If you’ve noticed “EM Heat” or “Emergency Heat” illuminated on your thermostat during a cold snap, you may be wondering whether your heating system is malfunctioning or if you should take action. Understanding what this indicator means can help you avoid unnecessary energy costs and identify when your system needs professional attention.
This guide explains what emergency heat is, when it activates, how it differs from auxiliary heat, and what steps you should take when you see this warning on your thermostat.
What Does EM Heat Mean on a Thermostat?
Emergency heat, abbreviated as “EM Heat” or “E-Heat” on most thermostats, is a backup heating mode that bypasses your heat pump and relies entirely on electric resistance heating or a secondary heating source. When activated, your heat pump stops operating and your system uses only the auxiliary heating strips or backup furnace, which is significantly less energy-efficient and more expensive to run than normal heat pump operation.
How Heat Pumps Normally Work
To understand emergency heat, you first need to know how a standard heat pump operates. A heat pump transfers heat from the outdoor air into your home, even when temperatures drop below freezing.
This process is remarkably efficient because it moves existing heat rather than generating it from scratch. However, when outdoor temperatures fall below a certain threshold, typically between 25 and 40 degrees Fahrenheit depending on your system, the heat pump struggles to extract enough warmth from the cold air.
Auxiliary Heat Versus Emergency Heat
Most heat pump systems include auxiliary heat that activates automatically when the outdoor temperature drops too low or when your thermostat calls for a temperature increase of more than two or three degrees. This supplemental heat works alongside your heat pump to help reach your desired temperature more quickly.
Emergency heat is different because it shuts off the heat pump entirely and runs only the backup system. You typically activate emergency heat manually using a switch on your thermostat, though some systems may switch to emergency mode automatically if the heat pump fails.
When Should You Use Emergency Heat?
Emergency heat exists as a safety feature for specific situations when your heat pump cannot operate. You should only use this setting when absolutely necessary.
Appropriate Times to Switch to Emergency Heat
- Your heat pump has stopped working entirely and you’re waiting for repair service
- Ice has formed on your outdoor unit and the defrost cycle isn’t clearing it
- The outdoor unit is making unusual grinding, squealing, or banging noises
- Your system is blowing cold air continuously instead of warm air
- A qualified HVAC technician has instructed you to use emergency heat temporarily
If none of these conditions apply, your system should remain in its normal heating mode. Running emergency heat unnecessarily can double or triple your heating costs compared to normal heat pump operation.
Why Is Emergency Heat So Expensive?
The cost difference between heat pump operation and emergency heat comes down to efficiency. A properly functioning heat pump can produce three to four units of heat for every unit of electricity it consumes, because it’s moving heat rather than creating it.
Electric resistance heating strips, which power most emergency heat systems, convert electricity directly into heat with a one-to-one ratio. This means emergency heat uses roughly two to three times more electricity to produce the same amount of warmth your heat pump would generate under normal conditions.
Monthly Cost Impact
If your heating bill normally runs $150 per month during winter with your heat pump, running emergency heat continuously could push that cost to $300 or more. The exact increase depends on your local electricity rates, outdoor temperatures, home insulation, and thermostat settings.
Even a few days of unnecessary emergency heat use can add $50 to $100 to your monthly utility bill. This is why you should switch back to normal heating mode as soon as your heat pump is repaired or the temporary problem resolves.
Common Reasons the EM Heat Light Comes On
If you see the emergency heat indicator and didn’t manually activate it, several issues might be triggering the backup system. Some thermostats display emergency heat when auxiliary heat runs, which can cause confusion.
Accidental Manual Activation
Many thermostats have a small switch or button that toggles emergency heat on and off. You or a family member may have accidentally flipped this switch while adjusting the temperature.
Check your thermostat’s emergency heat switch first before assuming a mechanical problem. This simple oversight accounts for a significant portion of emergency heat concerns.
Heat Pump Malfunction
A failing compressor, refrigerant leak, frozen coils, or electrical problem can prevent your heat pump from operating. When the system detects that the heat pump isn’t producing heat, some smart thermostats automatically switch to emergency heat to keep your home warm.
If your outdoor unit isn’t running but you’re still getting heat, your system is likely using emergency heat because the primary heating method has failed. You’ll need a licensed HVAC technician to diagnose and repair the underlying issue.
Defrost Cycle Confusion
Heat pumps periodically reverse their operation to melt frost and ice from the outdoor coil. During a defrost cycle, which typically lasts 5 to 15 minutes, the auxiliary heat may activate to prevent cold air from blowing through your vents.
Some homeowners see the auxiliary or emergency heat light during these cycles and worry something is wrong. This is completely normal operation and doesn’t require any action on your part.
Extreme Cold Temperatures
When outdoor temperatures drop significantly below your heat pump’s designed operating range, the system may struggle to keep up. If your home temperature falls more than three degrees below your thermostat setting, some systems will automatically engage emergency heat.
This response protects your home from dangerous temperature drops, but you should have an HVAC professional evaluate why your heat pump couldn’t maintain the temperature. You may need a larger system, better insulation, or thermostat adjustments to prevent this issue.
What to Do When You See EM Heat Activated
Your response depends on whether you intentionally turned on emergency heat or if the system activated it automatically. Taking the right steps quickly can prevent damage to your system and avoid excessive energy costs.
Step One: Check the Thermostat Settings
Look for an emergency heat switch, button, or setting on your thermostat. If it’s in the “on” or “emergency” position and you don’t recall changing it, switch it back to “auto” or “heat” mode.
Wait 10 to 15 minutes to see if your heat pump restarts and begins operating normally. You should hear the outdoor unit running and feel warm air from your vents.
Step Two: Inspect the Outdoor Unit
Go outside and visually examine your heat pump. Look for excessive ice buildup, which might indicate a stuck defrost cycle or low refrigerant.
Listen for unusual sounds like grinding metal, squealing, or rattling. Check that nothing is blocking airflow around the unit, including snow, leaves, or debris covering the coils.
Step Three: Check Your Air Filter
A clogged air filter restricts airflow and forces your system to work harder, sometimes triggering auxiliary or emergency heat unnecessarily. Replace or clean your filter if it appears dirty or hasn’t been changed in the past 30 to 90 days.
Proper airflow helps your heat pump operate efficiently and prevents ice buildup on the coils. This simple maintenance step can prevent many heating problems.
Step Four: Call for Professional Help
If your heat pump doesn’t restart after switching off emergency heat, or if you notice ice, strange noises, or no outdoor unit activity, contact a licensed HVAC technician. Heat pump repairs involve refrigerant, electrical components, and pressurized systems that require specialized training and tools.
Don’t attempt to chip ice off the coils, add refrigerant yourself, or disassemble the outdoor unit. These actions can damage the system and create safety hazards.
Preventing Unnecessary Emergency Heat Use
Regular maintenance and smart thermostat management help your heat pump run efficiently all winter without relying on expensive emergency heat. A few preventive steps can save hundreds of dollars in energy costs.
Schedule Annual Professional Maintenance
Have a qualified technician inspect your heat pump each fall before heating season begins. They’ll check refrigerant levels, test the defrost cycle, clean coils, lubricate moving parts, and verify electrical connections.
This service typically costs $100 to $200 but catches small problems before they become expensive failures. Well-maintained heat pumps rarely need emergency heat except during extreme weather.
Keep Your Outdoor Unit Clear
Maintain at least two feet of clearance around your outdoor unit. Remove leaves, grass clippings, snow, and ice that accumulate near the unit, but don’t hammer or chip ice directly off the coils.
Good airflow prevents ice buildup and helps your system run efficiently. Trim back shrubs and vegetation during warmer months to maintain proper spacing.
Avoid Large Thermostat Setbacks
Dropping your thermostat by more than two or three degrees overnight or when you’re away forces your system to work harder to recover the temperature. This often triggers auxiliary heat and increases energy consumption.
Set your thermostat to a consistent, comfortable temperature or make smaller adjustments of one or two degrees. Modern programmable and smart thermostats can learn your patterns and adjust gradually to minimize auxiliary heat use.
Improve Home Insulation and Sealing
Heat loss through poorly insulated attics, walls, and gaps around windows and doors forces your heat pump to run longer and work harder. Adding insulation and sealing air leaks reduces your heating demand and helps your system maintain temperature without backup heat.
These upgrades also improve comfort and lower energy bills year-round. Focus first on attic insulation and sealing gaps around doors, windows, and electrical outlets.
Can You Run Emergency Heat Continuously?
Technically, your system can operate in emergency heat mode indefinitely without immediate damage to the equipment. However, you should not run emergency heat for extended periods unless your heat pump is completely broken and you’re waiting for repair parts or service.
The high operating costs and energy consumption make emergency heat impractical for routine use. If you find yourself relying on emergency heat for more than a day or two, prioritize getting your heat pump repaired or consider whether your system is properly sized for your home’s heating needs.
Cold Climate Considerations
Homeowners in regions with prolonged freezing temperatures should ensure their heat pump is rated for cold-climate operation. Modern cold-climate heat pumps can operate efficiently down to -15 degrees Fahrenheit or lower, minimizing reliance on auxiliary or emergency heat.
If you frequently need emergency heat during typical winter weather, your system may be undersized, improperly installed, or not designed for your climate. A heating system evaluation can identify whether you need equipment upgrades or supplemental heating sources.
Understanding Your Specific Thermostat
Emergency heat indicators and controls vary by thermostat brand and model. Some display “Aux” for auxiliary heat and “Em” for emergency heat, while others use “E-Heat,” “Emergency,” or a light indicator without text.
Consult your thermostat’s manual or the manufacturer’s website to understand exactly what each indicator means for your model. What one brand calls auxiliary heat, another might label as emergency heat, causing confusion about whether you have a problem.
Smart Thermostat Features
Many smart thermostats track how often and how long your system uses auxiliary heat. This data helps you identify patterns, such as auxiliary heat running every morning when you raise the temperature or activating frequently during cold snaps.
Review this information in your thermostat’s app to determine whether your usage is normal or indicates a system problem. Excessive auxiliary heat use often signals that maintenance or repairs are needed.
Moving Forward with Confidence
Now that you understand what emergency heat means and when to use it, you can respond appropriately when you see that indicator on your thermostat. Remember that emergency heat is a temporary backup solution, not a routine heating mode, and should be turned off as soon as your heat pump is functioning properly again.
Take a moment to locate the emergency heat switch on your thermostat and verify that it’s in the correct position. If your system is currently using emergency heat and you’re not sure why, follow the diagnostic steps outlined here and contact a qualified HVAC professional if the problem doesn’t resolve quickly.
Whether you’re troubleshooting a heating issue, planning seasonal maintenance, or exploring ways to improve your home’s comfort and efficiency, we’ve gathered practical guidance to help you make informed decisions about your home systems and spaces.