Emergency Heat Explained: What the Setting Does and What to Do Next

The short answer: EM Heat means emergency heat

EM Heat is short for emergency heat. It is a backup heating mode found mainly on thermostats connected to heat pumps with a secondary heat source.

On many systems, manually selecting EM Heat bypasses normal heat-pump operation and heats the home with the configured backup source alone. That source might be electric resistance elements, a gas furnace, or an oil furnace. Emergency heat is intended primarily for times when the heat pump is damaged, frozen, or otherwise unable to operate—not for everyday heating (Lennox’s guide to emergency heat).

Seeing “EM Heat” on the thermostat does not by itself prove that the heat pump is broken. The display may simply indicate that someone selected the mode. Thermostat terminology and control behavior also vary: one model may show “EM Heat,” another may spell out “Emergency Heat,” and another may label backup operation differently.

Not every thermostat has this feature. It is associated primarily with heat-pump systems that have a secondary heating source.

The immediate rule of thumb is:

• Use Heat as the normal heating mode.
• Let AUX Heat operate automatically when the system calls for it.
• Select EM Heat manually only when the heat pump is known to be inoperable, the equipment manual recommends it, or a qualified HVAC technician instructs you to use it.
• If EM Heat was selected accidentally and no heat-pump failure is known, return the thermostat to the manufacturer’s normal heating mode. Depending on the model, that may be labeled Heat or, in some cases, Auto.

Think of emergency heat as a temporary backup plan—not an extra-power or faster-heating setting.

What changes inside the system when EM Heat is selected

In normal Heat mode, a heat pump uses a refrigeration cycle to transfer heat from outdoors into the home. It can obtain usable heat from outdoor air even when that air feels cold.

The thermostat and equipment controls determine how to meet the indoor temperature setting. During routine operation, the heat pump is the primary source. If the system needs more capacity, compatible equipment may activate auxiliary heat while continuing to use the heat pump.

Manual emergency mode commonly changes that sequence. Instead of allowing the outdoor heat pump to remain the primary source, the thermostat bypasses it and calls on the secondary source. In other words, automatic auxiliary heat generally assists the heat pump, while manually selected emergency heat generally operates without it (Valley Service’s explanation of EM Heat operation).

The backup source is not the same in every home. Common configurations include:

• Electric resistance elements: Often called heat strips, these are usually part of the indoor equipment.
• Gas furnace: Some dual-fuel systems pair a heat pump with a gas furnace.
• Oil furnace: Certain systems use an oil-fired appliance as the secondary source.

The distinction matters because the type of backup affects which equipment runs and what emergency operation may cost. Electric resistance heating and a gas or oil furnace do not necessarily have the same operating characteristics or economics.

Do not try to identify the backup source from the words “EM Heat.” The label describes a control mode, not the fuel or equipment producing the heat. Check the manuals and model information for the thermostat, heat pump, indoor air handler, and furnace, if present.

The exact display and operating sequence must therefore be confirmed in the documentation for the installed equipment.

Heat vs. AUX Heat vs. EM Heat

The simplest distinction is:

Normal Heat uses the heat pump, AUX helps it, and manually selected EM Heat commonly replaces it.

Mode	How it starts	Does the heat pump normally operate?	Main purpose	Cost considerations
Heat	Selected as the routine heating mode	Yes	Provides everyday heat through normal heat-pump operation	Serves as the baseline for comparison with backup operation
AUX Heat	Commonly activated automatically by the thermostat or equipment controls	Usually, although operation can vary during defrost and by system design	Adds capacity when the heat pump needs assistance	Can increase electricity or fuel use; the effect depends on the backup source and runtime
EM Heat	Commonly selected manually by the user or under technician guidance	Commonly no	Uses the backup source when normal heat-pump operation cannot or should not continue	Electric resistance backup can consume more electricity than normal heat-pump heating; other backup fuels have different costs

Normal Heat

Normal Heat is the everyday setting. The thermostat calls on the heat pump to transfer heat into the home. The system may still use supplemental heat when its controls determine that more output is needed; the homeowner does not ordinarily have to select emergency mode for that to happen.

Those observations alone do not establish that the system has failed.

AUX Heat

“AUX” means auxiliary. On many systems, AUX Heat activates automatically when the controls determine that the heat pump needs assistance. This can happen when:

• Indoor temperature is recovering slowly.
• The thermostat setting was raised substantially.
• The system is completing a defrost cycle.
• Conditions require additional heating capacity.

An AUX indicator is therefore not automatically an emergency. It may simply show that the system is using its normal control strategy. Craighead Electric Cooperative describes automatic AUX operation during slow recovery, substantial setpoint changes, and heat-pump defrost cycles (its explanation of auxiliary and emergency heat).

EM Heat

Manually selected EM Heat commonly stops using the heat pump and relies on the backup source instead. The backup is no longer merely assisting the primary heating process; it becomes the heating source.

That makes the mode useful when the heat pump is inoperable or should not continue running. It does not make emergency heat the preferred setting simply because the house feels cold.

Why the terminology can be confusing

Some educational sources and thermostat interfaces use “auxiliary heat” and “emergency heat” loosely. A particular thermostat may also label automatic backup operation differently from another model.

The comparison above is therefore a general framework. The manual for the exact thermostat and HVAC system controls when the display or behavior differs.

There is also no universal outdoor temperature at which every system must activate AUX Heat or at which a homeowner should select EM Heat. Heat-pump capacity, equipment sizing, controls, climate, backup configuration, and the home’s heating demand all affect operation. Fixed instructions to switch to emergency heat below 30, 35, or 40 degrees Fahrenheit are not reliable for every installation.

When to use emergency heat—and when not to

Manual EM Heat is generally appropriate as a temporary measure when the heat pump cannot provide normal heating and the backup system is available. Examples include:

• The outdoor heat-pump unit is known to have failed.
• The heat pump is operating but producing no effective heat, and the equipment instructions support changing modes.
• The outdoor unit has severe, persistent icing that it cannot clear through its normal operation.
• A technician directs you to use emergency heat while awaiting repair.
• The thermostat or HVAC manual recommends emergency mode for the condition shown.

Emergency heat can help maintain indoor warmth while service is arranged, but it does not repair the heat pump or identify the cause of the failure.

Cold weather alone is not a universal reason

Do not select EM Heat solely because the weather is unusually cold. Many systems automatically activate auxiliary heat when more capacity is needed. Manually switching to emergency mode may unnecessarily remove a functioning heat pump from operation.

There is no universal cutoff temperature for making that change. Coolray likewise advises leaving the system in normal heating mode during cold weather unless the heat pump stops heating the home (its cold-weather emergency-heat guidance).

EM Heat is not a “heat faster” button

Emergency heat is not intended to accelerate ordinary temperature recovery. Selecting it may force the system to rely only on the backup source while preventing the heat pump from contributing.

If the home is warming slowly, first determine what is actually happening:

• Is the indoor temperature rising, even if gradually?
• Does AUX appear temporarily?
• Was the thermostat setting recently raised substantially?
• Is heated air reaching the rooms?
• Is the thermostat displaying an error rather than an operating status?

Slow recovery alone does not prove that emergency mode is necessary.

When not to select it

Avoid switching manually to EM Heat merely because:

• The outdoor temperature crossed an arbitrary threshold.
• AUX appeared briefly.
• The heat pump is running a long cycle in cold weather.
• The air from a register feels warm rather than intensely hot.
• You want the room to reach its set temperature faster.
• You notice temporary frost, water vapor, or a change in sound at the outdoor unit.
• You are uncertain what a thermostat symbol means.

When the heat pump is operating normally, allow its controls to manage supplemental heat. When the heat pump is known to be inoperable, emergency heat may serve as a temporary bridge while qualified service is arranged.

What to do if EM Heat appears unexpectedly

First, read the thermostat display carefully. EM Heat, Emergency Heat, AUX, Stage 2, and similar labels may represent different operating states. Check whether someone may have manually changed the mode.

If EM Heat was selected accidentally and the heat pump is not known to have failed:

1. Use the thermostat instructions to select its normal heating mode—usually Heat, or Auto if the manual identifies that as the appropriate normal mode.
2. Avoid making repeated large temperature changes.
3. Allow the system time to resume ordinary operation.
4. Note whether the indoor temperature begins moving toward the setpoint.
5. Record any recurring alert, error code, or failure to heat.

If the display says AUX rather than EM Heat, the thermostat may simply be calling for supplemental heat automatically. Consult the manual if the label persists unexpectedly or the home does not warm.

Preliminary checks you can make

Without attempting an internal repair, you can make a few basic observations:

• Confirm that the thermostat is in the intended mode.
• Replace low thermostat batteries if the model uses replaceable batteries and its manual explains the procedure.
• Record any error code exactly as displayed.
• Check whether a relevant circuit breaker appears tripped, but do not repeatedly reset or cycle it.
• From a safe position, look for obvious snow, debris, or persistent ice around the outdoor unit.
• Note whether the outdoor unit appears to operate and whether the home is receiving heat.

These checks align with Lennox’s general troubleshooting guidance, which also advises against repeatedly cycling a malfunctioning system. Refrigerant work and heat-pump repairs should be left to qualified professionals (Lennox’s emergency-heat troubleshooting guidance).

Next, consult the manuals for the thermostat and HVAC equipment. Search by the full model number rather than by appearance. The documentation can clarify whether the message represents a manually selected mode, an automatic heating stage, a fault, or another system-specific condition.

Know what the display cannot diagnose

That distinction is important: observation is not diagnosis. Record what the thermostat and equipment are doing, but do not infer a specific failed component from the display alone.

Professional inspection is appropriate when:

• Normal heating does not resume.
• A backup-related display persists or returns unexpectedly.
• Backup heat operates frequently during mild conditions.
• An error code remains.
• The outdoor unit remains inoperable.
• The home does not warm adequately in either normal or emergency mode.

Frost, ice, steam, and defrost: what the thermostat cannot tell you

A heat pump’s outdoor coil can collect frost during heating operation. Compatible systems use a defrost cycle to remove it. During that process, auxiliary heat may operate to help maintain indoor warmth.

Defrost can produce unfamiliar sights and sounds. Water vapor may rise from the outdoor unit as frost melts, and a change in refrigerant flow can create a whooshing sound. Those observations can occur during normal defrost and do not, by themselves, prove that the system has failed (Keith Key Heating & Air’s comparison of defrost, AUX, and emergency heat).

At the same time, not every ice condition should be assumed normal. The relevant distinction is between frost that appears and clears during operation and persistent, substantial icing that the system does not clear.

Consider the full pattern:

• Does the frost later clear?
• Does the outdoor unit resume ordinary operation?
• Is the home maintaining or approaching the set temperature?
• Does AUX appear only temporarily?
• Is an error code displayed?
• Is ice continuing to accumulate?

Persistent icing combined with inadequate indoor heat or a nonoperating outdoor unit may justify temporary emergency heat while service is arranged, provided that the equipment manual or a qualified technician supports that response.

Frost, meltwater, vapor, sound, and thermostat labels are observations—not complete diagnoses. Defrost behavior varies by model and control setup, so the equipment documentation should govern when the sequence is unclear.

Cost implications and when to call an HVAC professional

Emergency heat can affect heating costs, but the effect depends on the backup equipment.

With electric resistance backup, electricity is used to generate heat directly. A heat pump instead uses electricity to transfer existing heat. Craighead Electric Cooperative explains that electric resistance heating produces one unit of heat for each unit of electrical energy consumed, while a heat pump can deliver more heat energy than the electrical energy used to operate it (the cooperative’s efficiency explanation).

As a result, using electric resistance elements as the sole heating source can consume substantially more electricity than normal heat-pump operation. That does not support a universal dollar estimate, however. The actual bill effect depends on:

• Backup-equipment type and capacity
• Electricity, gas, or oil prices
• Outdoor conditions
• Emergency-mode runtime
• Thermostat setting
• The home’s insulation and air leakage
• Equipment condition and sizing

Emergency heat is not equally expensive in every home. Electric resistance strips, gas furnaces, and oil furnaces have different operating economics, and the thermostat label does not reveal the local cost of running them.

When professional help is appropriate

Arrange qualified HVAC service when:

• The thermostat will not resume normal heating.
• The outdoor unit remains inoperable.
• Persistent ice does not clear during normal operation.
• Backup heat runs repeatedly in mild weather without an understood reason.
• An error code remains after following the manufacturer’s user-level instructions.
• The system repeatedly returns to a backup-related mode or status.
• The home does not warm even with EM Heat selected.

If emergency mode also fails to heat the home, the backup equipment may not be operating correctly, or another problem may be present. Do not respond by repeatedly cycling breakers or attempting internal electrical, compressor, or refrigerant repairs.

The appropriate response depends on the backup source, equipment configuration, building conditions, and reason the heat pump is unavailable. While awaiting service, follow the manufacturer’s instructions or the technician’s directions and monitor whether the home is actually maintaining adequate heat.

Use this decision path:

1. EM Heat was selected accidentally: Return to the thermostat’s normal Heat mode—or Auto if its manual specifies that mode—unless the heat pump is known to be inoperable.
2. AUX appears during normal heating: Let the controls manage it while watching for persistent, unexplained operation or inadequate heat.
3. The heat pump is known to have failed: Use EM Heat temporarily if the equipment instructions or a technician supports doing so.
4. The home remains cold or the problem returns: Consult the exact equipment manuals and arrange qualified diagnosis.

Normal Heat is the everyday setting. AUX commonly assists automatically. Manually selected EM Heat commonly replaces the heat pump when temporary backup heating is needed.

Frequently asked questions

Does EM Heat mean my heat pump is broken?

No. The display means that an emergency or backup-heating mode is selected or active; it does not identify why.

Someone may have selected the mode accidentally, or the thermostat may use backup-related terminology in a model-specific way. If no heat-pump failure is known, return to the manufacturer’s normal heating mode and monitor the system. If normal heating does not resume, consult the manuals and arrange service.

Should I use EM Heat when it is extremely cold outside?

Not solely because it is cold. Many heat-pump systems automatically add AUX Heat when supplemental capacity is needed. Manually selecting EM Heat may bypass a heat pump that can still contribute useful heat.

Do not rely on a universal outdoor-temperature cutoff. Use normal Heat unless the heat pump is known to be inoperable, the equipment manual directs otherwise, or a qualified technician recommends emergency mode.

Will emergency heat warm my house faster?

Not necessarily, and that is not its intended purpose. Emergency mode substitutes the backup source for normal heat-pump operation; it is not a rapid-recovery setting (Florida Breeze’s Heat versus Emergency Heat guide).

If the home is warming slowly, check the displayed mode and whether the temperature is rising. Consult the equipment instructions if heating remains inadequate.

How do I turn off EM Heat if it was selected accidentally?

Use the thermostat’s mode control to switch from EM Heat or Emergency Heat to its normal heating mode. This is usually Heat; use Auto only if the thermostat manual identifies it as the appropriate normal mode.

After changing the setting, allow the system to operate. If the backup-related display reappears, an error persists, or the home does not warm, record the display and contact a qualified HVAC professional.

How long can emergency heat stay on while I wait for a repair?

There is no universal maximum runtime for every emergency-heat system. It depends on the backup equipment, fuel, electrical configuration, building conditions, and reason the heat pump is unavailable.

Follow the thermostat and HVAC equipment manuals and any instructions from the service technician. Monitor whether the home is maintaining adequate heat and whether new errors appear. If emergency mode cannot keep the home warm or the system behaves abnormally, contact the service provider for further direction.