
Mild Climate Zones
In areas where winter temperatures rarely drop below freezing, such as the southern United States and Pacific coast, heat pumps operate efficiently most of the winter. You may see aux heat activate only a few times per season during unusual cold snaps.
These regions benefit most from heat pump technology because the systems rarely need backup heat and provide excellent cooling performance in summer.
Cold Climate Zones
Northern states and areas with prolonged freezing weather see more frequent auxiliary heat use. Traditional heat pumps may struggle when temperatures stay below 25 degrees Fahrenheit for extended periods.
Newer cold-climate heat pumps are engineered to operate efficiently down to minus 5 degrees Fahrenheit or lower, reducing backup heat dependency. If you live in a cold region and plan to install or replace a heat pump, look for models specifically rated for low-temperature performance.
Making Informed Decisions About Your Heating System
Understanding auxiliary heat helps you operate your system efficiently, recognize problems early, and make smart choices about maintenance and upgrades. While seeing the aux heat indicator does not always mean something is wrong, paying attention to patterns helps you catch issues before they become expensive.
Monitor how often auxiliary heat activates, keep up with simple maintenance like filter changes, and contact a professional whenever operation seems abnormal for your climate and usage patterns. These steps keep your heating costs manageable and your home comfortable all winter.
We hope this guide has helped you understand what auxiliary heat means and how to manage it effectively in your home. Explore more helpful Home and Garden articles for additional tips on maintaining your home’s comfort and efficiency throughout the year.