Auxiliary Heat vs Emergency Heat: Is It the Same Heat, and Should You Flip the Switch?

EcoFlow- 2026/10/27

Table of contents

It's 7 a.m., it's 12°F outside, and the thermostat says "Aux Heat On." Or there's an "Em Heat" setting nobody has touched, and someone asks if they should switch it on. Here's the short answer to auxiliary heat vs emergency heat. In most heat pump systems, both labels run the same backup heater. AUX turns on by itself, while EM heat is a manual override that shuts the heat pump off. Below, you'll find a table for the switch decision, real hourly power figures, and a plan for heat when the grid is down.

Same Heater, Different Trigger: What AUX and EM Mean on Your Thermostat

Most heat pump homes have a second heat source wired in, usually electric strips in the air handler and sometimes a gas furnace. Whether it runs as "auxiliary" or "emergency" heat depends on who turned it on: the thermostat or you.

Searches like "aux heat vs emergency heat" and "em heat vs aux heat" ask the same thing. So if you've wondered what's the difference between auxiliary heat and emergency heat, it comes down to the trigger.

Auxiliary heat: automatic help for the heat pump

Honeywell Home's thermostat support page calls AUX backup heat that helps the compressor hold your setting. The screen shows "Aux Heat On," and both can run at once.

Lennox's emergency and auxiliary heat FAQ says AUX applies only to heat pumps and comes on when the first stage can't meet demand. Electric strips run alongside the heat pump. On a dual-fuel system, the heat pump shuts off and the furnace runs alone. Lennox also gives 40°F as an example AUX lockout, meaning AUX runs only below that outdoor temperature. That's one brand's example, not a universal rule.

Emergency heat: a manual override that sidelines the compressor

Emergency heat starts only when you choose it. Honeywell Home says it runs when the system switch is moved to Em Ht, with the compressor off. It adds that both modes often use the same source, either strips or a fossil fuel furnace.

So is aux heat the same as emergency heat? Usually it's the same hardware under different rules. Google Nest's emergency heat guide says EM heat is for when your main heating isn't working, and it stays on until you turn it off.

The labels change by brand

If you've typed "is auxiliary heat and emergency heat the same" into a search bar, brand labels are part of the reason. Here's how they compare.

Thermostat or maker	Automatic backup	Manual override	Source
Honeywell Home	Aux Ht ("Aux Heat On")	Em Ht (system switch)	Honeywell Home
Google Nest	Auxiliary Heat (W1 wire labeled AUX on Thermostat E)	"Emer. Heat" in the menu, "Emergency Heat" in the Google Home app	Google Nest Help
Lennox	AUX (second stage) with lockout setting	Emergency heat (second stage alone)	Lennox FAQ
Older thermostats	Varies	"Emergency" switch or E terminal	Google Nest Help

Both modes still need grid power, which is one reason winter blackouts are more dangerous for heat pump homes. Knowing the labels is half of it, and the other half is knowing when the switch helps.

Should You Switch to EM Heat? Six Situations, Six Answers

Emergency heat is for a heat pump that can't do its job, which settles most emergency heat vs auxiliary heat questions. It isn't a boost button. Most of the time, the right move is to leave the thermostat alone.

What you're seeing	Is it normal?	Switch to EM heat?	What to do instead
Steam or a whoosh outside, brief cool air	Usually; it's a defrost cycle	No	Wait for the heat pump to resume.
"Aux Heat On" in a cold snap	Yes (Honeywell, Lennox)	No	Leave it in Heat.
"Aux Heat On" after a morning setpoint jump	Common	No	Avoid deep setbacks, which ENERGY STAR says don't save energy with heat pumps.
Outdoor unit stopped, iced solid, or damaged	No	Yes, as a stopgap	Call an HVAC technician. Nest says frequent EM use means service is needed.
Dual-fuel system in deep cold	Yes, the furnace takes over by design (Lennox)	Only if the heat pump failed	EM heat runs the furnace here (Honeywell).
Power outage	No heat from either mode	No	Plan backup power instead.

Before and after you flip the switch

Don't chip ice or open panels yourself. Defrost handles thin frost, but heavy ice or a unit that won't start needs a technician. Because Nest's EM heat stays on until you turn it off, switch back to Heat after the repair.

A heat pump that keeps leaning on backup heat in January is a good reason to prepare home energy for winter early. And in that last row, neither mode can help, so a battery must be sized for what it can really carry.

Heat in a Winter Outage: What a Home Battery Can and Can't Carry

A heat pump and its strips both run on 240V, and strips are the hungriest load in many homes. A battery doesn't turn EM heat into a backup feature or make the heat pump more efficient. It can keep the right circuits alive, if you choose them carefully.

The loads in play (from real nameplates)

Good planning starts with real numbers, not guesses. Here's what maker data shows for one 3-ton setup and its backup strips.

Heat pump: For the 3-ton Carrier 25HCE436C coastal outdoor unit paired with an FB4CNP036L indoor fan coil, Carrier product data 25HCE4C-03PD, page 18 lists the following at 70°F indoor dry-bulb temperature and 1,200 CFM airflow: at 17°F outdoors, 18,900 Btuh integrated heating capacity and 2.52 kW total system input; at 47°F outdoors, 35,000 Btuh and 2.85 kW. The page 19 notes define integrated capacity as net of defrost and include the compressor, outdoor fan and indoor blower in system kW; supplemental heaters are additional. The electrical table on page 8 lists 16.1A compressor RLA and 75.0A LRA at 208/230V.
Heat strips: KFCEH kits in the FX4D fan coil data run 5 to 20 kW and up at 240V, and about 75% of that at 208V.
Starting current: LRA is the surge a compressor pulls at startup, so an electrician must check it before backing up the heat pump.

Labeled estimates (capacity × 0.85 ÷ load)

These figures use 85% of battery capacity to allow for conversion losses. Treat them as rough planning examples, not promises about how long your own home stays warm.

Heat pump at 17°F, one 5kWh battery: 5,000 × 0.85 ÷ 2,520W is about 1.7 hours of nonstop running, or about 10.1 hours with 30kWh. Cycling changes real runtime.
5 kW strips, one battery: 5,000 × 0.85 ÷ 5,000W is about 0.85 hours.
10 kW strips: more than the 7,200W TITAN Plus delivers from its battery, so they can't run on battery alone. Even a 5 kW strip leaves only 2,200W for the blower and everything else.

Whether any strip stage may run on battery is a call for an HVAC professional, confirmed by your electrician. They may set a lockout or allow a single stage.

Matching the battery to the job

Are you trying to keep basics running, or the heat pump itself? Two TITAN models split this job by voltage, one for 120V essentials and one that can reach a 240V heat pump circuit.

EcoFlow TITAN 5000 Classic

• 4,000W Whole-Home Essential Power. Run your fridge, microwave, Wi-Fi, lighting, and CPAP all at once. •5–30kWh Expandable Capacity. Get 1 day to 1 week of essential backup. Add batteries anytime, with no installer needed. • <25 dB. Quiet enough for indoor use—quieter than a whisper. • ≤20 ms Automatic Switchover. The lights don't even flicker. • OASIS 3.0 — Your AI-based Home Energy Manager. Charges off-peak, powers your home during peak hours, and helps lower your electricity bill every day. • North America Highest Safety Standards. UL 9540 certified and tested to UL 9540A, UL 1741 compliant, with EV-grade LFP cells, 24/7 BMS monitoring, and a 5-year warranty.

Buy Now

If keeping the heat pump itself running matters more, you need split-phase 240V power and a careful load check. EcoFlow TITAN Plus fits a heat pump circuit that a load calculation shows it can carry, with strips reviewed separately.

EcoFlow TITAN 5000 Plus

• 100% Whole-Home Backup. 7.2–10kW output and 5kW single-circuit capacity power your 3-ton AC, refrigerator, and the rest of your home simultaneously, with no soft starter required. • 100% Whole-Home Savings. Save up to $3,100 per year, with ROI in as little as 3 years. •Only 30% of Traditional System Cost. Industry-first 200V architecture, custom high-rate LFP cells, and a pre-wired SHP10 help lower costs across the entire lifecycle, from upfront purchase and installation to long-term use. • OASIS 3.0 — Your AI Home Energy Manager. Charges off-peak, powers your home during peak hours, and helps lower your electricity bill every day. • Built to North America's Highest Safety Standards. UL 9540A tested and UL 9540 certified, with EV-grade LFP cells and 24/7 BMS monitoring.

Buy Now

EcoFlow TITAN 5000 Classic covers 120V essentials and can't serve a 240V heat pump at all. EcoFlow TITAN 5000 Plus fits a heat pump circuit a load calculation shows it can carry, with strips reviewed separately. Emergency heat isn't a battery feature, and stored power doesn't raise the heat pump's efficiency. Circuits connect through the matching Smart Home Panel 10, installed by a licensed electrician, and a short list of essential circuits keeps the plan realistic. Outages aside, most people meet EM heat on their bill. Real nameplates show why.

What Each Mode Costs to Run: Electricity per Hour From Real Equipment

Strips turn each kilowatt-hour into 3,412 Btu of heat. A heat pump moves heat instead, and ENERGY STAR's air-source heat pump page says it can deliver up to three times more heat than the electricity it uses.

Mode	Example equipment	kWh per hour	Heat per hour
Heat pump	Carrier 25HCE436C / FB4CNP036L, 47°F outdoors	2.85	35,000 Btu
Heat pump	Same combination, 17°F outdoors (net of defrost)	2.52	18,900 Btu
EM heat	KFCEH 5 kW kit, 240V	5	About 17,060 Btu
EM heat	KFCEH 10 kW kit, 240V	10.0 (7.5 at 208V)	About 34,120 Btu

The same heat, two ways (example)

Matching 18,900 Btu with strips takes 18,900 ÷ 3,412, or about 5.5 kWh, versus 2.52 kWh for the heat pump, so strips use roughly 2.2 times the power. That same table row works out to a COP of about 2.2, since 18,900 ÷ (2,520 × 3.412) ≈ 2.20.

The EIA Short-Term Energy Outlook table puts the U.S. residential price for the first quarter of 2026 at 17.83 cents per kWh. That's about $0.99 an hour on strips versus $0.45 on the heat pump, though your rate will differ.

Habits matter, and the simplest way to save electricity in winter is not forcing the backup heater on. With costs in view, a few thermostat beliefs fall apart.

Four EM Heat Myths That Run Up Strip Heat

Most extra strip heat starts with a mix-up about emergency heat vs aux heat. These four come up every winter, and each one quietly adds strip heat to your bill.

"EM heat warms faster." It's the same backup heater, minus the heat pump (Honeywell), so leave it in Heat.
"Turn it way down at night." ENERGY STAR says heat pumps don't save energy that way, and recovery can trigger AUX.
"Leave it on Auto." ENERGY STAR advises "heat" or "cool" so it doesn't switch modes on odd days.
"EM heat is for blackouts." Both modes need power, and Nest says frequent EM use means the heat pump needs service.

Conclusion

In the aux heat vs em heat question, both modes usually share one heater, but they work in different ways. AUX turns on by itself and helps the heat pump, while EM heat is a manual setting that shuts the heat pump off. Save EM heat for times when the outdoor unit has failed, call a technician, and switch back to Heat once it's fixed. For winter outages, ask an electrician to list your heat pump's LRA and strip kW first. Then compare EcoFlow TITAN Classic for 120V essentials with EcoFlow TITAN Plus for a load-checked 240V heat pump circuit.

FAQs

Is auxiliary heat the same as emergency heat?

Usually it's the same heater in a different mode. AUX starts on its own, while EM heat is a manual setting that keeps the compressor off (Honeywell Home, Lennox).

Is it bad to see "Aux Heat On" in cold weather?

No, it's normal in a cold snap or after a big setpoint jump. Your thermostat is asking for help to hold your setting, and a steady setpoint helps you see it less.

When should I switch to emergency heat?

Only when the outdoor unit has stopped, is heavily iced, or is damaged. Treat it as a stopgap that keeps the house warm until a technician arrives, not a fix.