If you’ve spotted a switch or setting labeled “Em Heat” on your thermostat and wondered whether you should flip it, you’re looking at your heating system’s backup plan. Understanding this feature can prevent surprise utility bills and help you respond correctly when your primary heat source fails.
This guide explains what emergency heat is, when you actually need it, how it differs from auxiliary heat, and what activating it means for your energy costs and home comfort.
What Does Em Heat Mean on a Thermostat?
Em Heat, short for emergency heat, is a setting that bypasses your heat pump’s normal operation and runs only the backup heating system, typically electric resistance coils or a gas furnace. You activate it manually when your heat pump malfunctions or cannot keep up with heating demand, allowing your home to stay warm while you arrange repairs.
How Heat Pumps Normally Work
A heat pump moves warm air from outside into your home during winter, even when outdoor temperatures feel cold. This process uses significantly less electricity than generating heat directly because the system transfers existing warmth rather than creating it.
Heat pumps work efficiently down to certain outdoor temperatures, typically between 25°F and 40°F depending on the model and climate zone. Below that threshold, the system struggles to extract enough heat from frigid outdoor air.
The Role of Auxiliary Heat
When outdoor temperatures drop low enough that your heat pump alone cannot maintain your set temperature, auxiliary heat kicks in automatically. This supplemental heating works alongside your heat pump, not instead of it.
You might see “Aux Heat” displayed on your thermostat during very cold weather. This is normal operation and nothing to worry about, though it does cost more to run than the heat pump alone.
Emergency Heat Versus Auxiliary Heat
These terms sound similar but describe completely different situations. Auxiliary heat is an automatic helper that supports your heat pump during extreme cold, while emergency heat is a manual override you control.
When you switch to Em Heat mode, you turn off the heat pump entirely. Only the backup system runs, usually electric resistance heating strips installed in your air handler.
Why the Distinction Matters for Your Bill
Heat pumps can deliver three times more heating energy than the electricity they consume because they move heat rather than generate it. Electric resistance heat, by contrast, converts electricity directly into warmth at a one-to-one ratio.
Running on emergency heat can double or triple your heating costs compared to normal heat pump operation. A home that typically costs $150 monthly to heat might see bills approaching $300 to $450 if emergency heat runs continuously.
When You Should Actually Use Emergency Heat
The name “emergency” tells you exactly when this setting belongs in use. You should activate Em Heat only when your heat pump has broken down and cannot run at all.
Common scenarios that justify using emergency heat include a failed compressor, refrigerant leak, frozen outdoor unit, or damaged reversing valve. If your thermostat is calling for heat but no warm air comes from the vents, and you’ve confirmed the system has power, you likely have a heat pump failure.
What Emergency Heat Is Not For
Do not switch to Em Heat just because your home feels cold or you want faster heating. Do not use it to “boost” warmth during a cold snap or because you see the auxiliary heat indicator.
Do not leave emergency heat on after a technician repairs your heat pump. Many homeowners forget to switch the thermostat back to normal heating mode, then wonder why their next utility bill doubled.
How to Activate Emergency Heat
Most thermostats have a physical switch, button, or menu option labeled “Em Heat,” “Emergency Heat,” or “E-Heat.” The location and method vary significantly by thermostat model.
On older mechanical thermostats, you’ll typically find a small switch on the face or side of the unit. On digital and programmable models, you may need to access a settings menu or system mode screen.
Steps for Common Thermostat Types
For manual thermostats: Look for a switch that toggles between Heat, Off, Cool, and Em Heat positions. Slide or flip it to the Em Heat setting.
For digital thermostats: Press the System or Mode button repeatedly until Em Heat appears on the display, or enter the settings menu and select emergency heat from the system mode options.
For smart thermostats: Open the associated app or use the thermostat touchscreen to navigate to system settings, then select emergency heat mode. Some models require you to disable a safety lock first.
What Happens When You Turn On Em Heat
The moment you activate emergency heat, your thermostat sends a signal that prevents the outdoor heat pump unit from running. Only the indoor backup heating elements or furnace receive power.
You should feel warm air from your vents within a few minutes, assuming your backup system functions properly. The air may feel warmer than what your heat pump typically delivers because resistance heating produces higher supply air temperatures.
Indicators That Emergency Heat Is Running
Most thermostats display “Em Heat,” “Emergency Heat,” or a similar message when this mode is active. Some models show a light or icon, often in red or orange to signal abnormal operation.
Your outdoor heat pump unit will remain completely silent and still. If you hear it running or see the fan turning while Em Heat is on, something is wrong with either the thermostat or the control wiring.
How Long You Can Run Emergency Heat
Emergency heat is safe to run continuously until you can get your heat pump repaired. The backup heating system is designed to handle full-time operation, though you should schedule service as quickly as possible to minimize energy costs.
If you must use Em Heat for more than a few days, monitor your system to confirm it maintains comfortable temperatures without cycling on and off too frequently. Undersized backup heat may struggle to keep up during severe cold.
Troubleshooting Before Using Emergency Heat
Before you switch to Em Heat, rule out simple problems that don’t require backup heating. Check your circuit breaker panel to ensure the heat pump breaker hasn’t tripped.
Go outside and look at the outdoor unit. If it’s covered in ice or snow, carefully clear the buildup, making sure not to damage the fins or fan blades with sharp tools.
The Defrost Cycle Confusion
Heat pumps periodically run a defrost cycle that melts frost buildup on the outdoor coil. During defrost, which lasts five to fifteen minutes, the outdoor unit may steam dramatically, and indoor air may feel cool.
This is completely normal. Wait fifteen minutes before assuming your heat pump has failed, as it will resume normal heating once defrost completes.
Systems Without Emergency Heat
Not every heating system includes an Em Heat option. Traditional furnaces that burn gas, oil, or propane don’t have this feature because they don’t use heat pump technology.
Some heat pump installations lack sufficient backup heat to warm the home independently. In these cases, the thermostat may not offer an emergency heat setting, or using it will provide inadequate warmth.
Dual Fuel Systems
Homes with dual fuel systems pair a heat pump with a gas furnace. These setups may label the backup differently, sometimes as “Fossil Fuel” or simply “Furnace,” though the concept remains the same.
The thermostat or control system automatically determines when to switch from electric heat pump to gas furnace based on outdoor temperature or efficiency calculations. You can still manually override to furnace-only operation if the heat pump fails.
Costs of Running Emergency Heat
Exact costs depend on your electricity rates, backup system size, home insulation, and outdoor temperatures. As a general estimate, expect emergency heat to cost two to four times more per hour than normal heat pump operation.
A 15-kilowatt backup heat system running continuously costs roughly $1.80 per hour at national average electricity rates of 12 cents per kilowatt-hour. That’s over $40 daily or $1,200 monthly if left on accidentally.
Minimizing Emergency Heat Expenses
Lower your thermostat a few degrees while running on Em Heat. Even a three-degree reduction can cut heating costs by ten to fifteen percent.
Close off unused rooms and focus heating on main living areas. Use draft stoppers at exterior doors and keep curtains closed at night to reduce heat loss.
Getting Your Heat Pump Repaired
Contact an HVAC technician as soon as you determine your heat pump isn’t working. Describe the symptoms clearly, mentioning whether the outdoor unit runs, whether you hear unusual noises, and when the problem started.
Many companies offer emergency service, though you’ll typically pay premium rates for evenings, weekends, and holidays. If temperatures are moderate enough that emergency heat keeps you comfortable, scheduling a regular appointment saves money.
What to Tell the Technician
Mention that you’ve switched to emergency heat and approximately how long the heat pump has been malfunctioning. Note whether the outdoor unit shows any visible damage, ice buildup, or unusual behavior.
Ask the technician to show you how to switch back to normal heating mode after repairs are complete. Confirm that both the heat pump and backup system are working properly before they leave.
Preventing Accidental Emergency Heat Use
Clearly label your thermostat if multiple household members might adjust it. A small note reading “Use only if heat pump breaks” near the Em Heat switch prevents accidental activation.
Some thermostats allow you to lock system settings or hide the emergency heat option behind a password or menu. Check your owner’s manual for these features if you want to prevent unintended switching.
Regular Maintenance Reduces Failures
Schedule professional heat pump maintenance every fall before heating season begins. A technician will check refrigerant levels, clean coils, test electrical connections, and catch small problems before they cause complete failures.
Keep the outdoor unit clear of leaves, grass clippings, and debris year-round. Trim vegetation back at least two feet on all sides to maintain proper airflow.
Understanding Your Specific System
Heat pump configurations vary significantly between installations. Your owner’s manual explains exactly how your model handles emergency heat, auxiliary heat, and backup systems.
If you don’t have the manual, search online using your thermostat’s brand and model number, usually printed on the faceplate or visible when you remove the cover. Manufacturer websites typically offer free PDF downloads.
Em Heat serves as a crucial safety net when your heat pump fails, keeping your home warm until repairs restore normal operation. Use it only for genuine equipment failures, schedule service promptly to minimize expensive backup heating costs, and remember to switch back to regular heating mode once your technician completes repairs.
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