Heat Pump Auxiliary Heat: How It Works and When to Use It

The article explains what heat pump auxiliary heat is, why it engages, and how homeowners can manage it to balance comfort and energy costs. It covers operation, types of auxiliary heat, thermostat strategies, troubleshooting, and tips to minimize unnecessary use of aux heat.

Topic	Quick Takeaway
What Is Aux Heat	Backup heat source activated when heat pump alone can’t meet demand
Common Types	Electric strip heat, gas furnace, dual-fuel systems
Cost Impact	Aux heat is often more expensive per kWh than heat pump operation
How To Reduce Use	Proper thermostat settings, maintenance, and system upgrades

What Is Heat Pump Auxiliary Heat

Heat pump auxiliary heat, often called “aux heat” or “emergency heat,” refers to a secondary heating source that activates when a heat pump cannot maintain the set temperature by itself. It typically supplements or replaces the heat pump during very cold weather, defrost cycles, or when the system detects a fault.

How Auxiliary Heat Works

A heat pump moves heat from outdoors to indoors using refrigerant. When outdoor temperatures drop significantly, the heat pump’s efficiency falls and its capacity may not meet thermostat demand.

At that point, the thermostat signals the auxiliary heat to engage. The system then provides heat via electric resistance elements, a gas furnace, or another backup method to reach or maintain the setpoint.

Types Of Auxiliary Heat

Electric Strip Heat

Electric strip heat uses resistive heating elements inside the air handler. It turns on in stages or all at once depending on thermostat control. It is simple and fast-acting but typically costly to run.

Gas Furnace (Dual-Fuel Systems)

Dual-fuel systems pair a heat pump with a gas furnace. The thermostat or control board switches to furnace heat under a temperature threshold to lower operating costs in very cold conditions.

Emergency Heat Mode

Emergency heat forces the system to use backup heat only, bypassing the heat pump. This mode is used when the heat pump fails or during certain service conditions. It is less efficient and should be temporary.

When Does Auxiliary Heat Activate

Auxiliary heat engages based on several triggers: outdoor temperature thresholds, indoor temperature drop relative to setpoint, compressor lockouts, defrost cycles, or manual activation by the homeowner or technician.

Many thermostats use an outdoor temperature sensor or a programmable balance point to determine when to switch to auxiliary heat, while basic thermostats rely on indoor temperature recovery time.

Thermostat Settings And Controls

Thermostat configuration affects when and how aux heat operates. Smart and programmable thermostats often allow setting a balance point or lockout temperature to prevent unnecessary auxiliary heat use.

Users should ensure thermostats are correctly wired for heat pump systems and that the installer configures recovery and cycle rates to avoid short-cycling auxiliary elements.

Balance Point And Lockout

The balance point is the outdoor temperature at which auxiliary heat becomes more economical or necessary. Setting an appropriate lockout prevents aux heat until the outdoor temperature goes below this threshold.

Energy And Cost Considerations

Auxiliary electric resistance heat can increase energy bills sharply because resistive heating costs more per unit of heat than a properly operating heat pump. Dual-fuel systems can reduce cost by switching to gas at very low temperatures.

Homeowners should compare local electricity and gas rates and consider heat pump efficiency metrics such as HSPF and COP to understand operating costs.

How To Minimize Unnecessary Aux Heat Use

• Verify thermostat settings and enable proper lockout or balance point controls.
• Replace old thermostats with heat-pump-compatible smart models to optimize switching.
• Insulate and air-seal the home to reduce heat load and reduce aux heat calls.
• Regularly maintain the heat pump: clean coils, replace filters, and ensure refrigerant charge is correct.
• Install a two-stage or variable-capacity heat pump to reduce reliance on auxiliary heat.

Maintenance And Performance Checks

Routine maintenance keeps heat pumps efficient and reduces unnecessary aux heat activation. Key tasks include cleaning or replacing filters, clearing debris from outdoor units, checking refrigerant levels, and inspecting electrical connections.

Seasonal professional tune-ups can catch issues like failing compressors or frozen coils that force auxiliary heat to run more often than needed.

Troubleshooting Common Aux Heat Issues

If auxiliary heat runs frequently or constantly, common causes include low refrigerant, thermostat misconfiguration, failed defrost control, or a failing compressor.

Homeowners should confirm thermostat mode (heat pump vs emergency heat), check for error codes on the thermostat or outdoor unit, and schedule a technician for diagnostic tests when faults persist.

Smart Controls And Automation

Smart thermostats and home automation can reduce auxiliary heat use by learning schedules, optimizing setbacks, and using weather forecasts. They can delay aux heat until necessary and provide alerts when the system engages backup heating frequently.

Integrating outdoor temperature sensors and utility rate data helps smart systems choose the most cost-effective heating source in dual-fuel setups.

Upgrades To Reduce Dependence On Aux Heat

Upgrading to a high-efficiency cold-climate heat pump or a variable-capacity unit can maintain higher output at lower temperatures, reducing auxiliary heat needs.

Other upgrades include adding duct improvements, zoning systems to avoid overheating unused spaces, and installing a properly sized backup furnace if a dual-fuel approach is preferred.

Safety And Code Considerations

Electric auxiliary heat elements require proper overcurrent protection and control wiring to meet code. Gas backup systems must follow combustion air, venting, and fuel-line regulations.

Installers should follow local code and manufacturer guidelines to ensure safe and reliable operation, and homeowners should request documentation of compliance after upgrades.

Frequently Asked Questions

Is Auxiliary Heat The Same As Emergency Heat?

Auxiliary heat and emergency heat are related but not identical. Auxiliary heat supplements the heat pump automatically, while emergency heat is a manual thermostat mode that forces the backup heat source to run without compressor assistance.

Why Is My Aux Heat Running Constantly?

Continuous aux heat often indicates a fault such as low refrigerant, compressor failure, thermostat misconfiguration, or extreme outdoor temperatures beyond the heat pump’s capacity. Schedule a professional inspection.

How Much More Expensive Is Aux Heat?

Electric resistance aux heat can be multiple times more expensive per delivered BTU than a heat pump operating at normal efficiency. The exact cost depends on local electricity rates and system performance.

Can A Homeowner Turn Off Aux Heat?

Thermostats typically allow switching to emergency heat, which effectively disables the heat pump, but manually preventing auxiliary heat without addressing root causes is not recommended for safety and comfort reasons.

Signs It’s Time To Call A Professional

• Auxiliary heat runs constantly or unexpectedly.
• Significant increase in heating bills without a change in usage.
• Unusual noises, frequent cycling, or error codes on the system.
• Thermostat wiring problems or persistent comfort issues across the home.

Practical Tips For Homeowners

• Set thermostat balance point thoughtfully and avoid excessive setback that forces long aux heat recovery periods.
• Use programmable or smart thermostats to reduce wasted aux heat runtime.
• Seal and insulate the home to reduce demand peaks on the heat pump.
• Schedule annual HVAC tune-ups before winter to maintain capacity and efficiency.

Understanding how auxiliary heat operates allows homeowners to optimize comfort while controlling energy costs. Proper thermostat setup, regular maintenance, and equipment upgrades are the most effective strategies to reduce unnecessary aux heat usage and improve overall heating performance.