Auxiliary Heat Running Reasons When Its Cause For Concern

Auxiliary heat—often labeled as \"Aux Heat\" or \"Emergency Heat\" on thermostats—is a backup heating function in heat pump systems designed to maintain indoor comfort during extreme cold. While it’s a necessary feature, seeing it activate frequently or remain on longer than expected can raise questions. Is it working as intended? Or is there an underlying issue with your HVAC system? Understanding the conditions under which auxiliary heat operates—and when it becomes a red flag—is essential for efficiency, cost control, and long-term system health.

How Auxiliary Heat Works in a Heat Pump System

Heat pumps transfer warmth from outdoor air into your home, even in cool weather. However, when outdoor temperatures drop below approximately 35–40°F, the efficiency of this process diminishes. At that point, the system engages auxiliary heat—typically electric resistance heaters located inside the air handler—to supplement warmth. This secondary heat source ensures indoor temperatures stay comfortable despite limited heat availability outdoors.

The thermostat monitors indoor temperature and compares it to the setpoint. If the gap exceeds about 2–3 degrees, the system triggers auxiliary heat to accelerate warming. Once the desired temperature is reached, auxiliary heat should disengage, returning control to the more efficient primary heat pump.

Tip: A brief activation of auxiliary heat during cold mornings or after prolonged thermostat setbacks is normal and not a cause for concern.

Common Reasons Auxiliary Heat Runs

Not all instances of auxiliary heat operation are problematic. Some are part of standard system behavior:

• Extreme Cold Weather: When outdoor temps fall below freezing, the heat pump struggles to extract sufficient heat, prompting regular aux heat use.
• Rapid Temperature Recovery: After turning the thermostat up significantly (e.g., following a vacation or nighttime setback), the system may use auxiliary heat to reach the target faster.
• Defrost Cycle: During winter, frost builds on the outdoor coil. The system temporarily switches to cooling mode to melt it, reversing valves and using auxiliary heat indoors to maintain comfort during this short phase (typically 5–15 minutes).
• System Design: In older or undersized heat pumps, auxiliary heat may engage more often due to lower heating capacity.

These scenarios reflect normal operation. The key is duration and frequency. Occasional use is expected; constant or prolonged activation is not.

When Auxiliary Heat Use Becomes a Concern

Persistent auxiliary heat operation increases energy consumption—electric resistance heating can cost two to three times more per unit of heat than a functioning heat pump. If you notice any of the following signs, further investigation is warranted:

1. Auxiliary heat runs continuously, even during mild weather (above 40°F).
2. The system fails to return to primary heat pump mode after reaching the set temperature.
3. You experience inconsistent indoor temperatures or weak airflow.
4. Your energy bills spike unexpectedly in winter without changes in usage patterns.
5. Frequent cycling between heat pump and auxiliary modes (\"short cycling\").

These behaviors suggest the heat pump isn’t performing as designed, forcing reliance on the less efficient backup system.

Underlying Causes of Abnormal Auxiliary Heat Operation

Several mechanical or environmental issues can trigger excessive auxiliary heat use:

Issue	Description	Impact
Low Refrigerant Charge	Insufficient refrigerant reduces heat transfer efficiency.	Heat pump can't warm air adequately; aux heat compensates.
Dirty Outdoor Coil	Debris, ice, or grime blocks heat exchange.	Reduces ability to absorb outdoor heat.
Failing Compressor or Reversing Valve	Mechanical failure prevents proper operation.	System defaults to auxiliary mode.
Thermostat Calibration Error	Incorrect temperature sensing leads to false aux heat calls.	Unnecessary activation even when heating is sufficient.
Airflow Restrictions	Clogged filters, blocked vents, or failing blower motor.	Reduced heat delivery; thermostat keeps calling for more heat.

“Continuous auxiliary heat is rarely normal. It’s often the first symptom of a deeper HVAC issue that, if ignored, can lead to costly repairs.” — Mark Reynolds, HVAC Technician with 20+ years of field experience

Step-by-Step Guide to Diagnose and Address the Issue

If you suspect abnormal auxiliary heat operation, follow this practical diagnostic sequence:

1. Check the Thermostat Display: Confirm whether “Aux Heat” or “Em Heat” is active. Note how often and under what conditions it turns on.
2. Inspect Air Filters: Replace if dirty. Restricted airflow is a common and easily fixable cause.
3. Clear the Outdoor Unit: Remove snow, leaves, or debris around the condenser. Ensure at least 18 inches of clearance on all sides.
4. Verify Thermostat Settings: Ensure it’s set to “Auto” (not “Emergency Heat”) and avoid drastic temperature adjustments.
5. Monitor Defrost Cycles: Observe if aux heat runs only briefly during defrost (normal) or remains on afterward (concerning).
6. Call a Licensed HVAC Technician: If problems persist, schedule a professional inspection focusing on refrigerant levels, electrical components, and system calibration.

Mini Case Study: The Winter Bill Shock

Sarah in central Ohio noticed her January electric bill had nearly doubled compared to the previous year. Her thermostat frequently displayed “Aux Heat,” even on days when temperatures were in the mid-40s. She changed the filter and cleared snow from the outdoor unit, but the issue continued. A technician diagnosed a refrigerant leak and repaired the line, recharging the system. Post-repair, auxiliary heat usage dropped by 70%, and her next bill returned to normal. The early intervention prevented compressor damage and saved hundreds in energy costs.

Tips to Minimize Unnecessary Auxiliary Heat Use

Tip: Set your thermostat between 68°F and 70°F during winter. Avoid cranking it up past 72°F, which forces auxiliary heat activation.

• Use programmable thermostats wisely: Gradual setbacks (no more than 5–7 degrees) reduce recovery strain.
• Seal air leaks in windows, doors, and ductwork to improve heating efficiency.
• Schedule annual HVAC maintenance to catch refrigerant, coil, and blower issues early.
• Ensure vents and registers are unobstructed by furniture or rugs.
• Upgrade to a dual-fuel system (heat pump + gas furnace) in very cold climates for better efficiency.

FAQ

Is auxiliary heat the same as emergency heat?

No. Auxiliary heat runs automatically to assist the heat pump when needed. Emergency heat must be manually activated by the user (via thermostat switch) and bypasses the heat pump entirely—used only if the primary system fails.

How much does auxiliary heat increase my energy bill?

Electric resistance heating (aux heat) typically costs $0.36–$0.50 per kWh to operate, while heat pumps average $0.12–$0.18 per kWh for the same output. Extended use can add $100–$300 monthly in colder regions.

Can I prevent auxiliary heat from turning on?

You cannot disable it safely—it’s a critical backup—but you can reduce reliance by maintaining your system, insulating your home, and avoiding large thermostat adjustments.

Conclusion: Take Control of Your Heating System

Auxiliary heat serves an important role in keeping homes warm during cold snaps, but it’s not meant to carry the full heating load. Recognizing when it runs unnecessarily empowers homeowners to act before small issues become expensive failures. By staying alert to warning signs, performing basic maintenance, and seeking professional help when needed, you can ensure your heat pump operates efficiently and economically all winter long.

🚀 Don’t wait for high bills or system failure. If your auxiliary heat seems overactive, take action today—inspect, maintain, and consult an HVAC expert. Your comfort and wallet will thank you.