Emergency heat is a mode on many heat pump systems that temporarily supplements the primary heating method. It activates when the heat pump cannot meet the desired indoor temperature, often due to severe outdoor conditions or a system fault. This guide explains what emergency heat is, when to use it, how it impacts energy usage, and how to operate it on common thermostats.
What Emergency Heat Is
Emergency heat, sometimes labeled as Em Heat or Aux (auxiliary) heat on a thermostat, is a backup heating method. In a heat pump system, the heat pump uses outside air to heat your home. When temperatures drop too low, the heat pump’s efficiency declines, and an electric resistance heater or another auxiliary system can kick in to provide the required warmth. Emergency heat bypasses the heat pump to deliver heat directly through the backup element, usually at a higher energy cost.
How Emergency Heat Works
In normal operation, the thermostat instructs the outdoor unit (the heat pump) to extract heat from outside air. The system uses a reversing valve to switch between heating and cooling modes. When emergency heat is engaged, the thermostat disables the heat pump’s primary cycle and turns on the backup heating source. The result is rapid, consistent warmth, but with reduced efficiency compared to normal heat pump operation.
When To Use Emergency Heat
Use emergency heat in these scenarios: outdoor temperatures are extremely cold and the heat pump struggles to maintain comfort, the heat pump is malfunctioning or in defrost mode for extended periods, or a temporary demand requires immediate warmth while awaiting service. Relying on emergency heat for extended periods significantly raises energy bills and can strain the backup system, so it should be a short-term solution until the heat pump is repaired or climatized.
Energy Implications And Efficiency
Typical heat pumps gain efficiency from extracting ambient heat from outside air. Emergency heat relies on electric resistance or other high-cost heat sources, which can quickly raise electricity usage. During emergency heat, homes may see higher utility bills and increased wear on the backup system. If you notice frequent or prolonged Em Heat usage, it’s wise to schedule a professional HVAC inspection to diagnose heat pump performance, refrigerant levels, airflow, or thermostat settings.
- Cost considerations: Emergency heat is usually the most expensive heating option in a heat pump system.
- Comfort consistency: Provides steady warmth, especially in extreme cold.
- System health: Frequent Em Heat cycles may indicate a failing heat pump or poor insulation.
How To Activate Emergency Heat On Common Thermostats
Most modern thermostats label the Em Heat or Aux Heat function clearly. Basic steps are similar across brands:
- Set the thermostat to Heat mode if it isn’t already.
- Look for a dedicated Emergency Heat, Aux, or Em Heat button or option in the menu.
- Activate Emergency Heat. The display may show “Em Heat” or “Aux” indicating backup heat is engaged.
- Monitor indoor temperature to ensure comfort is achieved. If the home becomes too warm or too cold, adjust the setpoint accordingly.
Note: If your thermostat doesn’t show a separate Em Heat option, consult the user manual or contact the HVAC installer. Some systems default to auxiliary heat automatically during very cold conditions, without a manual Em Heat toggle.
Common Thermostat Brands And Tips
Different brands label emergency heat differently. Here are quick references:
- <strongNest: Em Heat appears as Emergency Heat in the heat mode settings; may require you to toggle through options.
- Ecobee: Em Heat or Aux Heat is selectable within theHeat Confirmed menu; the badge on screen indicates backup heat in use.
- Lennox: Em Heat is typically shown on the main display when active; check the system status in the app for detailed readings.
- Carrier/Bryant/Trane: Em Heat is shown as Emergency or Aux in the thermostat interface; some models automatically switch to backup heat at low outdoor temperatures.
Troubleshooting Tips
If emergency heat is engaged unexpectedly or the system isn’t providing sufficient warmth, consider these checks:
- Check for air leaks or poor insulation that increase heating demand.
- Inspect air filters and ensure airflow is not restricted; clogged filters reduce heater efficiency.
- Verify outdoor unit is free of debris and there are no tripped breakers or blown fuses in the outdoor unit’s circuit.
- Confirm thermostat settings aren’t stuck on Em Heat; reprogram or reset the thermostat if needed.
- Schedule a professional maintenance visit to verify refrigerant levels, compressor operation, and thermostat calibration.
Preventive Measures To Reduce Reliance On Emergency Heat
Regular maintenance and smart home practices can minimize the need for emergency heat:
- Schedule annual HVAC tune-ups to ensure heat pump efficiency and refrigerant integrity.
- Replace air filters every 1–3 months during heavier use periods.
- Improve home insulation and seal air leaks around doors, windows, and ducts.
- Use programmable or smart thermostats to optimize heating schedules and avoid overheating or excessive cooling.