Emergency heat shuts your heat pump's compressor off entirely and runs the house on electric strip heat alone. It's meant for when the heat pump itself has failed, not for a cold morning. Auxiliary heat is different: it's supplemental strip heat that kicks in automatically alongside a working compressor when the house needs more heat than the heat pump can supply on its own.
Those two terms get used interchangeably online, and thermostat manufacturers don't help by putting them close together on the same menu. But they do different jobs, cost different amounts, and mean different things about the health of your system. This page walks through both.
Auxiliary heat: the backup that runs automatically
Auxiliary heat, usually labeled AUX on the thermostat, is a set of electric resistance coils built into the air handler. They work like a giant toaster element. When your thermostat calls for more heat than the compressor and outdoor unit can deliver on their own, the system energizes those coils and blends strip heat in with what the heat pump is already producing.
This happens automatically. You don't turn AUX on. The thermostat's logic decides when the heat pump alone can't close the gap between the current temperature and the setpoint, and it brings the strips in to help. Two situations trigger it most often:
A large setpoint jump, say bumping the thermostat up 5 degrees at once, asks for more heat faster than the compressor can supply, so AUX fills the gap temporarily.
Very cold outdoor air reduces how much heat the outdoor coil can pull from the atmosphere, so the compressor's output drops right when the house needs more. AUX compensates.
AUX heat costs more to run than the compressor alone, because electric resistance heat is inherently less efficient than a heat pump moving heat rather than generating it. Running for a few minutes during a temperature jump or a cold snap is normal and not something to worry about. AUX running for hours at a stretch, especially in mild weather, usually means the heat pump side isn't pulling its share of the load.
Emergency heat: the manual override for a failed heat pump
Emergency heat, labeled EM HEAT on most thermostats, is a setting you select yourself. Selecting it shuts the compressor off completely. The house then runs entirely on the same strip heat coils AUX uses, but as the sole heat source rather than a supplement.
EM heat exists for one situation: the heat pump itself has failed and a technician hasn't fixed it yet. A frozen outdoor unit that won't defrost, a dead compressor, a refrigerant leak that's left the system unable to move heat at all. In those cases, EM heat keeps the house warm on electric strips while you wait for repair.
It is not a setting for a cold day, a system that seems sluggish, or a house that feels a little cool at 6 a.m. Running EM heat for routine heating burns through electricity fast, since you're heating the entire house on resistance coils instead of a compressor that's several times more efficient at moving the same amount of heat. A homeowner who flips to EM heat because the air feels cool, without an actual failure, will see it on the power bill within a billing cycle.
If you've selected EM heat and the house is heating fine, that's expected. It should work, since it's just electric heat. The real question is whether you needed it in the first place.
The 85-95 degree confusion behind most of these calls
A huge share of people who search for emergency heat or auxiliary heat information do it because they put a hand up to a heat pump vent and the air felt cool. Here's the thing worth knowing before you touch the thermostat at all: air from a heat pump in normal heating mode typically runs 85 to 95 degrees. Your hand is closer to 98.6. Air that's 10 or 15 degrees below your skin temperature will always feel cool against your palm, even while the system is heating the house correctly.
A gas furnace pushes air closer to 120 degrees, which is why people expect "hot" air from a heating system and get suspicious when a heat pump doesn't deliver it. That expectation is the mismatch, not the equipment. Before assuming something's wrong and reaching for EM heat, check the room temperature against the thermostat setting instead of judging by hand. If the house is actually reaching the setpoint, the system is working as designed.
When auxiliary heat running constantly is a real problem
Short bursts of AUX during a cold snap or after a big setpoint change are routine. AUX that runs for hours on end, especially on a mild day, points to one of a few underlying issues:
Low refrigerant charge reduces how much heat the compressor can move, so AUX compensates constantly to make up the shortfall.
A failing or undersized compressor can't keep pace with the house's heat loss even in ordinary weather, so the thermostat leans on strip heat more than it should.
A dirty outdoor coil, caked with dust or debris, blocks the airflow the unit needs to pull heat from outside efficiently.
A stuck reversing valve can leave the system partly in the wrong mode, forcing AUX to cover more of the load than it's meant to.
Any of these warrants a technician visit rather than living with a higher power bill indefinitely. Our heat pump repair guide covers typical costs for these fixes, and our guide to a heat pump not heating walks through the same failure list from the angle of a system that isn't keeping the house warm at all.
The Arizona angle
Arizona heat pumps see mild winters compared to most of the country. Overnight lows in the 30s in the Valley, occasionally colder in higher-elevation parts of the state, are enough to trigger AUX briefly but rarely enough to push a healthy system toward EM heat territory. If you're in Phoenix, Mesa, or Gilbert and find yourself on emergency heat for more than a night while waiting on a repair, that's more about a genuine equipment failure than the outdoor temperature. Readers in colder climates researching this page will see AUX and EM heat come up far more often, since their systems work harder in heating mode for a longer stretch of the year.
The U.S. Department of Energy publishes background on how heat pump heating and backup heat interact, including efficiency tradeoffs between resistance heat and compressor-driven heat, at energy.gov.
Common questions
Is it bad to leave my thermostat on emergency heat?
Yes, if the heat pump is actually working. EM heat disables the compressor and runs everything on electric strips, which costs noticeably more to operate. It's meant as a temporary bridge until a technician fixes the heat pump, not a setting to leave selected long-term.
Why does my heat pump run auxiliary heat all the time?
Constant AUX usually means the heat pump can't keep up on its own: low refrigerant, a weak or failing compressor, a dirty outdoor coil, or a stuck reversing valve are the common causes. Brief AUX during cold snaps or big setpoint changes is normal. Hours of it on a mild day is not.
What's the difference between auxiliary heat and emergency heat?
Auxiliary heat is automatic supplemental strip heat that blends in alongside a working compressor when needed. Emergency heat is a manual setting that shuts the compressor off entirely and runs on strip heat alone. AUX is routine backup. EM heat means the heat pump itself isn't running.
Why does the air from my heat pump feel cool even when it's heating?
Heat pump air in normal heating mode runs 85 to 95 degrees, below your 98.6-degree hand and well below the 120-degree air a gas furnace produces. It feels cool by comparison even while heating the house correctly. Check the room temperature against the thermostat setting rather than judging by touch.
Should I switch to emergency heat myself if I think something's wrong?
Only as a stopgap once you know the heat pump has actually failed, like a frozen outdoor unit or a dead compressor confirmed by a technician. Switching to EM heat because the air feels cool or the house seems slow to warm usually isn't necessary and runs up the electric bill for no reason.
Get a technician's read before you touch the thermostat
If AUX is running for hours at a stretch, or you've already switched to EM heat and aren't sure why, that's worth a diagnostic visit rather than guesswork. Use the quote form below to get matched with licensed heat pump technicians in Chandler, Tempe, and Scottsdale. For a full breakdown of what typically fails and what it costs to fix, see our heat pump repair guide.