Aux heat vs emergency heat on a heat pump thermostat: what AUX HEAT and EM HEAT mean, which one is normal, and when constant aux heat is the real warning

If the outdoor unit is not running at all, or is frozen solid, this is the wrong page: Lennox says that is one of the three cases for emergency heat, and the next call is a technician, not a thermostat menu.

What is the thermostat telling you?

Auxiliary heat is the second heating stage a thermostat adds on top of a running heat pump. Emergency heat is the same backup source used by itself, with the compressor locked out. Honeywell Home says it is common for both to be the same electric heat strips or fossil fuel burner; what changes is whether the heat pump keeps running.

On a dual-fuel system (a heat pump paired with a gas or oil furnace) Lennox says the second stage turns off the first stage and runs alone, so “runs together” above applies to electric air handlers only.

When does aux heat come on by itself?

Three triggers, all automatic, and all printed by the makers.

Google’s 2023 configuration guide for the Nest Learning Thermostat and Thermostat E prints the ranges: auxiliary heat lockout default 40 F, adjustable from 0 to 90 F or “Always”; compressor lockout default -25 F, adjustable from -25 to 32 F; dual fuel breakpoint default 30 F. The Heat Pump Balance setting moves the lockout for you: Max Comfort (the default) gives a higher lockout, Max Savings a lower one, so AUX comes on later and less often.

Lennox gives the same idea in one sentence: if your AUX lockout is 40 F (4.4 C), auxiliary heat comes on when the outdoor air falls below it.

Why does the backup cost so much more?

Carrier’s page says electric auxiliary heat is 100% efficient but much more expensive to operate than a heat pump, which Carrier says can be 300% efficient. A U.S. Department of Energy article from January 2023 puts the same comparison the other way round: older electric resistance heating requires at least twice as much energy as a heat pump to keep a house warm. Google’s Nest help translates that into a bill: AUX heat can cost about 2 to 5 times as much as running the heat pump.

That is why DOE’s FEMP buying guide for air-source heat pumps says the resistance elements’ use “should be kept to a minimum”, and why it recommends thermostats designed for heat pumps, which ramp the temperature up slowly to avoid activating them. Emergency heat is the extreme case: with the compressor locked out, every hour is resistance heat. Lennox says the energy bill can climb quickly if emergency heat runs for days or weeks.

A thermostat setback is the owner’s own trigger. Carrier says to avoid dropping the temperature significantly at night, because recovering that heat in the morning often forces the auxiliary heat on. ecobee says heat pumps need a smaller setback and that keeping setbacks modest reduces the need for backup heating. Neither maker prints a number of degrees; the thermostat setback savings calculator covers what DOE does and does not claim for heat pumps.

When is emergency heat the right call?

Lennox lists three cases: a technician specifically instructs you to turn it on, the heat pump has completely failed, or the outdoor unit is frozen solid or damaged. Carrier says to use it only if the heat pump has physically broken down or malfunctioned. Google adds the test for everyone else: if you find you need emergency heat often, the heat pump may not be working as well as it should, and a local HVAC technician should test it.

Once on, it stays on. Google says a Nest uses emergency heat to reach the set temperature until you manually turn it off, then resumes heating normally. Lennox’s advice before the call is short: confirm the thermostat is on Heat or Auto rather than EM Heat, reset any tripped breaker and watch the system, and clear snow and debris from the outdoor unit without sharp tools or forcing ice off.

The wiring behind the two words is the AUX (or W2) and E terminals; the thermostat wire colors and terminal letters guide lists what each one switches.

What should you do first?

1. Read the mode, not the indicator: if the system switch is on Em Ht or Emergency Heat, move it back to Heat (Honeywell, Lennox).
2. Note the outdoor temperature each time AUX shows. Below the balance point or during defrost, it is normal (Carrier, Google).
3. AUX above about 40 F: replace the air filter and shrink the night setback (Carrier, ecobee).
4. On a Nest, check Heat Pump Balance and the auxiliary heat lockout, 40 F by default; Max Savings lowers it (Google).
5. Outdoor unit silent, iced solid, or AUX still constant in mild weather: book a technician. Carrier’s remaining causes are a refrigerant leak or a failed sensor, and neither is an owner repair.

Frequently asked questions

Sources

1. Honeywell Home Support, What is the difference between Aux Ht and Em Ht? (updated 2 September 2025) (manufacturer or vendor document)
2. Google Nest Help, Emergency Heat for heat pump systems (manufacturer or vendor document)
3. Google Nest Help, Heat Pump Balance (manufacturer or vendor document)
4. Google Nest, Configuring Heat Pump Settings, Nest Learning Thermostat and Nest Thermostat E (2023 PDF) (manufacturer or vendor document)
5. Carrier, What Is Auxiliary Heat? (manufacturer or vendor document)
6. Lennox Support FAQ, What is the difference between Emergency Heat and Auxiliary Heat? (manufacturer or vendor document)
7. Lennox Life, What is Emergency Heat and When to Use it? (manufacturer or vendor document)
8. ecobee, Does Turning Down Your Thermostat Save You Money? 6 Winter Heating Questions Answered (manufacturer or vendor document)
9. U.S. Department of Energy FEMP, Purchasing Energy-Efficient Residential Air-Source Heat Pumps (government or regulatory source)
10. U.S. Department of Energy, Heat Pumps Keep Homes Warm and Bills Low this Winter (18 January 2023) (government or regulatory source)