Heat pump auxiliary mode is a crucial setting found in many American homes, especially in regions experiencing cold winters. This article explores what auxiliary mode is, how it operates, when it should be used, and how you can optimize its efficiency. Whether you own a heat pump or considering installing one, this guide provides essential knowledge for maximizing comfort and managing energy costs.
Summary Table: Key Facts About Heat Pump Auxiliary Mode
| Feature | Description |
|---|---|
| What Is Aux Mode? | Backup heat source for heat pumps during extreme cold or demand spikes |
| Common Backup Types | Electric resistance coils, gas furnace, or oil furnace |
| Efficiency Impact | Less efficient; higher energy use and utility cost |
| When Does It Run? | Automatically when outdoor temps drop or system can’t meet set temp alone |
| Main Benefits | Ensures comfort even in very cold weather |
| Main Drawback | Increased electricity (or fuel) consumption |
What Is Heat Pump Auxiliary Mode?
Auxiliary mode (or AUX heat) refers to a heat pump’s backup method for providing warmth when outdoor conditions become too cold for the system to operate efficiently. Most heat pumps deliver excellent performance in mild to moderately cold weather, but their heating capacity drops as temperatures plummet. To keep your home comfortable, auxiliary mode activates a secondary heat source, such as electric resistance coils or a secondary furnace.
How Does Heat Pump Auxiliary Mode Work?
Heat pumps function by extracting heat from external air and transferring it indoors. However, as temperatures dip below around 35°F, there’s less available heat for the system to absorb. At a certain threshold—usually between 30°F and 40°F for most homes—the auxiliary heating system kicks in automatically to supplement the heat pump’s output.
This process is usually managed by the home’s thermostat. When the set temperature cannot be maintained by the standard heat pump cycle, the thermostat signals the auxiliary system to turn on, ensuring that your indoor temperature stays stable.
Types Of Auxiliary Heat Sources
- Electric Resistance Coils: The most common, these use electricity to generate heat directly. They operate similarly to space heaters and baseboard units.
- Gas or Oil Furnaces: Some dual-fuel systems use a furnace for auxiliary heating, switching over from electric to fuel as needed.
- Hydronic (Hot Water) Backup: In rare cases, hot water from a boiler supplements the heat pump.
The type installed will affect operating costs and system performance.
When Does Auxiliary Heat Turn On?
The auxiliary heat mode is designed to run only when needed. Common scenarios include:
- Outdoor temperatures fall below a set balance point (usually 30-40°F)
- Heat pump is in a defrost cycle (temporary backup to maintain warmth)
- Large temperature setbacks or jumps in setting (thermostat increased several degrees at once)
- Heat pump isn’t functioning efficiently due to maintenance issues
If AUX mode remains on for long periods, it may indicate a problem with the main heat pump system.
How To Tell If Auxiliary Heat Is Running
Most modern thermostats will display an indicator reading “AUX” or “EM HEAT” when auxiliary mode is active. Some smart thermostats provide alerts or energy usage reports noting when auxiliary heat has been used. Additionally, occupants may notice the following:
- Stable indoor temperature even in very cold weather
- Increase in energy use or utility bills during cold snaps
- Possible sensation of air warmth increasing more quickly
Staying aware of your system’s AUX mode usage can help manage energy expenses.
Auxiliary Heat Vs. Emergency Heat
Many heat pump thermostats have both auxiliary heat and emergency heat settings, but these modes serve different purposes:
| Auxiliary Heat | Emergency Heat |
|---|---|
| Activates automatically with cold or demand; operates with the heat pump | Activated manually; heat pump is turned off, and only the backup heat source runs |
| For routine cold-weather supplement | Used when the main heat pump is broken or frozen |
| Energy-efficient option for backup | Less efficient, only for emergencies |
Do not regularly use Emergency Heat except for breakdowns.
Energy Efficiency: Cost Of Running Auxiliary Heat
One of the biggest considerations for homeowners is the impact of auxiliary heat on energy costs. Electric resistance heat is far less efficient than normal heat pump operation. While a heat pump can deliver 200-300% efficiency (2-3 units of heat per unit of electricity), resistance coils convert electricity to heat at nearly a 1:1 ratio (100%). Gas or oil furnaces may have slightly higher costs depending on fuel prices.
During periods of extended auxiliary use—such as a deep freeze—expect energy use and bills to spike. Monitoring usage and optimizing settings can reduce costs.
How To Limit Auxiliary Heat Use And Save On Bills
Reducing dependency on auxiliary mode can lead to significant savings. Here’s how to optimize your system:
- Set Thermostat Responsibly: Avoid large “jumps” in temperature setting, especially in the morning. Gradually increase by 1-2°F increments.
- Use a Programmable or Smart Thermostat: These allow you to create a comfortable schedule without triggering AUX unnecessarily.
- Improve Home Insulation: Air seal attic, windows, and doors to preserve internal heat and reduce system load.
- Upgrade Old Heat Pumps: Modern high-efficiency units perform better at low temps, reducing AUX time.
- Zone Heating: Only heat occupied areas to reduce overall demand on the system.
- Routine Maintenance: Keep coils clean, filters changed, and refrigerant at proper levels to help the primary heat pump work effectively.
Smart management can cut back on the costly backup mode.
Common Problems With Auxiliary Heat
Auxiliary Mode Running Constantly
If the thermostat shows “AUX” running for an extended period—even in mild weather—there may be issues such as:
- Malfunctioning thermostat or control board
- Refrigerant charge issues
- Dirty coils or clogged filters
- Outdoor unit icing over (defrost issue)
Professional diagnosis may be required to address these faults.
Thermostat Not Switching Out Of AUX
Sometimes, a system remains in auxiliary mode during warmer weather, wasting energy. Troubleshooting steps include:
- Check thermostat settings and reprogram schedule
- Inspect for sensor problems or miswiring
- Call a qualified technician if the issue persists
Correct thermostat calibration is essential for efficient operations.
Benefits Of Heat Pump Auxiliary Mode
- Reliable Comfort: Ensures heating even during extreme weather events or technical issues
- Automatic Response: No user intervention needed under normal conditions—the thermostat manages the switch
- Home Value: Modern systems with seamless AUX support are attractive to home buyers in diverse climates
AUX mode bridges the gap during harsh conditions, improving year-round comfort.
When Should You Use Emergency Heat?
Emergency heat should be used only when the main heat pump system fails—such as a breakdown, refrigerant leak, or if the outdoor unit is disabled due to ice/frost. Manually setting the thermostat to “EM HEAT” or “EMERGENCY” ensures warmth while awaiting professional repair. Because it’s far less efficient, limit usage to genuine emergencies.
Can You Manually Turn Off Auxiliary Heat?
Most thermostats manage auxiliary heat automatically with no user override, but some models allow users to set a higher “balance point”—the temperature at which AUX is enabled. Advanced smart thermostats and dual-fuel systems offer more customization. Shutting off auxiliary heat entirely is not recommended as it may jeopardize comfort and cause the main heat pump to work too hard in harsh conditions.
Smart Thermostats And Auxiliary Heat Automation
Modern smart thermostats—including models from Ecobee, Nest, and Honeywell—offer advanced features for managing auxiliary heat. These may include:
- Learning Algorithms: Systems adjust when to use AUX heat based on weather forecasts and user habits
- Energy Usage Reports: Track the percentage of time spent in auxiliary vs normal mode
- Custom Setpoints: Define the minimum outdoor temperature for enabling AUX
These tools help maintain comfort while minimizing unnecessary energy use.
Dual-Fuel Heat Pump Systems Explained
A dual-fuel heat pump combines a standard heat pump with a gas or oil furnace as the auxiliary heating source. When temperatures fall below the heat pump’s optimal range, the system automatically switches to the fossil fuel furnace, which can be more cost-effective than electric resistance heat in many areas. These systems provide:
- Improved efficiency during extreme cold
- Lower utility bills in regions with high electricity prices
Homeowners should work with HVAC professionals to determine the best setpoints for system switchover.
Comparing Auxiliary Heat With Other Backup Heating Options
| Backup Type | Efficiency | Operating Cost | Best For |
|---|---|---|---|
| Electric Resistance | Low (~100%) | High electricity use | Areas with mild winters or infrequent deep cold |
| Gas Furnace | High (80-98%) | Moderate/low for gas | Colder climates, natural gas access |
| Oil Furnace | Medium (80-90%) | Higher fuel cost | Rural or oil-heated homes |
Heat Pump Sizing And It’s Impact On Auxiliary Use
Properly sizing a heat pump is crucial for minimizing auxiliary heating use. An undersized unit will rely on AUX more often, while an oversized system wastes energy and reduces comfort due to frequent cycling. A professional HVAC load calculation considers home size, insulation, window placement, and local climate data to select the right system capacity.
Climate And Regional Considerations For Auxiliary Mode
In the US, heat pump auxiliary mode use varies by region:
- Southeast and Mid-Atlantic: Mild winters; auxiliary heat is rarely used
- Midwest/Northeast/New England: Colder climates with frequent deep freezes; more AUX mode usage
- West Coast: Mediterranean and oceanic climates see minimal auxiliary operation
Local climate will determine the importance of auxiliary heat in your heat pump setup.
Maintenance Tips To Reduce Auxiliary Heat Dependency
- Schedule annual heat pump inspections and tune-ups
- Replace filters every 1-3 months
- Clear debris from outdoor unit and ensure unrestricted airflow
- Check ductwork for leaks; seal and insulate as needed
- Monitor refrigerant levels and electrical connections
Ongoing maintenance maximizes system performance and reduces auxiliary reliance.
Frequently Asked Questions About Heat Pump Auxiliary Mode
What Temperature Does Auxiliary Heat Typically Turn On?
Most systems switch to auxiliary heat when the outdoor temperature falls to 30-40°F. The exact “balance point” varies by home insulation, heat pump efficiency, and thermostat settings.
Is It Bad For Auxiliary Heat To Run A Lot?
While AUX mode ensures comfort, heavy or frequent use signals higher energy bills and possible equipment issues. If you notice constant auxiliary activity, schedule a service check to maintain optimal efficiency.
How Long Should Auxiliary Heat Stay On?
Auxiliary heat should operate only during brief periods of extreme cold or major setpoint changes. If it runs continuously for hours, investigate system performance and maintenance needs.
Does AUX Heat Provide The Same Comfort As The Heat Pump?
Yes, except in electric resistance systems the air may feel drier. Gas and oil furnaces in dual-fuel setups can deliver even more robust heat (hotter air) compared to the standard pump cycle.
When To Call A Professional
If you experience issues such as prolonged auxiliary use, inconsistent heating, or system error messages, contact an HVAC technician. Professionals can diagnose thermostat calibration problems, refrigerant leaks, or auxiliary heater faults to prevent spikes in utility bills and discomfort in your home.
Final Thoughts: Managing Auxiliary Mode For Comfort And Savings
Auxiliary heat mode is an essential safety net for American homes using heat pumps. Homeowners should understand when it activates, monitor energy bills, upgrade components as needed, and prioritize routine maintenance for the greatest savings and comfort throughout winter. Thoughtful use of thermostat schedules, insulation, and smart system controls can minimize unnecessary auxiliary use while ensuring a reliably warm home.