Use Emergency Heat when the heat-pump compressor or outdoor unit has failed and you need the installed backup heat to protect the home until service arrives. Do not select it just because the weather is cold or the thermostat briefly displays AUX Heat. Emergency mode normally locks out the heat pump and heats only with the backup source, which may be electric resistance strips or a gas/oil furnace.
Auxiliary heat and Emergency Heat are not the same. Auxiliary heat is automatically staged by a correctly configured thermostat when the heat pump needs help, such as during cold weather, a large setpoint increase, or defrost. Emergency Heat is a manual operating mode on many thermostats and is intended for a heat-pump problem, not normal cold-weather operation.
Emergency Heat vs. Auxiliary Heat
| Feature | Auxiliary heat (AUX) | Emergency Heat (EM Heat) |
|---|---|---|
| How it starts | The thermostat or system controls it automatically when backup capacity is needed. | The user normally selects the mode manually; some controls may also report a system lockout or fault. |
| Heat-pump compressor | May run with backup heat on all-electric systems; dual-fuel operation depends on system design. | Locked out on typical controls, so the outdoor heat-pump compressor does not provide heat. |
| Purpose | Normal supplemental heat during high demand, defrost, or low outdoor temperature. | Temporary heating when the heat pump cannot operate safely or needs repair. |
| Energy cost | Can increase energy use, especially with electric resistance backup. | Often costs more because the efficient heat-pump stage is disabled; actual cost depends on the backup fuel and rates. |
| What you should do | Allow normal automatic operation unless AUX runs unusually long in mild weather or comfort is poor. | Call for HVAC service and return to normal Heat mode only after the heat-pump problem is resolved. |
When You Should Turn On Emergency Heat
Emergency Heat is appropriate when all of the following are true:
- Your home has a heat pump with an installed, correctly configured backup heat source.
- The outdoor unit or compressor is not operating properly, is damaged, or has been shut down for repair.
- The thermostat and HVAC manual confirm that Emergency Heat disables the heat pump and operates the backup stage.
- You have contacted an HVAC professional and are using the mode temporarily while waiting for diagnosis or repair.
Examples include a confirmed compressor or refrigerant-system failure, storm or impact damage to the outdoor unit, a fan that is not turning when it should, a severe ice condition that does not clear through normal defrost, or a technician’s instruction to keep the outdoor unit off.
When Not to Use Emergency Heat
- Cold weather alone: a properly designed system uses its normal controls and auxiliary heat to meet demand.
- A brief AUX Heat indication: this can be normal during defrost, recovery from a large thermostat setback, or high heating demand.
- To warm the house faster: manually disabling the heat pump may increase operating cost without improving comfort.
- Normal frost on the outdoor coil: heat pumps periodically defrost themselves. Emergency Heat does not directly defrost the outdoor coil.
- A whole-home power outage: most electric backup heat cannot operate without power, and portable generators must never be connected or operated contrary to their safety instructions.
There is no universal outdoor temperature—such as 35°F—at which every homeowner should manually select Emergency Heat. Balance points, compressor limits, auxiliary staging, climate, system size, and thermostat configuration differ. The controls should manage normal low-temperature operation.
What Normal Defrost Looks and Sounds Like
In heating mode, the outdoor coil can develop frost. During a normal defrost cycle, the outdoor fan may stop, the system may make a whoosh or change in sound, steam may rise from the outdoor unit, and auxiliary heat may operate to reduce cool air from the indoor registers. The cycle should end automatically.
Do not chip ice with tools, pour boiling water on the coil, cover the unit, or change defrost controls. Clear loose snow and debris only while maintaining manufacturer clearances. Call for service if the unit becomes encased in ice, repeatedly enters defrost, the fan strikes ice, or the ice does not clear.
How to Use Emergency Heat Safely
- Confirm the system type. Emergency Heat is a heat-pump function and only works if a backup source was installed and configured.
- Check the thermostat manual. Mode names and menu steps vary. Do not rewire the thermostat or change installer settings.
- Record the symptoms first. Photograph error messages, note whether the outdoor fan/compressor was running, and record outdoor and indoor temperatures.
- Select Emergency Heat as documented. Confirm that the thermostat indicates EM Heat and that warm air begins after the system’s normal delay.
- Call for service. Emergency mode is a bridge to repair, not the repair itself.
- Monitor the home. If temperature keeps falling, the backup stage does not start, a breaker trips, or you notice gas/electrical warning signs, stop and obtain urgent professional help.
Do not open the outdoor disconnect, electrical panels, air handler, or heat-strip compartment. Electric backup heat uses high current. A dual-fuel system may use gas or oil and adds combustion and carbon-monoxide risks.
How Long Can Emergency Heat Run?
There is no safe universal number of hours. If the backup system is correctly installed and operating normally, it is designed to provide heat; however, Emergency Heat should remain a temporary response to a failed heat pump. Schedule repair promptly and follow the HVAC manufacturer or technician’s instructions.
With electric resistance strips, continuous operation can substantially increase electricity use. A 10kW strip heater uses 10 kilowatt-hours for every hour it is energized continuously. Actual cycling varies with heat loss, thermostat settings, and staging. A gas or oil backup has different fuel and operating costs, so “Emergency Heat is always electric” is not a safe assumption.
What If AUX Heat Stays On?
AUX may run longer during unusually cold weather, defrost, or recovery from a large thermostat change. Investigate when AUX remains on for an extended period in mild weather, the home cannot reach its setpoint, energy use changes sharply, the outdoor unit never operates, or the thermostat displays a fault.
- Check that the thermostat is in normal Heat mode, not Emergency Heat.
- Confirm the schedule did not make a large temperature jump.
- Inspect the air filter and accessible registers for airflow restriction.
- Look at the outdoor unit from a safe distance for blockage, unusual ice, fan trouble, or an error indicator.
- Call a technician rather than changing balance-point or installer settings.
Emergency and Safety Signs
Turn the system off and arrange urgent service for a burning electrical smell, sparking, a hot or discolored panel, repeated breaker trips, grinding, or severe vibration. If backup heat uses gas or oil and you smell fuel/rotten eggs or a carbon-monoxide alarm sounds, leave immediately and call emergency services or the fuel utility from outside. Do not operate switches or try to locate the leak.
Frequently Asked Questions
Does Emergency Heat turn on automatically?
On many conventional thermostats, Emergency Heat is a mode selected manually. Auxiliary heat is the automatic stage. Advanced controls may display fault or lockout information differently, so use the manual for the installed thermostat and heat pump.
Will Emergency Heat thaw the outdoor unit?
No. Selecting Emergency Heat generally locks out the heat-pump compressor and uses backup heat indoors. The heat pump’s own defrost controls manage normal outdoor-coil frost.
Why will Emergency Heat not turn off?
First confirm that the thermostat mode has been returned to normal Heat and allow the control’s normal delay. If EM Heat remains indicated or backup heat continues, do not replace the thermostat based on that symptom alone. Wiring, thermostat configuration, relays, sensors, or HVAC controls may be involved; call a qualified technician.
Sources and Editorial Notes
This guide was substantially rewritten and fact-checked on September 16, 2026. Heat-pump staging varies among all-electric, dual-fuel, variable-capacity, and communicating systems. The manuals for the exact heat pump, air handler/furnace, and thermostat take precedence.
- Trane: Auxiliary Heat and Emergency Heat
- Honeywell Home: EM HT Thermostat Setting
- Honeywell Home: Emergency and Auxiliary Heat Operation
- Carrier: Auxiliary Heat vs. Emergency Heat
- Carrier Heat Pump Not Heating Guide
