Why Is My Heat Pump Using Auxiliary Heat and What to Do

When a home heat pump switches to auxiliary heat, homeowners often notice higher energy bills and reduced efficiency. This article explains why a heat pump uses auxiliary heat, how to diagnose the cause, practical fixes, and when to call a professional. It focuses on common U.S. climate scenarios and provides actionable guidance to restore efficient operation.

Common Cause	How It Triggers Auxiliary Heat	Short-Term Fix	Long-Term Solution
Outdoor Temperature Drop	Compressor Cannot Maintain Setpoint	Increase Thermostat Setpoint Slightly	Install Cold-Climate Heat Pump Or Supplemental System
Defrost Cycle	Outdoor Unit Temporarily Reverts To Aux Heat	Wait For Cycle To End (10–20 Minutes)	Improve Defrost Controls Or Insulation
Thermostat Settings Or Wiring	Incorrect Mode Or Faulty Wiring Signals Aux Heat	Check Thermostat Mode & Reset	Reprogram/Replace Thermostat
Low Refrigerant Or Mechanical Fault	Insufficient Heat Transfer Leads To Aux Activation	Call Technician For Inspection	Repair Leak/Replace Components

How Heat Pumps Work And The Role Of Auxiliary Heat

Heat pumps move heat rather than generate it, using a refrigerant cycle to extract heat from outdoor air and deliver it indoors. In mild conditions, the heat pump compressor provides most or all heating needs with high efficiency. When outdoor temperatures are very low or when the system temporarily cannot meet demand, the thermostat engages auxiliary (backup) heat, typically electric resistance coils or a gas/propane furnace, to maintain the set temperature.

What Exactly Is Auxiliary Heat

Auxiliary heat (often labeled “aux” or “backup” heat) is a secondary heating source that supplements the heat pump. Common forms include electric resistance strips inside the air handler, a combustion furnace coordinated with the heat pump, or a hybrid setup where both systems work together. Auxiliary heat is less efficient than heat pump operation but provides reliable warmth when the heat pump can’t keep up.

Primary Reasons A Heat Pump Uses Auxiliary Heat

Cold Outdoor Temperatures

As outdoor temperatures fall, the heat pump’s coefficient of performance (COP) declines. Many standard heat pumps have reduced heating capacity below certain thresholds (commonly around 25°F to 35°F). When capacity is insufficient, the thermostat calls for auxiliary heat to meet the setpoint.

Defrost Cycle Activity

Frost or ice accumulates on the outdoor coil in cold, humid conditions. The outdoor unit cycles into defrost mode to melt ice, during which the heat pump temporarily reverses or stops heating. The thermostat may activate auxiliary heat during these intervals, often for short periods (10–20 minutes).

Thermostat Settings And Controls

Improper thermostat configuration—such as setting the system to emergency heat or having a high balance point—can cause persistent auxiliary heat use. Some thermostats have a configurable “heat pump balance point” or temperature differential that determines when to switch to auxiliary heat.

System Sizing And Load Issues

If the heat pump is undersized relative to the home’s heating load, it may be unable to maintain temperature during cold spells and will rely on auxiliary heat more frequently. Similarly, sudden large heating demands (e.g., after opening many windows) can trigger aux heat.

Mechanical Problems And Refrigerant Issues

Low refrigerant charge, failing compressors, clogged coils, or airflow restrictions reduce heat pump output. The thermostat senses insufficient heat delivery and calls for auxiliary heat to compensate. These issues also risk higher energy costs and equipment damage if not addressed.

Wiring Or Control Board Faults

Incorrect wiring between the heat pump and thermostat or a malfunctioning control board can mistakenly signal auxiliary heat. Common wiring errors include miswired W, W2, or AUX terminals or a thermostat that interprets sensor data incorrectly.

How To Diagnose Why Auxiliary Heat Is Running

Check The Thermostat Display And Mode

Confirm whether the thermostat indicates “Heat Pump,” “Aux,” or “Emergency Heat.” If it’s in emergency heat, the compressor is intentionally off and auxiliary heat is active. Resetting to standard heat pump mode may restore normal operation if the compressor is available.

Monitor Outdoor Temperature And Recent Cycles

Compare outdoor temperature to known performance thresholds of the heat pump. If aux heat runs only when it’s very cold or during defrost cycles, the behavior may be expected. Note frequency and duration of these cycles for technician review.

Inspect Airflow And Filters

Check and replace dirty air filters, ensure supply and return vents are unobstructed, and verify the outdoor unit is free of debris. Restricted airflow reduces heat transfer and can prompt auxiliary heat usage.

Listen For Unusual Noises And Smells

Hissing, grinding, or burning odors can indicate mechanical faults or electrical issues. Such symptoms paired with aux heat activation warrant immediate professional inspection to avoid safety hazards.

Review Thermostat Wiring And Settings

Inspect wiring for loose connections and ensure terminals match the heat pump manufacturer’s diagram. Confirm thermostat settings like balance point, temperature differential, and whether “Emergency Heat” is enabled.

Check Refrigerant And Outdoor Unit Operation

Visible signs like frost accumulation beyond expected or the outdoor compressor not running while aux heat is active suggest refrigerant problems or compressor failure. These issues require HVAC technician diagnosis and repair.

Practical Steps To Reduce Unnecessary Auxiliary Heat Use

Adjust Thermostat Strategy

Avoid large setpoint jumps and use gradual temperature changes. Set a reasonable balance between comfort and efficiency—raising the setpoint by a degree or two can sometimes prevent frequent aux activation without discomfort. Use programmable or smart thermostats optimized for heat pumps.

Maintain The Heat Pump Regularly

Schedule annual maintenance to check refrigerant levels, clean coils, lubricate motors, and confirm proper defrost controls. Regular maintenance preserves efficiency and prevents many causes of auxiliary heat reliance.

Improve Home Insulation And Air Sealing

Reducing heat loss lowers system load. Insulate attics, seal leaks around doors and windows, and add weatherstripping where needed. A tighter building envelope reduces the likelihood of aux heat triggering during cold spells.

Upgrade To Cold-Climate Heat Pumps If Needed

For homes in very cold regions, consider installing a cold-climate or multi-stage heat pump designed to maintain capacity at lower temperatures. These systems reduce auxiliary heat reliance and improve overall efficiency.

Optimize Defrost Settings And Outdoor Unit Placement

Ensure adequate airflow around the outdoor unit and that it is elevated above snow levels. If defrost cycles are frequent, a technician may adjust defrost controls or add shields to reduce ice buildup.

Correct Thermostat Programming And Wiring

Use a thermostat designed for heat pumps, and confirm proper wiring to avoid false aux heat calls. Professional thermostat setup ensures the balance point and staging behavior match the system’s capabilities.

When To Call An HVAC Technician

Contact a licensed HVAC technician when auxiliary heat runs continuously without cold outdoor temperatures, when the outdoor compressor does not operate, or if there are unusual noises, smells, or visible refrigerant leaks. Professional diagnostics should include refrigerant checks, electrical testing, airflow measurements, and control system inspection.

Cost And Efficiency Considerations

Auxiliary electric resistance heat can be multiple times more expensive per unit of heat than heat pump heating. Frequent auxiliary use significantly increases utility bills. Replacing inefficient auxiliary systems or upgrading to a modern heat pump with better low-temperature performance can yield long-term savings. Consider rebates and tax incentives available in many states for heat pump upgrades.

Common Misconceptions About Auxiliary Heat

My Heat Pump Is Broken If Aux Heat Turns On

This is not always true. Short aux cycles during defrost or extreme cold can be normal. Persistent or continuous aux heat often points to other issues that require attention.

Turning Off Aux Heat Saves Money

Disabling auxiliary heat without addressing the cause can allow indoor temperatures to drop or strain the heat pump. Emergency heat should only be used when appropriate; consult a technician before permanently disabling aux systems.

Frequently Asked Questions

How Long Should Auxiliary Heat Run During Defrost?

Defrost-related auxiliary heat typically runs for short periods, often 5–20 minutes, depending on outdoor conditions and system design. Frequent or extended defrost cycles indicate an underlying issue.

Can A Smart Thermostat Reduce Auxiliary Heat Use?

Yes, smart thermostats designed for heat pumps can optimize staging, learn home patterns, and avoid unnecessary aux activation. Correct configuration and compatibility with the heat pump are essential.

Is Emergency Heat Different From Auxiliary Heat?

Emergency heat typically forces auxiliary heat on and disables the compressor; it’s used when the heat pump is inoperable. Auxiliary heat may operate alongside the heat pump during normal staging without being labeled “emergency.”

Action Checklist For Homeowners

• Verify thermostat mode and wiring for correct heat pump operation.
• Inspect and replace air filters; clear vents and outdoor unit obstructions.
• Monitor when aux heat runs: cold weather, defrost, or continuously?
• Schedule HVAC maintenance for refrigerant, compressor, and control checks.
• Improve insulation and reduce heat loss to lower system load.
• Consider system upgrade for cold climates or adding a high-efficiency auxiliary source.

Addressing why a heat pump uses auxiliary heat involves understanding system behavior, verifying controls, and ensuring proper maintenance. Following diagnostic steps and corrective actions can restore efficient heat pump operation and reduce energy costs, while professional evaluation is recommended when mechanical or refrigerant issues are suspected.