Heat pumps offer energy-efficient heating, but when outdoor temperatures drop, auxiliary heat systems provide essential backup warmth. Understanding how auxiliary heat (aux heat) works, when it turns on, and how it affects energy bills is crucial for homeowners. This guide covers types of aux heat, when it activates, troubleshooting tips, and methods to optimize usage for greater efficiency and comfort.
What Is Auxiliary Heat In A Heat Pump System?
Auxiliary heat, also called “aux heat,” is a backup heating source within a heat pump system. When outside temperatures are too cold for the heat pump to extract enough warmth, the system automatically engages the auxiliary heating element to maintain your desired indoor temperature.
Unlike the primary heat pump, which transfers heat from outside air, the aux heat relies on electric resistance coils, gas, or oil burners, much like a traditional furnace.
How Heat Pumps And Auxiliary Heat Work Together
The Basic Operation
Heat pumps efficiently move heat from outdoor air into the home during winter. This process works well until outside temperatures dip to around 30-35°F. As the air becomes colder, the unit struggles to extract heat, and efficiency drops. To compensate, the thermostat triggers the aux heat to turn on.
When aux heat is running, the system combines—or “stages”—energy from the core heat pump and the auxiliary coils, ensuring the house stays warm but often at a higher energy cost.
Heat Pump Modes: “Heat,” “Aux Heat,” And “Emergency Heat”
- Heat: Primary energy-efficient heating. Heat pump only.
- Aux Heat: Supplemental heat automatically engaged when heat pump needs help.
- Emergency Heat: User-activated backup; only the auxiliary heat operates, typically used during malfunctions or system failures.
Types Of Auxiliary Heat Sources
Not all aux heat is equal. The main types include the following:
- Electric Resistance Coils: The most common, similar to an oversized hairdryer inside the air handler.
- Gas Or Oil Furnaces: Called “dual fuel” systems, these provide heat through fossil fuels when needed, increasing output and efficiency in cold climates.
- Hydronic (Water-Based) Coils: Uses hot water from a boiler to supplement heating (less common in American residential systems).
Most homes rely on electric resistance strips as auxiliary heat since they are easy to integrate and require no additional fuel.
When Does Auxiliary Heat Turn On?
Triggers For Aux Heat Activation
- Outdoor Temperatures Drop: Generally, below 35°F, the heat pump can’t pull enough heat from the air.
- Rapid Temperature Rise Required: If you adjust your thermostat significantly higher, aux heat may kick in to reach setpoint quickly.
- Defrost Cycle: During periodic defrosting, the outdoor coil reverses and cools. Auxiliary heat maintains indoor comfort during this process.
- Malfunction: If the heat pump cannot operate, the aux heat acts as the primary (emergency) heat source.
Some advanced systems allow you to view or adjust the aux heat triggers using a smart thermostat.
Aux Heat Modes: Automatic Vs. Manual
The aux heat setting is typically controlled by the thermostat:
- Automatic Mode: The system decides when to use auxiliary heat, based on temperature difference and outdoor weather.
- Manual/Emergency Heat Mode: Homeowners can switch to emergency heat if the heat pump is broken and only the aux heater is working.
Normal operation keeps aux heat in automatic mode because it’s more efficient and less expensive when used sparingly.
Auxiliary Heat And Energy Efficiency
How Aux Heat Affects Your Utility Bills
Electricity powers most auxiliary heating systems. Because electric resistance coils are far less efficient than the heat pump itself, using aux heat can rapidly increase your energy consumption and raise your monthly bills—potentially doubling or tripling costs during extreme cold snaps.
Dual-fuel models help mitigate these costs in colder climates by switching from electric aux heat to natural gas or oil, which is often cheaper per BTU produced than electricity.
Understanding What “Aux Heat” Means On Your Thermostat
When your thermostat displays “AUX HEAT” or “EMER”/“EMERGENCY”, it signals that the secondary heating element is active. If you notice this light on for extended periods—even during milder weather—it may indicate a system issue.
Short, periodic AUX HEAT cycles are normal during extreme cold, but extended aux heating means your heat pump may need service.
The Difference Between Auxiliary And Emergency Heat
While often confused, these terms differ:
- Auxiliary Heat: Automatically engages as needed alongside the heat pump.
- Emergency Heat: Only runs if the homeowner or technician manually activates it (or the main heat pump fails), relying solely on the backup heater.
Running in EMERGENCY HEAT mode is much more costly and should be reserved for actual emergencies.
Choosing The Right Size And Configuration For Auxiliary Heat
The correct size for your auxiliary heat depends on climate, home size, and heat pump capacity. HVAC contractors calculate required output in kilowatts (kW) using Manual J load calculations.
Oversized aux heat systems consume more power than needed, while undersized systems may struggle to keep you warm during severe weather.
How To Optimize Auxiliary Heat Usage
Reduce Reliance On Auxiliary Heat: Practical Tips
- Seal Drafts: Improve insulation and seal leaks around windows and doors to minimize heat loss.
- Upgrade Your Thermostat: Smart thermostats with adaptive recovery can better manage aux heat activation.
- Avoid Drastic Thermostat Changes: Raising the setpoint just 1-2°F reduces aux heat demand versus big temperature swings.
- Regular Maintenance: Have your HVAC system serviced each fall to maximize performance and detect problems early.
- Use Ceiling Fans In Reverse: Circulate warm air downwards to improve comfort at lower thermostat settings.
Small adjustments and preventive maintenance can significantly reduce aux heat runtime and costs.
Common Problems With Aux Heat (And Their Solutions)
Auxiliary heat issues can lead to uncomfortable rooms and expensive energy bills. Typical problems include:
- Aux Heat Running Constantly: Usually caused by a faulty thermostat, undersized heat pump, refrigerant leaks, or wiring problems. Professional diagnosis is recommended.
- Aux Heat Not Working: Can indicate a tripped breaker, blown fuse, or defective heating element/coils. Check breaker box and call a technician if unsure.
- Heat Pump Blowing Cold Air: May occur during defrost cycles or indicate a problem with the reversing valve or compressor.
If the “AUX HEAT” light won’t go off, have a professional HVAC contractor inspect your system.
Can You Turn Off Auxiliary Heat?
In most homes, aux heat activates automatically and cannot be “turned off” without special programming. However, certain advanced thermostats allow users to limit or delay aux heat operation, though excessive limiting may risk frozen pipes or discomfort during cold weather.
Consult a licensed HVAC technician before adjusting or disabling aux heat features.
Smart Thermostats And Aux Heat Management
Modern smart thermostats—like Nest, Ecobee, Honeywell, or Emerson Sensi—offer more control over when and how long auxiliary heat runs. Settings let homeowners:
- View real-time aux heat usage statistics
- Set lockout temperatures to delay aux heat activation
- Program efficient temperature setbacks and recoveries
These features can empower you to save on energy costs while maintaining comfort, especially in regions with wide daily temperature fluctuations.
Is Auxiliary Heat Required In Every Climate?
Aux heat is essential in colder U.S. states (like the Midwest, Northeast, and Rockies), where temperatures routinely drop below freezing. In southern states, homeowners might rarely see aux heat activate except during cold snaps.
Building codes, local climate, and heat pump design determine if and how much aux heat is required.
How Much Energy Does Auxiliary Heat Use?
Electric aux heat strips consume approximately 1 kilowatt-hour (kWh) for each 3,400 BTUs produced. An average U.S. home with a 10 kW aux heat system could draw 10,000 watts per hour, increasing your monthly bill sharply when running continuously.
| Climate | Aux Heat Runtime (Monthly) | Potential Extra Cost |
|---|---|---|
| Deep South | 0-10 hours | $0-$30 |
| Midwest | 30-80 hours | $40-$180 |
| Northeast / Rockies | 60-150 hours | $80-$300 |
Electric aux heat can be several times more expensive than standard heat pump operation.
Comparing Heat Pump With Aux Heat To Other Heating Systems
| Type | Primary Energy Source | Backup Source (Aux Heat) | Efficiency | Cost To Run (Estimate)* |
|---|---|---|---|---|
| Standard Heat Pump | Electricity | Electric Resistance | 250-300% | Low-Moderate |
| Dual-Fuel Heat Pump | Electricity | Natural Gas/Oil Furnace | 200-300% | Low in Moderate Climates, Moderate in Cold |
| Electric Furnace Only | Electricity | None | 90-100% | High |
| Gas Furnace | Natural Gas | None | 85-98% | Moderate |
*Estimates vary by climate, home size, and energy prices.
Heat pumps with aux heat are typically more efficient than electric furnaces, especially when the heat pump is the primary source.
How To Tell If Your System Is Using Aux Heat
- Many thermostats display “AUX HEAT” or “SECOND STAGE” when active.
- Some systems run a fan at higher speed, while others may produce warmer air (due to resistance coils).
- Modern smart thermostats and HVAC apps often show aux heat usage in the dashboard or device settings.
If in doubt, check your system manual or consult an HVAC professional for guidance.
Expert Tips For Lowering Auxiliary Heat Use In Winter
- Set back your thermostat at night, but not too far—big jumps increase aux heat demand in the morning.
- Keep vents and air returns unobstructed for better airflow and heat delivery.
- Consider weatherizing your attic, basement, and crawl space for added insulation.
- Replace air filters every 1-3 months for maximum airflow and heating efficiency.
Combining these tips with regular professional maintenance can reduce aux heat usage and lower seasonal energy bills.
Should You Upgrade To Dual-Fuel Or Advanced Heat Pump Systems?
If you live in a zone with frequent sub-freezing temperatures, investing in a dual-fuel system may deliver long-term savings. These setups automatically switch between heat pump and gas furnace based on outdoor temperatures, optimizing your monthly energy spend.
Dual-fuel and cold-climate heat pump models can provide superior comfort with less reliance on expensive electric auxiliary heating.
Key Takeaways For American Homeowners
- Auxiliary heat is a vital backup for heat pumps in cold weather, but it costs more than standard heat pump operation.
- Optimize your system by sealing drafts, managing thermostat settings, and upgrading to a smart thermostat.
- Watch for excessive aux heat usage, as it could signal a system issue or inefficient settings.
- Consider professional maintenance, and consult your HVAC provider if you experience comfort problems or high bills.
- In cold climates, dual-fuel or advanced heat pump systems offer the best blend of comfort and economy.
By understanding and optimizing heat pump auxiliary heat, homeowners can maximize comfort while keeping winter energy costs in check.