When Does a Heat Pump Switch to Emergency Heat and How to Manage It

A heat pump switches to emergency heat when the heat pump can’t meet the thermostat’s demand for heat, or when the system detects a fault, low outdoor temperature, or specific thermostat settings trigger the backup. This article explains the triggers, thermostat behavior, signs of emergency heat, testing methods, costs, and maintenance to avoid unnecessary operation.

Trigger	What Happens	Action
Outdoor Temperature Too Low	Heat pump becomes inefficient; emergency heat engages	Monitor temp limits or install cold-climate heat pump
Thermostat Manual Selection	User selects EM/H, backup heat activates	Switch back to heat pump when temps rise
Compressor Or Defrost Fault	System forces backup to prevent loss of heat	Schedule HVAC service
Power Or Relay Failure	Backup system may run or system shuts down	Check breakers and call technician

How Heat Pumps And Emergency Heat Work

Heat pumps move heat between outdoor and indoor environments using a compressor and refrigerant. In heating mode, they extract outdoor heat and transfer it inside. Emergency heat, often called backup heat, is usually electric resistance coils or a gas furnace that provides heat when the heat pump cannot operate effectively.

Emergency heat is strictly a backup and is typically more expensive per unit of heat than the heat pump. It ensures continuous indoor comfort during extreme cold, equipment failure, or situations where the heat pump cannot defrost or maintain setpoint.

Primary Triggers For Switching To Emergency Heat

Low Outdoor Temperatures And Efficiency Limits

Most conventional air-source heat pumps lose efficiency as outdoor temperatures drop. When the heat pump can’t deliver enough heat to maintain the thermostat setpoint, the thermostat or control board may stage in emergency heat. This is common when temperatures fall below the manufacturer-specified balance point, often between 25°F and 40°F depending on system design.

Thermostat Settings And Manual Selection

Many thermostats include a switch or menu option labeled EM/HEAT or AUX/EMERGENCY. When set to EM, the thermostat intentionally disables the compressor and forces the backup heat. Users sometimes choose EM for immediate warm air or during perceived heat pump failure, but manual selection should be temporary.

System Faults And Compressor Failure

If the compressor fails, the control board may automatically engage the backup heat to maintain temperature. Similarly, control faults, refrigerant leaks, or sensor failures can prompt a safety routine that uses emergency heat until repairs are made.

Defrost Cycle Confusion

During defrost cycles the heat pump temporarily reverses to melt ice on the outdoor coil, which can reduce heat output. The thermostat may add auxiliary resistance heat to prevent indoor temperature drops. This is typically brief and should not be confused with full emergency heat operation.

Power Problems And Electrical Failures

Partial electrical faults can disable the outdoor unit while leaving indoor electric backup available. In some configurations, loss of outdoor unit communication results in the thermostat switching to emergency to preserve heat delivery.

How Thermostats Decide Between Heat Pump And Emergency Heat

Modern thermostats use logic based on sensor inputs, stage demands, and time delays. Staging often employs an auxiliary heat lockout that prevents backup heat use unless the heat pump cannot reach setpoint within a defined time or when outdoor temperature is below a configured threshold.

Smart thermostats may learn how long the heat pump will take to reach setpoint and allow backup heat only when necessary, reducing energy waste. Older thermostats use simpler timers or manual switches and may be more likely to call for emergency heat.

Signs That The System Is Running In Emergency Heat

• Warmer, drier air: Electric resistance heat feels hotter than the softer output from the heat pump.
• Markedly higher utility bills: Emergency heat costs more per BTU, so energy usage and bills often spike.
• Thermostat display: Many thermostats show “EM” or “Aux Heat On.”
• Outdoor unit inactivity: If the outdoor compressor is off but indoor heat continues, emergency heat may be running.

How To Test Whether Emergency Heat Engages Correctly

Testing helps confirm that the system switches and returns correctly. To test, set the thermostat to heat and lower the setpoint to allow the heat pump to run, then raise the setpoint quickly to observe staging. On manual thermostats, flip to EM briefly to see backup heat engage. Avoid prolonged use of emergency heat during testing.

During tests, monitor the outdoor unit. If the outdoor compressor never runs but the home warms, the system may be stuck in emergency heat or the compressor may be inoperative.

Costs And Energy Considerations

Emergency heat is typically provided by electric resistance coils or a fossil-fuel furnace. Electric resistance is the most expensive per delivered BTU and can double or triple heating costs if used heavily. Gas backup can be cheaper but depends on local gas prices and system efficiency.

Minimizing emergency heat use preserves energy and reduces bills. Strategies include installing a cold-climate heat pump, improving home insulation, and configuring thermostat lockouts and setpoints to favor heat pump operation until it reaches its effective balance point.

When Emergency Heat Is Normal Versus A Sign Of Trouble

Short activations during extreme cold or defrost cycles are normal. Extended or frequent emergency heat use, especially when outdoor temperatures are within the pump’s rated range, suggests a problem such as compressor failure, refrigerant leak, control board fault, or a misconfigured thermostat.

If emergency heat runs constantly, schedule professional diagnostics to identify mechanical faults, improper wiring, or incorrect thermostat setup.

Preventing Unnecessary Emergency Heat Engagement

• Install a thermostat with heat pump-specific programming and auxiliary heat lockout.
• Upgrade to a cold-climate heat pump designed for low-temperature efficiency.
• Ensure regular HVAC maintenance, including refrigerant checks, coil cleaning, and electrical inspections.
• Improve home envelope performance: insulation, air sealing, and proper ventilation reduce load on the heat pump.

Troubleshooting Checklist For Homeowners

1. Check thermostat mode: Ensure it’s not manually set to EM/AUX.
2. Inspect outdoor unit: Look for ice, unusual noises, or inactivity.
3. Check breakers and disconnect switches to ensure power to outdoor compressor.
4. Observe cycles: Note whether auxiliary heat engages during defrost or continuously.
5. Review utility usage: A sudden bill increase points to prolonged emergency heat.
6. Call an HVAC pro if the compressor won’t run, or if refrigerant or electrical faults are suspected.

When To Call A Professional

Contact licensed HVAC service if the outdoor unit does not run when the thermostat calls for heat, if ice accumulation persists, if emergency heat runs continuously without reason, or if unusual sounds or smells occur. Technicians can test compressors, controls, refrigerant levels, and thermostat wiring.

Timely repair prevents reliance on costly emergency heat and reduces equipment strain and utility costs.

Upgrades And Long-Term Solutions

To reduce emergency heat dependence, consider a variable-speed inverter heat pump that performs better in cold conditions or a dual-fuel system that automatically uses a gas furnace efficiently when temperatures fall below a set threshold.

Adding zoning controls, supplemental insulation, and a smart thermostat also helps optimize heat pump use and minimize backup operation.

Key Takeaways For Managing Emergency Heat

Emergency heat activates when the heat pump cannot meet demand due to low outdoor temperatures, faults, thermostat settings, or power issues. Short activations can be normal; prolonged or frequent use signals trouble or inefficient system design.

Homeowners should monitor thermostat indicators, understand the behavior of their specific system, maintain HVAC equipment, and consult professionals for persistent issues to avoid unnecessary energy costs and ensure reliable heating. For further guidance, reputable resources include the U.S. Department of Energy and industry HVAC specialists.