Heat Pump Emergency Heat vs. Aux Heat: What They Mean and Cost

Quick answer

AUX heat is your heat pump’s backup heat, usually electric heat strips in the indoor unit. The thermostat turns it on by itself in cold weather, during defrost, or after a big jump in the setting, and that is normal. EM HEAT (emergency heat) is a mode you pick by hand that shuts off the heat pump and runs only the backup heat; use it only when the heat pump has failed, and call for service. Backup heat costs more: at the July 2026 U.S. average price, 10 kW of strips costs about $1.83 an hour, versus roughly 61¢ to $1.05 for a heat pump making the same heat (depending on efficiency and weather).

Aux heat vs. emergency heat at a glance

What to know	AUX (auxiliary) heat	EM HEAT (emergency heat)
Who turns it on	The thermostat, automatically	You, by changing the thermostat’s mode
What runs	Backup heat, usually together with the heat pump	Backup heat only; the compressor is locked out
Heat source	Electric heat strips, or a furnace in a dual-fuel system	Usually the same backup source
Normal when	Cold weather, defrost, a jump of a few degrees in the setting	The heat pump is broken or damaged and you are waiting for repair
Warning sign	Stays on for long stretches above 40°F	You need it often, or it is on and nobody chose it
Display may show	“Aux Heat On” (Honeywell Home), a flame with a plus sign (Trane Home)	“Em Ht” (Honeywell Home), a flame with an “E” (Trane Home)

For central (ducted) heat pumps with backup heat. In a dual-fuel system the backup is a furnace, and Lennox says the heat pump turns off while the furnace runs. Source: Honeywell Home, Carrier, Trane, Lennox, and Google Nest pages listed in the sources

What AUX heat means

Aux is short for auxiliary: backup heat. Trane says every heat pump should have one, and the thermostat picks the most efficient source at any given time, usually the heat pump. The backup is one of two things:

• Electric heat strips in the indoor air handler, which Carrier compares to the coils in a toaster. Trane says almost all dealers install them with a heat pump, and Honeywell Home notes that aux heat and the compressor may run at the same time.
• A gas or oil furnace in a dual-fuel system (Trane, Carrier). Here, Lennox says, the heat pump shuts off while the furnace runs.

When aux heat normally comes on

Trane, Carrier, Lennox, and Google Nest list three normal triggers:

1. Cold weather. Trane says the thermostat calls for aux heat below 40°F outside, or below a trigger temperature set on the thermostat. Carrier says it varies by system, depending on where the heat pump’s output drops to what the house needs; its variable-speed Infinity 23 is designed to heat down to 13°F below zero.
2. Defrost. While the heat pump briefly reverses to melt frost off the outdoor coil, aux heat keeps the house warm (Lennox, Carrier). See the heat pump defrost cycle.
3. A big jump in the setting. A heat pump raises the temperature slowly. A jump of more than 3 or 4 degrees can bring on aux heat, according to Trane and Lennox; Carrier puts the line at 2 or 3.

None of these is a fault. Trane says aux heat is a problem only if it stays on when it is above 40°F.

What EM HEAT means

Emergency heat is a mode you select. It runs only when you move the system switch to Em Ht, and the compressor stays off in that position (Honeywell Home, with Lennox and Carrier agreeing). Because emergency and aux heat are commonly the same source, switching to EM HEAT usually doesn’t add heat; it takes the heat pump out of the job.

Not every system has it. Google Nest suggests looking for an “Emergency” switch or button, checking the heat pump’s manual, or asking an HVAC professional. Where to find it:

• Honeywell Home: the system switch or mode, set to Em Ht.
• Google Nest Thermostat and Nest Learning Thermostat (4th gen): Settings, then Emergency Heat. It stays on until you turn it off, Nest notes.
• Lennox smart thermostats: Modes, then Emergency heat.

When to use emergency heat

Use it when:

• The heat pump has failed completely and you need heat until a technician can fix it (Lennox, Honeywell Home).
• The outdoor unit is frozen solid or damaged, for example by storm debris or ice (Lennox). See heat pump freezing up.
• A technician tells you to while you wait for a repair or parts (Lennox).

Don’t use it just because it’s cold. Aux heat already comes on by itself while the heat pump keeps helping. Carrier says emergency heat is for a heat pump that has broken down, not a substitute for normal operation. Google Nest and ecobee tie it to a heat pump that can’t keep the house warm, and Nest adds that needing it regularly means the system may need service.

If you do turn it on:

• Call for service right away (Trane, Lennox). Long stretches on emergency heat mean steep bills, more wear on the heating elements, and a higher risk of burnout, Lennox warns, though occasional use during a repair or cold snap won’t harm the system.
• Switch back to Heat once the heat pump is fixed.
• See EM HEAT but didn’t choose it? Check that the mode wasn’t set to Emergency Heat by mistake, and don’t switch the system on and off repeatedly (Lennox).

What aux and emergency heat cost to run

Electric aux heat is 100% efficient, Carrier says: one unit of heat per unit of electricity. A heat pump moves heat instead of making it, so it can deliver up to three times more heat energy than the electricity it uses (ENERGY STAR).

Strip size. Kits come in fixed sizes: 5 to 20 kW for Goodman’s HKR air handler kits and 5 to 25 kW for Carrier’s EHKMB kits. The examples here use 10 kW, though Goodman rates its 10 kW kit at 9.5 kW at 240 volts, so real kits can run a little below nominal. Your installer knows your size.

The math. At the EIA’s U.S. average residential price for July 2026, 18.31¢ per kWh, 10 kW for one hour = 10 kWh × $0.1831 = $1.83. That hour makes 34,120 Btu of heat (EIA: 1 kWh = 3,412 Btu). The table shows what a heat pump pays for the same heat at three ENERGY STAR efficiency levels. COP is heat out divided by electricity in. HSPF2 is a season’s heat in Btu per watt-hour of electricity, so HSPF2 ÷ 3.412 gives a season-average COP.

Heat source	COP	Electricity	Cost
Electric heat strips (aux or emergency heat)	1	10.0 kWh	$1.83
Heat pump at 5°F, ENERGY STAR cold-climate minimum	1.75	5.7 kWh	$1.05
Heat pump, season average at the ENERGY STAR split-system minimum (HSPF2 7.8)	2.29	4.4 kWh	$0.80
Heat pump at ENERGY STAR’s “up to three times”	3	3.3 kWh	$0.61

Heat pump rows use ENERGY STAR minimums and its upper estimate, not a specific model. Source: EIA Electric Power Monthly, Table 5.6.A (U.S. residential average, July 2026: 18.31¢/kWh); ENERGY STAR heat pump criteria; EIA Btu conversion

So strip heat costs about 1.75 to 3 times as much per unit of heat as a working heat pump at these ratings; Google Nest’s estimate is wider, about 2 to 5 times. If your backup is a gas furnace, Trane says it may or may not cost more, depending on gas prices compared with electricity. Our heat pump vs. gas furnace cost guide compares them by state.

What strip heat costs in your state

The table below assumes 10 kW of strips running 3 hours a day, the default limit for ecobee’s Aux Heat Runtime alert (3 hours of aux heat in 24). At the U.S. average, that is about $5.49 a day and $164.79 for 30 days, versus about $72 for the same heat from a heat pump at the ENERGY STAR season-average minimum. July 2026 prices ran from 12.72¢ per kWh in Louisiana (about $114 for 30 days) to 48.00¢ in Hawaii ($432).

Cost to run a 10 kW electric heat strip kit 3 hours a day, by state (July 2026 rates)

State	Rate (¢/kWh)	Per hour	Per day	Per month (30 days)
U.S. average	18.31¢	$1.83	$5.49	$164.79
Alabama	16.40¢	$1.64	$4.92	$147.60
Alaska	28.83¢	$2.88	$8.65	$259.47
Arizona	15.38¢	$1.54	$4.61	$138.42
Arkansas	14.33¢	$1.43	$4.30	$128.97
California	33.61¢	$3.36	$10.08	$302.49
Colorado	17.00¢	$1.70	$5.10	$153.00
Connecticut	24.16¢	$2.42	$7.25	$217.44
Delaware	18.48¢	$1.85	$5.54	$166.32
District of Columbia	25.21¢	$2.52	$7.56	$226.89
Florida	15.03¢	$1.50	$4.51	$135.27
Georgia	16.27¢	$1.63	$4.88	$146.43
Hawaii	48.00¢	$4.80	$14.40	$432.00
Idaho	13.73¢	$1.37	$4.12	$123.57
Illinois	19.22¢	$1.92	$5.77	$172.98
Indiana	16.73¢	$1.67	$5.02	$150.57
Iowa	15.99¢	$1.60	$4.80	$143.91
Kansas	15.27¢	$1.53	$4.58	$137.43
Kentucky	13.81¢	$1.38	$4.14	$124.29
Louisiana	12.72¢	$1.27	$3.82	$114.48
Maine	32.41¢	$3.24	$9.72	$291.69
Maryland	21.41¢	$2.14	$6.42	$192.69
Massachusetts	30.49¢	$3.05	$9.15	$274.41
Michigan	23.05¢	$2.31	$6.92	$207.45
Minnesota	17.45¢	$1.74	$5.23	$157.05
Mississippi	14.54¢	$1.45	$4.36	$130.86
Missouri	16.09¢	$1.61	$4.83	$144.81
Montana	14.99¢	$1.50	$4.50	$134.91
Nebraska	13.78¢	$1.38	$4.13	$124.02
Nevada	12.77¢	$1.28	$3.83	$114.93
New Hampshire	26.60¢	$2.66	$7.98	$239.40
New Jersey	25.19¢	$2.52	$7.56	$226.71
New Mexico	16.09¢	$1.61	$4.83	$144.81
New York	29.90¢	$2.99	$8.97	$269.10
North Carolina	15.16¢	$1.52	$4.55	$136.44
North Dakota	13.41¢	$1.34	$4.02	$120.69
Ohio	19.45¢	$1.95	$5.84	$175.05
Oklahoma	14.35¢	$1.43	$4.31	$129.15
Oregon	15.97¢	$1.60	$4.79	$143.73
Pennsylvania	21.72¢	$2.17	$6.52	$195.48
Rhode Island	28.29¢	$2.83	$8.49	$254.61
South Carolina	15.47¢	$1.55	$4.64	$139.23
South Dakota	15.37¢	$1.54	$4.61	$138.33
Tennessee	13.71¢	$1.37	$4.11	$123.39
Texas	15.88¢	$1.59	$4.76	$142.92
Utah	13.12¢	$1.31	$3.94	$118.08
Vermont	23.75¢	$2.38	$7.13	$213.75
Virginia	17.55¢	$1.76	$5.27	$157.95
Washington	14.71¢	$1.47	$4.41	$132.39
West Virginia	15.78¢	$1.58	$4.73	$142.02
Wisconsin	19.06¢	$1.91	$5.72	$171.54
Wyoming	14.36¢	$1.44	$4.31	$129.24

The table assumes all 10 kW are on. Goodman’s kits are built to permit staging, and some aux systems have two stages (ecobee), so partial use costs less. A 20 kW kit at full power costs twice as much.

How to tell if aux heat runs too much

Long aux run times are expected in a hard freeze (Trane). In milder weather, the makers’ rules of thumb differ a little:

• Trane: AUX HEAT showing for more than an hour or two above 40°F is worth investigating; if it stays on, call for service.
• Lennox: aux heat that comes on often or stays on more than about 30 minutes at a time means the heat pump can’t handle its workload.
• Carrier: aux heat above 40°F can point to a dirty filter, a refrigerant leak, a malfunctioning sensor, or a thermostat raised too quickly.
• ecobee: besides the 3-hour runtime alert, an Aux Outdoor Temp Alert flags aux heat above an outdoor temperature you set.

Quick checks you can do

1. The mode. It should be Heat, not Em Heat (Lennox) or Auto (ENERGY STAR).
2. A recent setting change. Lower the setting by several degrees (Trane). AUX should go away while the heat pump keeps running; if it stays on when it isn’t that cold, the system may not be switching heat sources properly.
3. The outdoor unit. Make sure snow isn’t covering or surrounding it (Trane). Turn the system off at the thermostat before clearing it, and keep hands away from the fan and fins.
4. The air filter. Dirty filters can force the system onto backup heat even in mild weather (Carrier). Lennox suggests checking it monthly in heavy use and replacing it every 1 to 3 months.
5. The breakers. A tripped breaker can shut down the outdoor unit and leave the system on backup heat (Lennox).

More detail: heat pump not heating or blowing cold air.

Thermostat settings that control aux heat

• Google Nest Heat Pump Balance. Max Comfort, the default, brings on aux heat sooner and more often; Max Savings uses it as little as possible; Balanced sits in between. Your system still decides how much it needs, Nest notes.
• ecobee thresholds (set on the thermostat, not in the app). Aux Savings Optimization sets how far the room must fall below your setting before aux heat starts (ecobee recommends the 2.9°F default), and Aux Heat Max Outdoor Temperature keeps aux heat off above a chosen outdoor temperature.
• ecobee compressor lockout. Compressor Min Outdoor Temperature stops the heat pump below a chosen outdoor temperature, leaving aux heat to do all the work. The default is 35°F on ecobee’s support page (disabled in its 2022 Premium manual). Ask your heat pump’s manufacturer for the safest setting before changing it, since running the compressor colder than it can handle can damage it (ecobee).
• Trane Home systems. An HVAC technician must configure aux and emergency heat, including lockout temperatures (Trane Home).

Habits help too: keep one steady temperature (ENERGY STAR), and raise the setting a degree or two at a time (Trane), since recovering from a deep night setback often brings on aux heat (Carrier). See our winter thermostat settings guide.

How we calculated these numbers

• Price: EIA Electric Power Monthly, Table 5.6.A, July 2026 (released September 24, 2026): 18.31¢ per kWh, a U.S. residential average across all customers, not your exact plan.
• Formulas: as shown above, with all 10 kW on and season-average COP = HSPF2 ÷ 3.412.
• Rounding: to the cent. We recalculate when the EIA publishes new monthly prices.