How Heat Pump Aux Mode Works and When to Use It

Heat pump auxiliary (aux) mode is a backup heating feature found in many modern heat pump systems that provides supplemental warmth during extreme cold, defrost cycles, or when the primary heat pump cannot meet demand. This article explains how aux mode works, when it activates, how it affects efficiency and costs, and best practices for homeowners to optimize comfort and energy use.

Feature	What It Does	Impact
Aux Heat Source	Electric resistance or secondary gas/oil furnace	Higher operating cost but reliable heat
Activation Triggers	Temperature differential, compressor fault, defrost	Automatic based on thermostat settings
Thermostat Role	Controls staging between heat pump and aux	Critical for efficiency and comfort

What Is Heat Pump Aux Mode?

Heat pump aux mode, commonly called “aux heat,” refers to supplemental heating that engages when the heat pump alone cannot maintain the set indoor temperature. The auxiliary source is typically electric resistance coils in the air handler or a secondary fossil-fuel furnace tied into the system.

Aux Mode Purpose: Provide immediate heat during very cold outdoor conditions, cover temporary shortfalls during defrost cycles, or act as a backup if the heat pump is malfunctioning.

How Heat Pumps Normally Operate

Standard heat pumps transfer heat from outdoors to indoors using a refrigerant cycle and a compressor. They are most efficient when outdoor temperatures are moderate because the system moves heat instead of producing it.

When outdoor temperatures drop, the heat pump’s capacity to extract heat declines and the compressor works harder, potentially failing to reach the thermostat setpoint. At that point, auxiliary heating becomes relevant.

Common Types Of Auxiliary Heat

Auxiliary heat typically comes in one of three forms: electric resistance heat, a gas or oil furnace used as a backup, or hydronic (hot water) backup systems.

• Electric Resistance Heat: Built-in heating elements in the air handler provide immediate heat and are easy to control but are the most expensive to run per BTU.
• Gas Or Oil Furnace: Acts as a backup by firing when the heat pump can’t meet demand; more cost-effective than electric resistance in many regions.
• Hydronic Backup: Uses a boiler to supply hot water for coils or radiant systems; less common but efficient in some setups.

When Does Aux Mode Activate?

Aux heat activation is controlled by the thermostat and the heat pump’s performance. Common triggers include insufficient heat output, a significant temperature drop, a compressor failure, or the defrost cycle, which temporarily reverses the heat pump to melt ice on the outdoor coil.

Typical Activation Logic: If the thermostat senses the indoor temperature is below setpoint and the heat pump cannot close the gap within a set time or temperature differential, it calls for aux heat.

Thermostat Settings And Staging

Thermostats manage staging between the heat pump and aux heat. Entry-level thermostats might immediately enable aux heat under a set temperature threshold, while more advanced models use adaptive logic to minimize aux use.

Staging Strategies: Dual fuel thermostats coordinate a heat pump with a gas furnace to prioritize the most efficient source based on outdoor temperature and utility costs.

Impact On Efficiency And Energy Costs

Aux heat is generally less efficient and more costly than the heat pump alone. Electric resistance heat can be several times more expensive per unit of heat than a functioning heat pump.

Efficiency Considerations: Relying heavily on aux heat raises energy bills and increases greenhouse gas emissions if powered by fossil-fuel-generated electricity.

Signs The Aux System Is Running Too Often

Homeowners might notice higher-than-expected energy bills, frequent cycling between heat modes, or the thermostat displaying “aux” or “em heat.” Persistent indoor temperature swings despite heating calls can also indicate overuse.

Diagnostic Tips: Check the outdoor unit for frost, ice buildup, or unusual noises; verify thermostat settings; and consult HVAC diagnostics for fault codes.

How To Reduce Reliance On Aux Mode

Reducing auxiliary heat usage improves efficiency and lowers bills. Strategies include improving insulation, sealing air leaks, and optimizing thermostat settings.

• Weatherization: Seal gaps, add insulation, and upgrade windows to reduce heat loss.
• Thermostat Programming: Use setback schedules and avoid large temperature differentials that prompt aux heat.
• Maintenance: Keep the heat pump clean and serviced; replace filters, clear debris from the outdoor unit, and ensure refrigerant charge and airflow are correct.
• Upgrade Controls: Consider an intelligent or dual-fuel thermostat that minimizes aux use by adapting to outdoor temperatures and system performance.

When Aux Mode Is Beneficial

Aux mode is valuable in extreme cold snaps when the heat pump capacity is insufficient and for immediate comfort during defrost cycles. It also serves as essential backup when the primary system fails.

Practical Scenarios: In regions with short but severe cold spells, aux heat prevents comfort loss. For homes with older tight envelopes, aux heat can bridge gaps when demand spikes.

Costs And Utility Considerations

Electricity rates and local energy mix influence how expensive aux mode is. In areas with high electric rates or carbon-intensive grids, aux heat can be particularly costly and less environmentally friendly.

Cost Mitigation: Use programmable thermostats, take advantage of off-peak rates if available, and consider dual-fuel options that switch to gas when electricity prices spike.

Troubleshooting Common Aux Mode Problems

Frequent unnecessary aux activation often stems from thermostat misconfiguration, sensor issues, or heat pump underperformance. Basic troubleshooting can identify common problems before calling a technician.

1. Confirm Thermostat Settings: Ensure the thermostat is set to “heat pump” mode and that emergency heat is not locked on.
2. Inspect Outdoor Unit: Remove snow, ice, and debris; clear at least two feet around the unit.
3. Check Airflow: Replace filters and ensure vents and returns are unobstructed.
4. Run Diagnostics: Look for fault codes on the HVAC system or thermostat for compressor or defrost errors.

When To Call An HVAC Professional

If aux mode engages frequently despite correct thermostat settings and routine maintenance, a licensed HVAC technician should evaluate the system. Possible issues include refrigerant leaks, failing compressors, reversing valve faults, or incorrect control wiring.

Professional Services Include: System diagnostics, refrigerant recharge, control board testing, and replacing failing components to restore primary heat pump function.

Smart Thermostats And Energy Management

Smart thermostats can reduce aux heat use by learning household patterns, staging heat more intelligently, and integrating weather forecasts. These devices often provide insights and alerts when the heat pump struggles.

Benefits: Reduced energy costs through optimized staging, remote monitoring of aux activations, and automated efficiency adjustments based on real-time conditions.

Dual-Fuel Systems And Optimal Switching

Dual-fuel systems combine a heat pump with a gas furnace and use a thermostat to decide which source is most cost-effective. The thermostat typically switches to the furnace below a set outdoor temperature known as the “balance point.”

Balance Point Strategy: Properly set balance points and utility-aware algorithms ensure the system uses the heat pump when efficient and the furnace when it saves money.

Safety And Emergency Heat

Thermostats sometimes offer an “emergency heat” setting that forces the backup heat source on and disables the heat pump. This setting is intended for true heat pump failures and should not be used for normal operation.

Emergency Heat Reminder: Using emergency heat long-term increases costs and may indicate the need for system repair rather than continued reliance on auxiliary heat.

Retrofits And Upgrades To Reduce Aux Dependence

Homeowners can reduce aux usage by upgrading to higher-efficiency heat pumps, adding variable-speed compressors, or installing better controls. Heat pumps with enhanced low-temperature performance delay or eliminate aux engagement.

Upgrade Options: Cold-climate heat pumps, inverter-driven compressors, and improved coils can maintain capacity at lower temperatures and reduce the need for backup heat.

Environmental Impacts

Aux heat, especially electric resistance, can increase a household’s carbon footprint if electricity comes from fossil fuels. Choosing more efficient backups or coordinating with cleaner grids minimizes environmental impact.

Low-Carbon Strategies: Install heat pumps with superior low-temperature efficiency, use dual-fuel systems with lower-carbon fuels, or pair systems with on-site renewables like solar PV to offset higher electricity use during aux operation.

Key Takeaways For Homeowners

Understand The Role Of Aux Mode: It is a necessary safety and comfort feature, not a sign of immediate system failure.

Minimize Use Through Maintenance And Controls: Regular HVAC care, improved home insulation, and an intelligent thermostat reduce reliance on auxiliary heat and lower costs.

Upgrade When Appropriate: Consider cold-climate heat pumps, inverter technology, or dual-fuel setups where aux heat is a recurring expense.

Resources For Further Reading

• U.S. Department Of Energy — Heat Pump Systems
• ENERGY STAR — Heat Pumps
• Industry Articles On Auxiliary Heat