Heat Pump Emergency Heat: Causes, When to Use It, and Troubleshooting

Heat pump emergency heat is a backup heating mode that activates when the primary heat pump cannot meet indoor temperature needs or has failed. This article explains how emergency heat works, when to use it, common causes, troubleshooting steps, and tips to minimize energy costs while keeping a home safe and comfortable.

Topic	Quick Facts
What It Is	Auxiliary heat source used when heat pump can’t provide heat
Common Triggers	Outdoor unit failure, defrost cycle issues, thermostat settings
Typical Backup	Electric resistance coils, gas furnace, or hydronic systems
Energy Impact	Costs can be 2–5× higher than normal heat pump mode
When To Call Tech	Persistent emergency heat, unusual noises, no heat at all

How Heat Pump Emergency Heat Works

A heat pump extracts heat from outdoor air and moves it indoors during heating mode. When the outdoor temperature or a system fault prevents adequate heat transfer, the thermostat can switch to emergency heat. Emergency heat relies on a secondary source such as electric resistance coils, a gas furnace, or a boiler, which provides direct heat without using the heat pump compressor.

Emergency heat differs from auxiliary (aux) heat in that emergency mode typically disables the heat pump entirely and forces full reliance on the backup heat source. Auxiliary heat may run alongside the heat pump when extra heat is briefly required.

When The Thermostat Automatically Uses Emergency Heat

Thermostats may automatically call for emergency heat if they detect the heat pump cannot maintain the set temperature. Typical triggers include repeated failure to reach setpoint, sensor errors, or communication faults between thermostat and outdoor unit.

The thermostat also goes into emergency mode if manual controls are set to “EM” or “Emergency Heat.” Users should confirm the thermostat setting before assuming a system breakdown.

Common Causes For Emergency Heat Activation

Outdoor Unit Failure Or Low Refrigerant

A malfunctioning compressor, locked rotor, or refrigerant leak reduces heating capacity. When the heat pump cannot move enough heat indoors, the thermostat may switch to emergency heat to maintain comfort.

Defrost Cycle Problems

During cold, humid weather the outdoor coil can ice over. Normal defrost cycles melt the ice, but a stuck or failed defrost control can leave the unit iced and ineffective, prompting emergency heat.

Reversing Valve Or Control Board Faults

The reversing valve enables the system to switch between heating and cooling. If it sticks or the control board fails, the heat pump might not enter heating mode, so the system falls back on emergency heat.

Thermostat Communication Or Sensor Issues

Incorrect thermostat wiring, bad sensors, or outdated firmware can misinterpret system status and call for emergency heat. Diagnosing thermostat errors early avoids unnecessary use of costly backup heat.

Extreme Cold Or Demand Surges

While modern cold-climate heat pumps operate efficiently at low temperatures, prolonged extremes or sudden temperature drops can reduce output and trigger emergency heat to maintain setpoint.

How To Tell If Emergency Heat Is On

The thermostat display usually indicates “EM Heat,” “Emergency Heat,” or a similar message. Other signs include the outdoor unit being off while indoor electric coils or a furnace runs, or a sudden spike in energy use on utility bills.

Homeowners may also notice a different air delivery smell or warmer-than-usual outlet temperatures from electric strip heaters. If uncertain, check the thermostat mode and inspect the outdoor unit for operation.

Energy And Cost Considerations

Emergency heat—especially electric resistance heat—costs significantly more than heat pump heating because it converts electricity to heat at nearly 100% efficiency but without the heat pump’s coefficient of performance (COP). Expect costs two to five times higher than heat pump mode depending on local electricity rates and the backup system type.

Using emergency heat for extended periods can rapidly increase utility bills. Gas furnace backups may be more economical where natural gas is cheaper than electricity, but they still represent higher operating costs compared with an optimally functioning heat pump.

Troubleshooting Steps Before Calling A Technician

Check Thermostat Settings

Confirm the thermostat is not set to “Emergency Heat” or “EM.” If it is, switch back to “Heat” and set a slightly lower temperature to allow the heat pump time to restore normal operation.

Inspect Outdoor Unit

Look for obvious issues: heavy ice accumulation, debris blocking airflow, unusual noises, or a completely inactive compressor. Clear light snow and leaves; do not chip away thick ice—call a technician if ice persists.

Reset Power To System

Turn off the thermostat, switch off the HVAC breaker for 30 seconds, then restore power. This can reset controls and communications. If the system resumes normal heat mode, monitor for recurrence.

Check Air Filters And Indoor Vents

Restricted airflow strains the heat pump. Replace dirty filters and ensure vents are open and unobstructed. Improved airflow can reduce calls for emergency heat.

Monitor Outdoor Temperature And Defrost Cycles

If the outdoor temperature is very low, the heat pump may be operating but running auxiliary heat intermittently. Review the system’s behavior through a full defrost cycle; if cycles are abnormal, schedule service.

When To Call A Professional

Contact an HVAC professional if emergency heat remains active after basic troubleshooting, if the outdoor unit shows ice buildup that won’t clear, if there are unusual noises or smells, or if the thermostat indicates repeated errors. Persistent reliance on emergency heat indicates a system fault that requires diagnosis of refrigerant, compressor, reversing valve, defrost controls, or electrical components.

Choose a licensed technician experienced with heat pumps and cold-climate models. Request diagnostic documentation and an estimate for repairs versus replacement when equipment is older.

Preventive Maintenance To Avoid Emergency Heat Use

Regular maintenance reduces failures that trigger emergency heat. Recommended tasks include annual professional inspections, cleaning and replacing air filters monthly or as needed, and keeping outdoor units clear of vegetation and debris.

Seasonal checks before winter should verify refrigerant charge, defrost control operation, thermostat calibration, and electrical connections. Early detection of refrigerant leaks and compressor wear prevents sudden loss of heating capacity.

Smart Thermostats And Remote Monitoring

Smart thermostats can log system behavior, detect irregularities, and alert homeowners when emergency heat engages. Many models provide remote mode changes and usage reports that help identify whether emergency heat activation is transient or persistent.

Integrating home energy monitoring can reveal sudden consumption spikes tied to emergency heat, allowing faster responses and informed troubleshooting with technicians.

Safety Considerations

Electric resistance heat strips and older gas furnaces require attention to prevent hazards. Ensure adequate clearance around indoor units, listen for electrical arcing sounds, and be alert to unusual odors that could indicate combustion or wiring issues.

Shut off power and contact a technician if there is smoke, burning smells, or tripped breakers. Never attempt complex electrical repairs without professional training.

Minimizing Cost While Emergency Heat Is Active

If emergency heat must run temporarily, minimize costs by lowering the thermostat a few degrees, using zonal heating or programmable schedules, and improving insulation and window sealing to reduce overall load.

Use ceiling fans on low to distribute warm air evenly and wear warmer clothing indoors. If the backup is a gas furnace, consider setting the fan to “auto” to avoid unnecessary circulation losses.

Upgrading To Reduce Emergency Heat Usage

Upgrading to a modern cold-climate heat pump with higher low-temperature performance reduces reliance on emergency heating. Variable-speed compressors and improved refrigerants maintain capacity at lower temperatures.

Adding a backup that is more efficient than electric resistance—such as a gas furnace or an integrated air-source heat pump with hybrid systems—can decrease emergency heat operating costs during outages or extreme conditions.

Common Myths About Emergency Heat

Myth: Emergency Heat Is Better For Faster Warm-Up

Emergency heat can warm air quickly, but using it to accelerate heating is inefficient and expensive. Heat pumps maintain temperature more economically in most conditions.

Myth: Emergency Heat Extends Heat Pump Life

Using emergency heat does not reduce wear on the heat pump; in fact, frequent emergencies indicate underlying problems that may shorten system life if unaddressed.

Myth: Emergency Heat Runs Only During Breakdowns

Emergency heat can activate during extreme cold or during short defrost cycles. It is not solely indicative of a total failure.

Key Takeaways For Homeowners

• Emergency heat is a backup mode used when the heat pump cannot meet the setpoint or has a fault.
• Identify emergency heat by thermostat indicators, the outdoor unit being off, and higher-than-normal energy use.
• Perform simple checks first: thermostat mode, filters, outdoor unit clearance, and a power reset.
• Call a qualified HVAC technician for persistent emergency heat, ice buildup, refrigerant issues, or electrical faults.
• Regular maintenance and modern heat pump upgrades reduce emergency heat reliance and lower long-term costs.

Resources And Further Reading

Reliable sources for deeper information include the U.S. Department of Energy on heat pumps, ASHRAE guidance for residential HVAC, and manufacturer resources for specific brand diagnostics. Homeowners should consult local utility programs for rebates on cold-climate heat pump upgrades that minimize emergency heat dependence.

For immediate issues, contact a licensed HVAC service provider and reference system model numbers and thermostat make/model to speed diagnosis.