Heat Pump Auxiliary Heat vs Emergency Heat: Key Differences and When to Use

Heat pump systems have two backup heating modes—auxiliary (aux) heat and emergency heat—but homeowners often confuse their functions and when to use each. This article clarifies how aux heat and emergency heat operate, why systems switch between them, costs and efficiency differences, thermostat settings, troubleshooting tips, and best practices for American households.

Feature	Auxiliary Heat	Emergency Heat
Primary Use	Automatic supplemental heating when heat pump output is insufficient	Manual fallback when heat pump fails or is off
Control	Activated automatically by thermostat	Activated manually by user on thermostat
Cost	Higher energy cost due to electric resistance or fossil fuel backup	Usually highest cost; used only during heat pump failure
Efficiency	Lower than heat pump but intended for temporary supplement	Lowest efficiency; bypasses heat pump entirely

How Heat Pumps Heat And Why Backup Modes Exist

Heat pumps transfer heat from outside to inside using refrigerant and a compressor, providing efficient heating down to moderate cold temperatures. As outside temperatures fall, a heat pump’s ability to extract heat diminishes and the system may require supplemental heat. Auxiliary heat supplements the heat pump automatically when demand exceeds the heat pump’s output, while emergency heat is a manual override that disables the heat pump and relies solely on backup heat sources.

What Is Auxiliary (Aux) Heat?

Auxiliary heat, commonly called aux heat or backup heat, typically uses electric resistance heaters, a dual-fuel gas furnace, or another secondary heat source integrated with the heat pump system. The thermostat switches to aux heat when the heat pump cannot meet the setpoint or during rapid temperature recovery cycles.

When Aux Heat Activates

Aux heat activates under several common conditions: when the outdoor temperature is very low and the heat pump’s capacity drops, during defrost cycles when the heat pump temporarily reduces output, or when a sudden large thermostat setback requires fast recovery to the setpoint. The thermostat monitors both room temperature and compressor runtime to decide when to engage aux heat.

How Aux Heat Affects Energy Use

Auxiliary heat is generally less efficient than the heat pump because electric resistance heating converts electricity to heat at nearly 100% efficiency but lacks the multiplier effect of heat transfer. Energy bills can rise when aux heat is used frequently, especially in all-electric setups with resistance strips.

What Is Emergency Heat?

Emergency heat is a thermostat setting that forces the system to use only the backup heating source and prevents the heat pump from operating. It’s intended for situations when the heat pump is malfunctioning, frozen, or needs servicing. Emergency heat ensures the home remains warm but at the expense of efficiency and higher operating cost.

When To Use Emergency Heat

Emergency heat is appropriate when the heat pump stops producing heat due to mechanical failure, compressor issues, or control problems. It should also be used when outdoor units are iced over and cannot recover through normal defrost cycles, and only after basic troubleshooting confirms the heat pump cannot be safely used.

Risks Of Prolonged Emergency Heat Use

Relying on emergency heat for extended periods can dramatically increase energy bills and wear backup heating components. Because emergency heat bypasses heat pump operation, it eliminates the efficiency advantage and may lead to higher maintenance costs if used instead of addressing the root cause.

Thermostat Settings: Aux Heat Vs Emergency Heat

Thermostat models vary, but most modern programmable thermostats and smart thermostats provide both “Aux Heat” and “Emergency Heat” options. Aux heat is automatic and typically labeled “Aux” or “Aux/E” on the display. Emergency heat is a manual user selection often labeled “Emergency Heat” or “EM Heat.”

Common Thermostat Behaviors

• When set to normal heating, the thermostat cycles the heat pump and adds aux heat automatically as needed.
• When set to emergency heat, the thermostat disables the compressor and only runs the backup heat source.
• Smart thermostats may display alerts explaining why aux heat engaged, using runtime and temperature algorithms.

Costs And Efficiency Comparison

A clear understanding of costs helps homeowners make informed decisions. Heat pumps are typically the most efficient option, aux heat less efficient, and emergency heat the least efficient. Cost differences depend on the backup heat type: electric resistance, natural gas, or oil.

Heating Mode	Typical Efficiency	Relative Operating Cost
Heat Pump (Normal)	High (COP 2–4+)	Lowest
Auxiliary Heat (Electric Strips)	Low (COP ~1)	High
Auxiliary Heat (Gas Furnace)	Moderate to High (depends on AFUE)	Moderate
Emergency Heat	Lowest (often uses resistance or furnace operated alone)	Highest

Troubleshooting When Aux Or Emergency Heat Appears

Unexpected use of aux or emergency heat often signals a problem. Basic troubleshooting steps can identify common issues before calling a technician.

Quick Troubleshooting Checklist

• Check thermostat mode—ensure it is set to “Heat” and not “Emergency Heat.”
• Replace thermostat batteries and verify correct wiring and settings for heat pump type.
• Inspect outdoor unit for ice, debris, or blocked airflow; clear snow and ensure adequate clearance.
• Check air filters and indoor airflow; restricted airflow can reduce heat pump efficiency.
• Look for error codes on the thermostat or HVAC control board indicating compressor or sensor faults.

When To Call A Professional

If troubleshooting does not restore normal heat pump operation, a licensed HVAC technician should diagnose compressor problems, refrigerant leaks, control board issues, or defrost cycle malfunctions. Professional service can prevent unnecessary energy costs and further equipment damage.

Best Practices For Reducing Aux And Emergency Heat Reliance

Reducing unnecessary use of backup heating improves comfort and lowers costs. Preventive maintenance and proper thermostat programming help ensure the heat pump handles most heating demands.

• Schedule annual professional maintenance to inspect refrigerant levels, defrost controls, and electrical components.
• Use a programmable or smart thermostat to avoid large temperature setbacks that trigger aux heat frequently.
• Improve home insulation and seal air leaks to reduce heating load and minimize backup heat use.
• Consider a dual-fuel system pairing a heat pump with a high-efficiency gas furnace for cost-effective backup in very cold climates.

Dual-Fuel Systems And How They Use Aux Heat

Dual-fuel systems combine a heat pump with a gas furnace so the thermostat or control system can switch from electric heat pump to gas furnace at a pre-set outdoor temperature designed to optimize cost and efficiency. In this setup, the gas furnace often functions as both auxiliary and emergency heat.

Advantages Of Dual-Fuel Configurations

• Cost Optimization: Automatically switches to the most economical heat source based on outdoor temperature or energy prices.
• Improved Comfort: Provides reliable heating during extreme cold while maintaining heat pump efficiency during milder weather.
• Reduced Electric Load: Limits use of expensive electric resistance strips.

Smart Thermostats And Energy Monitoring

Smart thermostats enhance control over aux and emergency heat by providing alerts, runtime reports, and adaptive algorithms. These devices can identify inefficient aux heat cycles and advise homeowners how to adjust settings or schedule maintenance.

Useful Smart Thermostat Features

• Mode alerts when emergency heat is selected or when aux heat runs excessively.
• Energy usage estimates for different heating modes to compare costs.
• Remote controls and scheduling to avoid unnecessary setbacks or rapid recovery demands.

Common Misconceptions About Aux And Emergency Heat

Misunderstandings often lead to improper use. Clarifying these points helps homeowners avoid unnecessary expense.

• Myth: Emergency heat is the same as aux heat. Fact: Emergency heat disables the heat pump and uses only backup heat; aux heat supplements the heat pump automatically.
• Myth: Aux heat indicates a broken heat pump. Fact: Aux heat can be a normal response to low outdoor temperatures or rapid recovery needs.
• Myth: Always keep emergency heat enabled in winter. Fact: Prolonged use of emergency heat is costly and only recommended when the heat pump is malfunctioning.

Practical Examples And Scenarios

Example 1: On a cold morning when the thermostat is set from 60°F to 70°F, the heat pump may run but aux heat will engage temporarily to accelerate recovery. Example 2: If the outdoor unit fails due to a compressor fault, the homeowner sets the thermostat to emergency heat to keep the home warm until repairs are completed.

Key Takeaways For Homeowners

Auxiliary heat is an automatic supplement used to maintain setpoint when the heat pump can’t meet demand, while emergency heat is a manual override used only when the heat pump is inoperable. Understanding these distinctions helps manage energy costs, maintain comfort, and identify when professional HVAC service is needed.

Resources And Further Reading

Reliable resources include the U.S. Department of Energy for heat pump basics, manufacturer manuals for system-specific guidance, and local utility programs that offer incentives for efficient heat pump installations and dual-fuel systems.

U.S. Department Of Energy — Heat Pump Systems ASHRAE