Emergency Heat vs Auxiliary Heat: What's the Difference?
If your thermostat suddenly says AUX, it doesn’t automatically mean your heat pump is broken. Auxiliary heat is backup heat that the system brings on automatically when needed. Emergency Heat is usually a setting you select to heat the house with backup equipment while keeping the heat pump’s compressor off.
That’s the main difference in emergency heat vs auxiliary heat: automatic assistance versus a backup-only operating mode. On many systems, both use the same heating equipment, but they use it differently.
During a Southwest Tennessee cold snap, seeing AUX occasionally can be normal. Selecting Emergency Heat just because it’s cold outside usually isn’t necessary—and with electric backup heat, it can raise operating costs.
What Auxiliary Heat Does
A heat pump normally heats your home by moving heat from outdoor air indoors. There’s still heat available outside in cold weather, although how much the equipment can deliver depends on its design, outdoor conditions, and condition.
As temperatures fall, your house generally loses heat faster. If the heat pump can’t meet that demand by itself, the controls can call for supplemental heat. Your thermostat may display AUX, Aux Heat, or a similar message.
Auxiliary heat commonly comes on for three reasons:
Cold-weather demand: The house needs more heat than the heat pump can currently provide.
Temperature recovery: You raise the thermostat, and the controls bring on backup heat to help reach the new setting.
Defrost operation: Some systems use backup heat while the outdoor unit clears frost from its coil.
Exactly when this happens depends on thermostat programming, equipment staging, outdoor sensors, and system design. Some controls respond to the difference between room temperature and the setting; others also consider how long the equipment has been running.
AUX is an operating status, not a diagnosis. It tells you backup heat has been requested, not why it was needed or whether every heating component is working.
What Emergency Heat Does
Emergency Heat—sometimes labeled EM Heat or E Heat—is generally a manually selected thermostat mode. It tells a compatible system to stop using the heat pump compressor and rely on its backup heat source.
This can provide temporary heat when the outdoor heat pump has a problem, provided the backup equipment and necessary indoor components still work.
For example, a technician may recommend Emergency Heat while an outdoor-unit repair is pending. But it isn’t a cure for every no-heat situation. A failed indoor blower, control problem, or loss of electrical power can prevent backup heating too.
Emergency Heat isn’t a “turbo” setting. It also isn’t a power-outage setting. Even fuel-burning backup equipment generally needs electricity for its blower and controls.
Do both modes use electric heat strips?
Often, yes. Many heat pumps in Tennessee have electric resistance elements, commonly called heat strips, inside the air handler. In auxiliary operation, these can supplement the heat pump. In Emergency Heat mode, they provide heat without the compressor running.
Some homes instead have a dual-fuel system: a heat pump paired with a gas or propane furnace. Those controls typically switch from the heat pump to the furnace rather than running both heat sources together. The thermostat’s labels and available modes vary.
Don’t assume every AUX display means electric strips are running. Your equipment manual or an HVAC technician can confirm what your system uses.
Should You Switch to Emergency Heat When It Gets Cold?
Normally, leave the thermostat in Heat and let the controls manage backup heating. A properly configured system should bring on auxiliary heat or switch heat sources when needed.
There isn’t one outdoor temperature at which every homeowner should manually select Emergency Heat. Heat pumps have different cold-weather capabilities, and houses have different heating demands. Insulation, duct leakage, airflow, and equipment size all affect the point where backup heat is needed.
A heat pump may run for long stretches during freezing weather without having a problem. Long operation by itself isn’t a reason to disable it.
A familiar Pickwick-area example
Consider a lake home near Pickwick where the thermostat is kept lower while the house is empty. The owner arrives on a cold Friday evening and raises the setting several degrees. AUX appears because the controls are trying to warm the house back up.
If the rooms reach the setting and auxiliary heat cycles off, that can be normal recovery. If AUX stays on through milder afternoons, or the house keeps getting colder, the system deserves a closer look.
For heat pumps with electric backup, a fairly steady setting or smaller setbacks may reduce strip-heat use during recovery. Thermostat recovery features and equipment type matter, so there’s no single schedule that fits every house.
Why AUX Can Appear During Defrost
Southwest Tennessee winters bring damp, chilly weather as well as hard freezes. Under the right conditions, moisture freezes onto the outdoor coil while the heat pump is heating.
The system periodically reverses operation to warm that coil and melt the frost. During this defrost cycle, backup heat may run to keep the air delivered indoors from feeling cold.
You might notice a brief change in sound, the outdoor fan stopping, or a cloud of water vapor. Those signs can be normal during defrost.
Light frost that clears is different from thick ice that stays on the equipment. If the coil remains encased in ice, heating drops off, or the fan appears to be striking ice, arrange service. Don’t chip the ice away or pour hot water over the unit.
Which Costs More: Emergency Heat or Auxiliary Heat?
With electric heat strips, the bigger question is how much resistance heat runs and for how long. A heat pump moves heat rather than producing all of it through resistance elements, so it generally uses less electricity to deliver the same amount of heat within its normal operating range.
Auxiliary operation may use the compressor and only the backup stages needed. Emergency Heat takes the compressor out of the heating job and relies on backup, which often increases electricity use.
The thermostat label alone won’t tell you the bill impact. Outdoor temperature, heating demand, strip capacity, staging, runtime, and utility rates all matter. With dual-fuel equipment, fuel prices and changeover settings also affect the comparison.
Brief AUX operation isn’t a reason to panic. A new pattern of extended backup heating is worth investigating.
What You Can Safely Check
Before calling for heating repair, a few observations can help separate normal backup operation from a problem:
Check the mode. If Emergency Heat was selected accidentally and there’s no known equipment fault, return to normal Heat.
Review recent adjustments. A large increase in the setting may explain AUX operation.
Check an accessible filter. Replace a dirty filter with the correct type and size. Don’t run without one.
Look at vents. Keep supply registers and return grilles open and clear of furniture or rugs.
Observe outdoor conditions. Note visible ice or debris blocking the unit. Stay outside equipment panels.
Track room temperature. Is the house holding its setting, recovering, or steadily losing temperature?
Don’t disable auxiliary heat, change installer-level lockouts, or alter thermostat wiring to make the AUX message disappear. Those changes can leave the home without enough heat.
When Backup Heat Needs Professional Attention
Call for heat pump service if AUX use increases noticeably in similar weather, the house won’t reach its setting, or the outdoor unit appears inactive through ordinary heating cycles. Brief pauses and defrost can be normal; persistent failure to heat isn’t.
Possible causes include restricted airflow, thermostat configuration, failed heating stages, defrost faults, electrical problems, refrigerant-related issues, or heat loss through ductwork. Diagnosis should check both the heat pump and its backup—not just the thermostat display.
If there’s smoke, sparking, a strong electrical burning odor, or repeated breaker trips, stop operating the equipment and seek help. Don’t switch to Emergency Heat as a workaround for a suspected electrical hazard. For a gas odor or carbon monoxide alarm, leave the home and contact emergency services from outside.
Bottom Line
Leave a working heat pump in normal Heat mode, and judge AUX by the weather, recent thermostat changes, and whether the house stays warm. Use Emergency Heat as temporary backup for an appropriate equipment failure, ideally with professional guidance. Once repairs are complete, return to normal Heat.
Harbin Heating & Air Conditioning can check why backup heat is running and whether your heat pump controls are operating correctly.
Call Harbin Heating & Air Conditioning at 731-689-3651 or visit us at 5910 Hwy 57, Counce, Tennessee 38326 for help with HVAC replacement, heat pumps, and humidity control in Counce, Shiloh, Pickwick, and the surrounding area.
