When winter temperatures plummet, heat pumps rely on auxiliary and emergency heat modes for backup. But what really sets aux heat apart from emergency heat? Understanding their differences can help American homeowners optimize energy use and comfort. This guide explores how each system works, when to use them, and how choosing the right settings can lower costs and avoid system issues.
| Feature | Auxiliary Heat | Emergency Heat |
|---|---|---|
| Activation | Automatic | Manual |
| Purpose | Supplemental Heat | Backup for Malfunction |
| Energy Use | Moderate to High | Highest |
| Control | Thermostat Controlled | User Selected |
| System Condition | Normal Operation | Main Heat Pump Failure |
How Heat Pumps Operate In Cold Weather
Heat pumps work by transferring heat from the outside air to inside the home, even when it’s cold outside. They offer an efficient alternative to traditional furnaces. However, their performance can suffer as temperatures drop below freezing, leading to the need for backup heating sources.
When the outdoor temperature is above about 35°F, the heat pump efficiently maintains indoor comfort. But as the temperature drops, the system’s ability to extract heat declines. This is where auxiliary and emergency heating modes come into play, ensuring continued warmth when the heat pump alone can’t meet demand.
Defining Auxiliary Heat (Aux Heat)
Auxiliary heat refers to supplemental heating activated automatically by the heat pump’s thermostat. It typically consists of electric resistance heating coils installed inside the air handler. When the outdoor unit struggles to keep up—often during extremely cold conditions or rapid temperature drops—the thermostat adds aux heat to maintain your set temperature.
The integration is seamless: you may not even notice when aux heat is running, except for an increase in energy consumption. The system automatically returns to standard heat pump mode as soon as conditions improve. Aux heat bridges any temporary gap in the heat pump’s capacity, ensuring the comfort of the household.
Primary Functions And Limitations Of Aux Heat
Auxiliary heat kicks in when:
- Outdoor temperatures are too low for the heat pump to operate efficiently (typically below 32–35°F).
- The heat pump enters a defrost cycle, briefly rendering it unable to supply heat.
- There’s a sudden spike in thermostat setting (e.g., raising temperature by more than a couple of degrees quickly).
While aux heat protects against cold spots, it’s much less efficient compared to standard heat pump operation. Extended aux heat use can result in higher energy bills, so home efficiency tactics like weatherproofing and gradual thermostat adjustments help minimize reliance on aux mode.
How A Heat Pump Defrost Cycle Relates To Aux Heat
In freezing temperatures, frost can build up on the outdoor coil. The heat pump responds with a defrost cycle, temporarily reversing operation to melt frost. During this time, no heat transfers indoors. To avoid a temperature drop in your home, aux heat automatically switches on, providing warmth until normal operation resumes. This feature prevents chilly drafts and keeps the home temperature steady during winter’s harshest spells.
What Is Emergency Heat?
Emergency heat is the backup mode that homeowners manually select on their thermostat. It directly engages the secondary heating source—most commonly electric resistance coils, but sometimes gas or oil auxiliary heaters if the system is equipped. Emergency heat bypasses the heat pump compressor entirely, making it the sole source of heating.
This setting is designed for situations where the heat pump itself is malfunctioning, or when the outdoor unit is inoperable due to ice, mechanical failure, or maintenance concerns. Running emergency heat long-term can be extremely expensive, since it lacks the energy efficiency of standard heat pump operation.
Key Differences Between Auxiliary And Emergency Heat
| Aspect | Auxiliary Heat | Emergency Heat |
|---|---|---|
| How It’s Activated | Automatic, by thermostat sensors | Manual, selected by homeowner |
| Operational Condition | Heat pump still running, just supplemented | Heat pump out of service or failed |
| Main Use Case | Temperature boost during extreme cold or quick changes | Primary source when system malfunction occurs |
| Energy Cost | High, but less than emergency heat | Highest among heating options |
Auxiliary heat supports the heat pump automatically when needed, while emergency heat is a user-controlled, last-resort measure for equipment failure or outdoor unit issues.
Signs That Your Heat Pump Has Switched To Aux Or Emergency Heat
Most modern thermostats feature indicators—either lights, icons, or text—that show when aux or emergency heat is in operation. The display may read “AUX HEAT,” “EM HEAT,” or use a colored light, depending on the model. It’s worth consulting the thermostat’s manual to understand the specific indicators. Moreover, if you notice a sudden spike in energy bills during a cold spell, increased runtime coupled with these indicators often means AUX or EM heat has been engaged.
When To Use Emergency Heat
- Your heat pump fails entirely or does not turn on.
- The outdoor unit is damaged, covered in heavy ice, or requires repairs/maintenance.
- Instructed by a licensed HVAC professional during troubleshooting or repairs.
It is never recommended to use emergency heat as a primary everyday heating solution. It’s intended as a temporary fix until the heat pump is repaired.
Smart Thermostats And Heat Pump Backups
Today’s smart and programmable thermostats can control both auxiliary and emergency heat. Many can optimize usage, only triggering AUX heat when absolutely necessary, and even suggest energy-saving settings. Advanced models display notifications to alert homeowners when aux or emergency heat is engaged, supporting smarter energy usage and maintenance decisions.
Energy Costs: Aux Heat Vs Emergency Heat
| Heating Method | Efficiency (COP) | Average kWh Consumption | Relative Cost |
|---|---|---|---|
| Heat Pump (Normal) | 2.0–3.0 | Lowest | Low |
| Aux Heat | 1.0 | High, but variable | High |
| Emergency Heat | 1.0 | Very High | Very High |
Both aux and emergency heat rely on electric resistance heating, which is less efficient than the heat pump. However, emergency mode operates 100% on this less efficient heat, while auxiliary heat supplements rather than replaces the pump entirely.
Reducing Reliance On Backup Heating
To minimize bills and environmental impact, there are proven ways to reduce the frequency and duration of aux and emergency heat operation:
- Weatherize your home with insulation and air sealing to retain warmth.
- Set the thermostat to heat gradually rather than making abrupt changes.
- Schedule routine maintenance on your heat pump system each fall.
- Regularly inspect and change air filters for unobstructed airflow.
- Consider upgrading to a two-stage or variable-speed heat pump for better cold-weather performance.
Consistent system care and home upgrades can dramatically reduce the need for aux and emergency heating.
Frequently Asked Questions About Aux Vs Emergency Heat
Does Aux Heat Automatically Switch Off?
Yes, aux heat turns off automatically when the heat pump catches up or outside temperatures rise. The system reverts to primary heat pump operation as soon as possible to save energy.
What Should You Do If Emergency Heat Is Needed?
If you need to switch to emergency heat, schedule professional HVAC service immediately. Do not continue using emergency mode long-term, as this is costly and may signal a larger problem with your heat pump.
Is Emergency Heat Bad For The System?
No, emergency heat won’t harm your HVAC system if used as intended—for emergencies and repairs. Extended use, however, is energy inefficient and expensive.
How Can You Tell If The Heat Pump Or The Backup Is Running?
Check your thermostat display for “AUX HEAT” or “EM HEAT” indicators. Some systems also use indicator lights or mobile app notifications to inform you which system is active at any given time.
Heat Pump System Settings: Best Practices For Homeowners
Optimizing settings can help avoid unnecessary aux or emergency heat usage:
- Set a reasonable temperature (68°F or lower is recommended by the U.S. Department of Energy).
- Don’t set the thermostat back more than 2–3°F for “setbacks” during your absence.
- If returning to a warmer setpoint, do so in small increments rather than one large step.
- Use the “heat” setting for daily operation. Only set “emergency heat” when instructed or absolutely necessary.
Smart, gradual adjustments keep both comfort and energy costs under control.
Potential Issues If Emergency Heat Is Misused
- Elevated energy bills due to resistance heater inefficiency.
- Overloading electrical systems if used for extended periods.
- Failure to address the root cause of a heat pump malfunction.
- Possible warranty risks if the main unit is damaged and left unrepaired.
Only use emergency heat as a temporary solution and seek professional repairs promptly for system issues.
Summary Table: Heat Pump Aux Heat Vs Emergency Heat
| Feature | Auxiliary Heat | Emergency Heat |
|---|---|---|
| Automatic Activation | Yes | No |
| Manual Selection | No | Yes |
| Main Use | Supplemental when needed | Primary during unit failure |
| Typical Duration | Short-term, situational | Short-term only |
| Energy Cost | Higher than normal, but lower than EM heat | Highest |
| Thermostat Control | Triggered by system | Chosen by user |
Professional Tips: Maintaining Comfort And Efficiency
- Schedule annual service to ensure both heat pump and backup systems are working optimally.
- Clear snow and debris around the outdoor unit for efficient operation.
- Upgrade to smart thermostats for better backup control and timely alerts.
- Monitor energy bills; a sudden rise during winter may indicate reliance on auxiliary or emergency heat.
With an understanding of aux vs emergency heat, homeowners can maintain comfort, save money, and extend their heat pump’s lifetime—even during the harshest cold spells.