Emergency heat, often abbreviated as “Em Heat” or “Aux Heat,” is a supplemental heating system built into heat pumps to provide warmth when primary heating methods struggle. This secondary heating system activates either manually through your thermostat’s emergency heat setting or automatically when temperatures drop below a certain threshold. Understanding when and how to properly use emergency heat can significantly impact your home’s comfort and energy bills during extreme cold weather. This backup system typically uses electric resistance heating elements, gas, or oil furnaces to generate additional heat when your heat pump can’t extract sufficient warmth from the outside air.
Emergency heat refers to a secondary or auxiliary heating system that serves as a backup when a heat pump can’t operate efficiently. Unlike the standard heat pump operation that transfers heat from outside air to inside your home, emergency heat generates new heat through alternative means. It’s essentially a safety net designed to keep your home warm when outside temperatures become too extreme for normal heat pump operation.
Most heat pumps are designed to operate efficiently down to about 25-30°F (-4 to -1°C). Below these temperatures, the heat pump struggles to extract sufficient heat from the outside air, making the emergency heat function necessary. This backup system ensures your home stays warm even during the coldest weather conditions.
Emergency heat typically comes from one of three sources: electric resistance heating elements (most common), a gas furnace, or an oil furnace. In dual-fuel systems, the backup is often a fossil fuel heating source that kicks in when electricity-based heat becomes less efficient.
How Emergency Heat Works
Heat pumps normally work by extracting heat from outside air and transferring it indoors. However, when temperatures drop significantly, this process becomes less efficient. Emergency heat bypasses the normal heat pump operation entirely, shutting down the outdoor unit and relying solely on the secondary heating source.
There are two primary ways emergency heat activates:
- Automatic activation: Most modern heat pumps automatically engage auxiliary heat when outside temperatures fall below a preset threshold (typically around 35°F or 1.7°C).
- Manual activation: Users can manually switch to emergency heat mode through their thermostat when they notice insufficient heating or when the heat pump is malfunctioning.
When emergency heat runs, you’ll often notice an indicator light on your thermostat. This mode is significantly less efficient than normal heat pump operation, as it’s creating new heat rather than moving existing heat from outdoors to indoors.
Types of Emergency Heat Sources
| Heat Source | Efficiency | Operating Costs | Common Applications |
|---|---|---|---|
| Electric Resistance | 100% conversion efficiency | High | Most common in all-electric homes |
| Natural Gas Furnace | 80-98% efficiency | Moderate | Dual-fuel systems in cold climates |
| Oil Furnace | 80-90% efficiency | High | Older homes or areas without natural gas |
| Propane Furnace | 80-98% efficiency | High | Rural areas without natural gas service |
When to Use Emergency Heat
Knowing when to use emergency heat is crucial for balancing comfort and energy costs. Emergency heat should only be manually activated in specific situations, not as a regular heating mode during cold weather. Here are appropriate scenarios for using emergency heat:
- Extreme cold: When temperatures fall well below your heat pump’s efficient operating range (typically below 20°F or -6.7°C)
- Heat pump malfunction: If your outdoor unit is damaged, frozen, or not functioning properly
- During repairs: While waiting for heat pump repairs when the outdoor unit needs service
- Ice accumulation: When excessive ice has built up on the outdoor unit and the defrost cycle can’t keep up
It’s important to note that many modern heat pumps automatically engage auxiliary heat when needed, making manual activation unnecessary in most circumstances. Using emergency heat when not required can lead to significantly higher energy bills.
Temperature Guidelines for Emergency Heat
| Heat Pump Type | Standard Operating Range | Consider Emergency Heat Below |
|---|---|---|
| Standard Air Source | Down to 25-30°F (-4 to -1°C) | 20°F (-6.7°C) |
| Cold Climate Air Source | Down to -13°F (-25°C) | -10°F (-23°C) |
| Geothermal/Ground Source | Any outdoor temperature | Only during malfunction |
| Dual-Fuel Systems | Varies by fuel type | System switches automatically at optimal point |
Energy Consumption and Costs of Emergency Heat
One of the most significant considerations when using emergency heat is the increased energy consumption and associated costs. Emergency heat can use 2-3 times more electricity than standard heat pump operation, leading to substantially higher utility bills when used extensively.
Electric resistance heating, the most common form of emergency heat, is 100% efficient at converting electricity to heat. However, this is much less efficient than a heat pump, which can produce 2-3 units of heat for every unit of electricity consumed under optimal conditions.
Comparative Cost Analysis
| Heating Mode | Energy Efficiency | Approximate Cost per Hour* | Monthly Cost Impact |
|---|---|---|---|
| Standard Heat Pump | 200-300% (COP 2-3) | $0.50-$1.50 | Baseline |
| Emergency Heat (Electric) | 100% efficiency | $1.50-$4.50 | +100% to +300% |
| Emergency Heat (Natural Gas) | 80-98% efficiency | $1.00-$2.50 | +50% to +150% |
| Emergency Heat (Oil) | 80-90% efficiency | $2.00-$5.00 | +100% to +300% |
*Costs vary by region, energy rates, home size, insulation quality, and outside temperature
To minimize these costs, only use emergency heat when absolutely necessary, and address any issues with your heat pump promptly. Investing in a cold-climate heat pump or dual-fuel system can also reduce reliance on emergency heat in colder regions.
Signs Your Heat Pump Needs Emergency Heat
Recognizing when your heat pump is struggling can help you decide when emergency heat might be necessary. Pay attention to these warning signs that indicate your system might benefit from switching to emergency heat mode:
- Continuous running: Your heat pump runs constantly without cycling off
- Cold air from vents: The air coming from your vents feels cool or lukewarm instead of warm
- Inability to maintain temperature: Your home temperature falls several degrees below the thermostat setting
- Excessive frost or ice: The outdoor unit has significant ice buildup that isn’t clearing during defrost cycles
- Strange noises: The outdoor unit makes unusual grinding, scraping, or squealing sounds
- Outdoor unit failure: The outdoor component isn’t running or has visible damage
If you notice these signs during extreme cold, switching to emergency heat temporarily may be appropriate while you arrange for professional service. However, persistent issues require HVAC professional diagnosis rather than simply relying on emergency heat as a long-term solution.
How to Activate Emergency Heat
Most modern heat pumps will automatically engage auxiliary heat when needed, but there may be situations where manual activation is necessary. The process for activating emergency heat is straightforward but varies slightly depending on your thermostat type.
Digital Programmable Thermostats
- Locate the “Emergency,” “Em Heat,” or “Aux” setting on your thermostat
- Switch the thermostat from “Heat” mode to “Emergency Heat” or “Aux Heat”
- Confirm activation – most thermostats show an indicator light (often red) when emergency heat is running
- Set your desired temperature
Smart Thermostats
- Open your thermostat app or access the touchscreen interface
- Navigate to mode settings or system settings
- Select “Emergency Heat” or “Aux Heat” from the available options
- Confirm your selection when prompted
- Monitor system status through the app or display
Remember to switch back to normal “Heat” mode once temperatures rise or the issue with your heat pump is resolved. Leaving your system in emergency heat mode unnecessarily will result in significantly higher energy bills.
Troubleshooting Emergency Heat Issues
Even emergency heating systems can experience problems. If your emergency heat isn’t working properly, try these troubleshooting steps before calling a professional:
- Check power sources: Ensure circuit breakers haven’t tripped for your air handler or electric heating elements
- Verify thermostat settings: Confirm the thermostat is set to “Emergency Heat” and the temperature setting is above the current room temperature
- Replace batteries: If your thermostat uses batteries, try replacing them
- Check air filters: Clogged filters can restrict airflow and reduce heating effectiveness
- Inspect gas or oil supply: For dual-fuel systems, verify the fuel supply is uninterrupted
- Reset the system: Turn the system off at the thermostat, wait 30 seconds, then turn it back on
If these steps don’t resolve the issue, it’s time to contact an HVAC professional. Emergency heat failure during extreme cold weather is considered an urgent service need by most heating contractors.
Common Emergency Heat Problems and Solutions
| Problem | Possible Causes | Solutions |
|---|---|---|
| No heat at all | Power issues, failed heating elements | Check breakers, verify power supply, professional inspection |
| Insufficient heat | Dirty filters, undersized system | Replace filters, add supplemental heating, improve insulation |
| Emergency heat won’t turn off | Thermostat issues, stuck relay | Reset thermostat, professional electrical inspection |
| System short-cycling | Thermostat placement, airflow issues | Relocate thermostat, clean vents, check for blockages |
| Burning smells | Dust buildup, electrical issues | Turn off system immediately, professional service required |
Preventing Emergency Heat Activation
The best approach to emergency heat is minimizing how often you need it. Proper maintenance and home improvements can significantly reduce your reliance on emergency heat, saving money and extending your system’s lifespan.
- Regular maintenance: Schedule professional heat pump service before each heating season
- Change filters: Replace or clean air filters monthly during peak usage seasons
- Improve insulation: Upgrade attic and wall insulation to retain more heat
- Seal air leaks: Caulk windows, weatherstrip doors, and seal ductwork
- Install a cold-climate heat pump: Consider upgrading to models designed to work efficiently at lower temperatures
- Program your thermostat wisely: Avoid large temperature swings that might trigger emergency heat
- Keep outdoor unit clear: Remove debris, snow, and ice that could restrict airflow
Some homeowners install supplemental heating sources like space heaters or wood stoves for extreme weather events. This allows them to raise the ambient temperature enough to avoid emergency heat mode while still maintaining comfort.
Emergency Heat vs. Auxiliary Heat: Understanding the Difference
The terms “emergency heat” and “auxiliary heat” are often used interchangeably, but they represent different functions in a heat pump system. Understanding the distinction between these two modes can help you operate your system more efficiently.
Auxiliary Heat
Auxiliary heat activates automatically when:
- Your heat pump can’t maintain the desired temperature alone
- The temperature differential between the current temperature and thermostat setting is large (typically more than 2-3 degrees)
- The outdoor temperature falls below the heat pump’s efficient operating range
- The system is in defrost mode (temporarily reversing to clear ice from the outdoor coil)
This supplemental heat works alongside your heat pump to provide additional warmth when needed. The heat pump continues to operate, and the system balances efficiency by using both heating sources.
Emergency Heat
Emergency heat, in contrast, is manually activated and:
- Completely shuts down the outdoor heat pump unit
- Relies solely on the backup heating source (electric, gas, or oil)
- Is significantly less efficient than normal operation
- Should only be used when the heat pump is malfunctioning or during extreme cold beyond the heat pump’s capabilities
The key difference is that auxiliary heat supplements the heat pump automatically, while emergency heat bypasses it entirely as a manual override. Modern systems often use the same physical heating elements for both functions, but they operate in different modes.
Heat Pump Innovations Reducing Emergency Heat Reliance
Technology advances have significantly improved heat pump performance in cold weather, reducing the need for emergency heat. Recent innovations have extended the efficient operating range of heat pumps to much lower temperatures than was possible with older models.
Cold Climate Heat Pump Technology
Modern cold climate heat pumps incorporate several key technologies:
- Variable-speed compressors: Adjust output based on heating demands instead of simple on/off cycling
- Enhanced vapor injection: Improves efficiency and heating capacity at low temperatures
- Better defrost controls: More effective ice removal with less reliance on backup heat
- Improved refrigerants: New formulations that perform better in cold conditions
- Smart controls: Algorithms that optimize when to use heat pump versus backup heat
Leading manufacturers now offer heat pumps that can operate efficiently down to -13°F (-25°C) or lower, a significant improvement over traditional models that struggled below 30°F (-1°C).
Comparative Performance of Heat Pump Technologies
| Heat Pump Type | Effective Operation Range | Efficiency at 32°F (0°C) | Efficiency at 0°F (-18°C) | Approximate Cost Premium |
|---|---|---|---|---|
| Standard Air Source (Pre-2010) | Down to 30°F (-1°C) | COP 2.0-2.5 | Below efficient range | Baseline |
| Standard Air Source (Current) | Down to 20°F (-7°C) | COP 2.5-3.0 | COP 1.5-1.8 | +10-20% |
| Cold Climate Air Source | Down to -13°F (-25°C) | COP 2.5-3.5 | COP 1.8-2.2 | +30-50% |
| Geothermal/Ground Source | Any temperature | COP 3.0-5.0 | COP 3.0-4.5 | +100-300% |
COP (Coefficient of Performance) represents efficiency – higher numbers indicate better performance
For homeowners in colder climates, investing in a modern cold-climate heat pump can significantly reduce reliance on emergency heat, providing both comfort and long-term energy savings despite the higher initial cost.