Heat Pump Emergency Heat vs. Aux Heat: What They Mean and When to Switch

A heat pump thermostat has two backup-heat labels, and they mean different things. AUX (auxiliary heat) comes on by itself to help the heat pump, and seeing it on a cold morning is normal. EM HEAT (emergency heat) is a setting you switch on by hand, and it shuts the heat pump off. Trane puts it plainly: emergency heat is "a heat pump thermostat setting that you should only use in a true emergency - as in, your heat pump has failed and it's really cold outside."

This guide covers what each one does, when the makers say to use emergency heat, why it costs more to run, and when to call a technician.

Aux Heat vs. Emergency Heat

The backup itself is usually electric heat strips in the indoor air handler. Lennox calls it "electric resistance heating, similar to how a space heater or toaster works," and says "some dual-fuel systems use a gas or oil furnace as the backup instead." Google Nest sums up the difference: "Emergency heat must be turned on manually while Nest thermostats can use Auxiliary Heat automatically as needed."

When to Switch to Emergency Heat

Rheem's owner manual says emergency heat "is a back-up heating mode to be used only if there is a suspected problem with the outdoor unit." Lennox lists the cases:

• Your HVAC technician tells you to, while you wait for a repair or parts.
• The heat pump has completely failed.
• The outdoor unit is "frozen solid or damaged (from storm debris, ice buildup, etc.) and cannot operate safely until inspected."

Cold weather alone isn't on the list. A heat pump that runs for long stretches and calls for AUX on a frigid day is doing its job. And Trane says that once you switch to emergency heat, you should "call for HVAC service right away."

Why Emergency Heat Costs More

A heat pump moves heat in from outdoors; resistance heat makes heat from electricity directly. The U.S. Department of Energy says heat pumps provide "up to three times more heat than the energy they use" and cut electricity use for heating by about half compared with electric furnaces or baseboard heaters. Rheem warns that "sustained use of electric resistance heat in place of the heat pump will result in an increase in electric utility costs," and Nest calls emergency heat "usually expensive and inefficient." Leave it on only as long as the outdoor unit is out of action.

What's Normal and What Isn't

• Defrost. In cold, damp weather the outdoor coil frosts over and the heat pump melts it off. Rheem says the outdoor fan stops and "some steam may rise from the outdoor unit." Lennox says the cycle "usually lasts between 5 and 15 minutes" and that a whooshing sound and steam are both normal. Bryant adds that the system "may temporarily deliver cooler air indoors." Our frozen heat pump guide shows normal frost next to ice that needs a technician.
• AUX on a cold day. Trane says the thermostat calls for aux heat when it's below about 40°F outside or below your system's set trigger, during defrost, or when you raise the temperature by more than 3 or 4 degrees.
• AUX on a mild day. Trane says that if it's above 40 degrees and the thermostat shows AUX HEAT "for more than an hour or two, you might want to investigate."
• Needing EM HEAT often. Nest says that "if you need to run it regularly, your system may need servicing from a qualified HVAC technician."

If the Outdoor Unit Is Buried in Snow or Ice

• Lennox says to clear at least two feet of space on all sides and to brush light snow off the top with a broom, "never use sharp tools."
• Don't chip at ice. Lennox warns that tools "can damage the delicate coils, fins, and fan blades," and says never to pour hot water on the unit. Bryant says the same about hard tools.
• Don't cover the sides. Lennox says the unit needs open airflow.
• If it's frozen solid and not defrosting, that's one of the cases for emergency heat while you wait for a technician. Trane says a heat pump that "keeps running but hasn't defrosted after an hour" may need repair.

Thermostat Setbacks

A big setback can cost you on a heat pump, because the morning catch-up can bring on the strips. The Department of Energy's Energy Saver Guide says not to set a heat pump back manually "if it causes backup electric-resistance heating to come on," and not to use larger setbacks without a thermostat designed for heat pumps. DOE's federal buying guidance points to thermostats that "ramp the temperature up slowly to avoid activating the electric resistance heating elements." For how warm to keep the house when you're away, see what temperature to keep your house in winter; the floor is 55°F.

When to Call a Technician

• You've had to switch to emergency heat. Trane says to call right away.
• The outdoor unit won't turn on at all, or it hasn't cleared its ice after an hour of running.
• AUX stays on for hours in mild weather.
• A breaker for the heat pump or air handler trips. Bryant says to reset it once, and if it trips again, "leave it off and contact a qualified HVAC professional." Many systems have separate breakers for the outdoor unit and the indoor air handler, so check both.

Frequently Asked Questions

Is it bad to leave a heat pump on emergency heat?

It won't hurt the heat pump, which is switched off, but it is expensive. Rheem warns that running resistance heat instead of the heat pump raises your electric bill, so use it only until the outdoor unit is fixed or cleared, and call for service.

Why is my thermostat showing AUX HEAT?

Usually because it's cold out, the heat pump is in defrost, or you raised the setting by a few degrees. Trane lists all three as normal triggers. If AUX stays on for more than an hour or two above about 40°F, have it checked.

Should I switch to emergency heat when it's really cold?

Not just for the cold. The makers reserve it for a heat pump that has failed or an outdoor unit that's damaged or frozen solid. In cold weather the thermostat brings on AUX by itself when the heat pump needs help.

Does a dual-fuel system have emergency heat?

Yes, but the backup is the furnace rather than electric strips. Lennox notes that some dual-fuel systems use a gas or oil furnace as the backup. The rule is the same: switch to it by hand only when the heat pump can't run.

The Bottom Line

AUX is the thermostat helping the heat pump, and on a cold day it's normal. EM HEAT is the switch you flip only when the outdoor unit has failed, is damaged, or is frozen solid, and it means calling for service. Keep the outdoor unit clear of snow with a broom, never chip at ice, and avoid big setbacks unless your thermostat is made for heat pumps.

Sources

• Trane — heat pump auxiliary heat (aux vs. emergency heat; heat strips; aux triggers; call for service after switching to emergency heat; defrost after an hour; AUX above 40°F) and extreme winter weather preparation (clear heavy snow off the top).
• Lennox — what is emergency heat (backup types; when to activate it), heat pump defrost cycle (5 to 15 minutes; whooshing and steam are normal), and heat pump frozen (two feet of clearance; broom only; no chipping or hot water; don't cover the sides).
• Rheem — heat pump owner manual (PDF) (EM. HT. locks out the outdoor unit and is for a suspected outdoor-unit problem; higher utility costs; defrost fan stop and steam; disconnect both power supplies before maintenance).
• Bryant — heat pump not heating (defrost 5 to 15 minutes and cooler air; don't chip ice; reset a tripped breaker once).
• Google Nest — emergency heat and auxiliary heat (emergency heat is manual and usually expensive; regular use suggests servicing).
• U.S. Department of Energy — Energy Saver Guide 2022 (PDF) (up to three times more heat than the energy used; about 50% less electricity than electric furnaces or baseboard; setback guidance for heat pumps) and FEMP air-source heat pump purchasing (thermostats that ramp up slowly to avoid resistance heat).

Last reviewed October 24, 2026. General information, not professional advice — refrigerant, electrical, and control repairs on a heat pump belong to a licensed HVAC technician.

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