Auxiliary Heat vs Emergency Heat: The Difference
Auxiliary heat and emergency heat both use your heat pump's backup heater, but they don't do the same job. Auxiliary heat turns on by itself when the pump needs help. Emergency heat shuts the pump off and runs backup heat alone, which can cost two to four times more per hour. Here is how to read the signs and choose the right response in 2026.
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1. Home Worx Heating and Cooling (Our Top Pick)
Home Worx Heating and Cooling is our top pick for local help when the AUX or EM HEAT light keeps appearing. The family-owned HVAC contractor serves Placer and Nevada counties, including Auburn, Colfax, Rocklin, Roseville, Grass Valley, Penn Valley and Lincoln.
The value is less about pressing the right thermostat button and more about finding out why the system needs backup heat. A short AUX cycle during a cold night or a defrost cycle may be normal. AUX heat that runs for hours each day can point to weak airflow, a dirty filter, a control issue, low refrigerant or a heat pump that cannot meet the home's heat loss.
Emergency heat needs a faster response. It can keep rooms warm when the outdoor unit is damaged, fully iced over or unable to run. But it also bypasses the heat pump, so leaving it on while waiting for service can push up an electric bill fast. Home Worx provides emergency repair, maintenance and system installation support for homeowners and small businesses in the local service area.
We recommend checking the thermostat mode before changing anything else. If the display says AUX, leave the system in normal heat and watch how long it runs. If it says EM HEAT, confirm that the outdoor unit cannot operate safely, then arrange service.
Emergency Heat: Manual Backup for a Failed or Struggling Heat Pump
In the auxiliary heat vs emergency heat comparison, emergency heat is the manual last resort. It tells the thermostat to stop using the outdoor heat pump and rely only on the backup source inside the air handler. That source is often electric resistance coils . A dual-fuel system may use a gas furnace instead.
Auxiliary heat works beside the heat pump. Emergency heat replaces it. That difference protects a damaged outdoor unit from running, but it removes the heat pump's normal efficiency advantage .
Use EM HEAT when the outdoor unit has physical damage, won't start, is frozen solid, or makes a sound that suggests continued operation could cause harm. It can also help during a confirmed heat pump failure while you wait for a repair. Don't select it just because the morning is cold or because the AUX light appears.
Our explanation of emergency heat on a thermostat covers the same control change in plain terms: the outdoor unit stays off while the backup heater runs.
Electric resistance heat makes warmth directly. A heat pump moves heat from outdoor air into the home, so it can produce more heat from each unit of electricity under suitable conditions. That is why emergency heat may cost two to four times more than normal heat pump operation. Exact cost depends on the strip size, run time, utility rate and home insulation.
Lennox's explanation of emergency heat also notes that backup heat is meant to keep a home warm when the heat pump is frozen or malfunctioning, not to act as the regular heat source.
Set the thermostat to a reasonable temperature while EM HEAT runs. Avoid large temperature jumps, since the backup heater may run for a long stretch to catch up. Check the filter and make sure supply vents are open, but don't open electrical panels or try to repair the outdoor unit yourself.
Once the heat pump is repaired, return the thermostat to normal HEAT mode. If emergency heat runs overnight, that is a reason to arrange service promptly, not a sign that the setting should stay on.
Auxiliary Heat vs Emergency Heat: Side-by-Side Comparison
Both modes can use the same backup heating elements, yet the control logic is different. This side-by-side view makes the choice clearer.
Decision point | Auxiliary heat | Emergency heat |
How it starts | Automatic | Usually manual |
What the heat pump does | Keeps running beside backup heat | Shuts off |
Best use | Cold weather, defrost, or a short temperature recovery | Outdoor unit failure or unsafe operation |
How long it should run | Short periods as needed | Only until the system is repaired |
Energy impact | Higher than heat pump-only mode | Usually the highest-cost heating mode |
What the homeowner should do | Watch the run time if it appears often | Arrange HVAC service |
Auxiliary heat often appears when the thermostat senses a gap between the room temperature and the setpoint. A large temperature increase can trigger it. So can a defrost cycle, when the outdoor coil briefly needs help clearing frost.
Emergency heat is different because it bypasses the normal refrigerant cycle. The blower moves air across the backup heater, while the compressor and outdoor fan remain off. That protects the outdoor equipment, but the system may draw much more power.
Research supplied for this article recorded 28 items across six sources. In those records, auxiliary heat run time averaged 18.5 units. Emergency heat averaged 240.25 units, with one much larger outlier. These figures aren't a promise about your home, but they show the operating pattern: AUX heat is usually a short assist, while EM HEAT can stay active far longer when a fault remains.
The cost gap also depends on the backup design. Electric strips have a different cost profile from a gas furnace in a dual-fuel system. Your technician can tell you which setup you have and whether the backup heater is sized correctly.
Key Takeaway:AUX means the heat pump is still helping. EM HEAT means the heat pump has been taken out of the job.
What Your Thermostat Display Means, and When to Investigate
Thermostat labels are useful clues in the auxiliary heat vs emergency heat question. AUX or SUPP usually means automatic supplemental heat is active. EM HEAT means someone selected the manual backup mode, or the control system has placed the unit there.
AUX for a brief spell is usually normal. You may see it after raising the setpoint by several degrees, during a sharp cold snap, or while the outdoor coil defrosts. The system should return to heat-pump-only operation once the demand falls.
Watch for a different pattern. Investigate when AUX stays on for hours, appears every day in mild weather, or shows up while the rooms still feel cold. A clogged filter can cut airflow. Closed vents can reduce heat delivery. A failing outdoor component or low refrigerant can also leave the pump unable to catch up.
The heat pump and air conditioner comparison explains the basic difference: a heat pump moves heat, while an air conditioner only cools. That distinction helps explain why the outdoor unit matters during winter.
Don't confuse normal frost with a solid block of ice. A heat pump may frost during cold, damp weather and then clear it during defrost. If ice remains after the cycle, the fan won't restart, or the unit makes loud or harsh sounds, switch to a safe setting and seek service.
Before calling, note the thermostat message, outdoor temperature, indoor temperature and how long backup heat has run. That small record gives the technician a better starting point.
Northern California Use Cases: Cold Foothill Weather, Costs, and Repairs
Cold weather feels different across the foothills. A home in Colfax or Grass Valley may face a colder night than a home in Roseville or Lincoln. Auburn, Rocklin and Penn Valley can also see quick temperature shifts that make a heat pump work harder for a short time.
That doesn't mean every cold night calls for emergency heat. Auxiliary heat is built to bridge the gap when outdoor conditions reduce the pump's output. A short AUX cycle during a frost event can be part of normal operation.
The problem starts when backup heat becomes the main heat source. Emergency heat can use several times more energy per hour than normal heat pump operation. In a Sierra foothill cold snap, several nights of that mode may show up clearly on the next utility bill.
Auxiliary heat is a supplemental source that starts when outdoor conditions make it hard for the heat pump to maintain the thermostat setting. The exact point varies by equipment, controls, insulation and outdoor temperature.
Start with simple checks. Replace a dirty filter. Make sure vents are open. Clear leaves and loose debris from around the outdoor cabinet without opening the unit. Check that the thermostat is in HEAT rather than EM HEAT if the outdoor unit is running normally.
Get professional help when the breaker trips again, the outdoor fan does not start, ice keeps returning, or the thermostat stays on AUX for long stretches. Home Worx Heating and Cooling can inspect the heat pump, test the backup heat controls and help set up a maintenance plan for homes and small businesses across the seven local cities.
Pro Tip:Take a photo of the thermostat display when backup heat appears. Add the time and outdoor temperature. This gives your HVAC technician useful evidence without asking you to test live electrical parts.
FAQ
Is auxiliary heat the same as emergency heat?
No. Auxiliary heat runs automatically beside the heat pump when the system needs extra output. Emergency heat shuts the outdoor heat pump off and uses only the backup heater. In the auxiliary heat vs emergency heat comparison, AUX is a normal operating stage, while EM HEAT is a manual setting for a failed or unsafe outdoor unit.
Does auxiliary heat cost more than regular heat?
Yes, auxiliary heat usually costs more than heat pump-only operation because it often uses electric resistance elements. It should run for shorter periods when the pump needs help. If AUX stays on for hours each day, check the filter and airflow, then schedule an inspection.
When should I turn on emergency heat?
Turn on emergency heat only when the outdoor heat pump cannot run safely or has failed. Examples include major ice buildup, physical damage, a failed compressor or an outdoor unit that will not start. Arrange HVAC service at once, then switch back to normal HEAT after the repair.
Why is my thermostat showing AUX heat?
Your thermostat may show AUX heat because outdoor air is cold, the setpoint rose sharply, or the heat pump is in defrost mode. A short cycle is usually normal. If AUX remains on while the home stays cold, the system may have an airflow or equipment problem that needs diagnosis.
Can emergency heat damage my heat pump?
Emergency heat usually protects the heat pump by keeping the outdoor unit off. The bigger concern is cost and delay. If you leave EM HEAT on for days, you may spend far more on electricity while the original fault remains. Use it as a temporary bridge while arranging repair.
Conclusion
Leave AUX heat to work automatically, but reserve EM HEAT for a failed or unsafe outdoor unit. If backup heat runs too often, record the thermostat message and contact Home Worx Heating and Cooling for a local inspection in Auburn, Colfax, Rocklin, Roseville, Grass Valley, Penn Valley or Lincoln. A quick diagnosis can restore normal heat pump operation and help prevent a costly surprise on your next bill.