Is it bad if my thermostat shows AUX heat?

No. Auxiliary heat is designed to come on by itself in cold weather, during defrost, or when you raise the setpoint a lot at once. It costs more to run than the heat pump alone, so it is worth knowing why it is on. See our page on why a thermostat shows AUX.

When should I use emergency heat?

Only when the heat pump itself has stopped working. Carrier says to use it if the heat pump has broken down or malfunctioned, and Trane says to call for HVAC service right away if you switch to it. Google says you should not need it often.

Why does emergency heat cost more?

It turns the heat pump off and heats with the backup source alone, which on most systems is electric heat or a furnace. Google describes running emergency heat as usually expensive and inefficient compared with letting the heat pump do most of the work.

Are AUX heat and emergency heat the same heater?

Often, yes. Honeywell says it is common for emergency heat and auxiliary heat to be the same source, such as electric heat strips. The difference is how they are used: AUX helps the heat pump automatically, while emergency heat replaces it until you switch it off.

AUX heat vs emergency heat

Auxiliary heat (AUX) turns on automatically to help a heat pump when it cannot keep up, and the heat pump keeps running alongside it. Emergency heat (EM heat) is a mode you pick by hand that shuts the heat pump off and heats with the backup source alone. It is meant for a heat pump that has failed.

What is the difference between AUX heat and emergency heat?

Both use the backup heat that comes with many heat pump systems. What differs is who turns it on and whether the heat pump keeps running.

Auxiliary heat (AUX) is automatic. The thermostat brings it in when the heat pump alone cannot keep the house at the setpoint, and the heat pump keeps running at the same time. Honeywell says both your auxiliary heat and your compressor may be on at the same time.

Emergency heat (EM heat) is manual. You choose it, and the thermostat then locks out the heat pump and heats with the backup source only. Honeywell's wording for its Em Heat setting: when the thermostat is in this position, the compressor does not run, only the emergency heat runs.

Google sums up the split the same way for Nest: emergency heat must be turned on manually, while Nest thermostats can use auxiliary heat automatically as needed.

AUX heat and emergency heat side by side
	Auxiliary heat (AUX)	Emergency heat (EM heat)
Who turns it on	The thermostat, automatically	You, by choosing the mode
Heat pump	Keeps running	Locked out
Heat source	Backup heat, together with the heat pump	Backup heat only
When it stops	By itself, once the heat pump can keep up	Only when you switch the mode back
Meant for	Cold snaps, defrost, big setpoint increases	A heat pump that has stopped working
Running cost	Higher than the heat pump alone	Highest, because the heat pump does nothing

Summarized from Honeywell Home, Google Nest, Carrier and Trane, listed under Sources.

Why does a heat pump need backup heat at all?

A heat pump moves heat from the outdoor air into the house. On very cold days, during the defrost cycle, or when you ask for a big jump in temperature, it may not move heat fast enough on its own. Carrier describes AUX as automatic: it runs alongside the heat pump to boost heating power.

The backup heat is usually electric heat strips or a furnace. Honeywell notes it is common for the emergency heat and the auxiliary heat to be the same source, which is why the two terms get mixed up. Same heater, two different ways of using it.

When should you switch to emergency heat?

Rarely, and only for a real fault. Carrier says to use EM heat if your heat pump has physically broken down or malfunctioned. Trane calls it a setting for a true emergency, and adds that if you engage it, you should call for HVAC service right away. Google says you should not need emergency heat often, typically only in very cold weather, and that needing it regularly can mean the system needs servicing.

Two practical points follow from how the mode works:

Emergency heat stays on until you turn it off. Google says a Nest thermostat will use it to reach your set temperature until you manually turn it off, and the Lux P621U manual says the mode even survives a power outage.
It is not a faster way to warm the house. With the heat pump locked out, the system loses its most efficient heat source. Google describes running emergency heat as usually expensive and inefficient.

If the house is cold and you are not sure the heat pump has failed, leave the thermostat in Heat and let it decide when to use AUX. Our page on a thermostat that says heat on but nothing happens covers the checks you can make first.

What does each brand call these modes?

The names on the screen vary, and some brands only offer one of them:

How each brand labels auxiliary and emergency heat
Brand	Automatic backup	Manual backup only mode
Honeywell Home	"Aux Heat On" on the display	"Em Heat" system mode; "Em Heat On" while it runs
Google Nest	AUX heat, managed by Heat Pump Balance	"Emergency Heat" in the thermostat or Home app settings
ecobee	Aux heat, controlled by thresholds	"Aux" system mode, which uses only the backup heat
Emerson Sensi	Aux heat, controlled by Aux Lockout and Balance Point	"Aux Heat" mode, which makes the backup the primary heat
Amazon Smart Thermostat	Not stated	"Emergency Heat" in the Alexa app, Thermostat Configuration

From each manufacturer's support pages and manuals. Not stated means the pages we checked do not describe it.

On a Nest Thermostat E set up for a dual fuel system, Google says you will see "Alt heat" instead of AUX heat, and there is no emergency heat option. On the Amazon Smart Thermostat, Amazon notes you cannot use its lower-emissions priority setting while emergency heat is running.

Which one costs more to run?

Emergency heat, because it uses only the backup source for every call for heat. AUX heat also costs more than the heat pump alone, which is why thermostats limit it with settings such as outdoor lockout temperatures. Google puts a figure on it for Nest systems: AUX heat can cost about 2 to 5 times as much as running the heat pump.

How and when a thermostat decides to use AUX, and how to tell whether it is coming on more than it should, is covered in why your thermostat shows AUX when it is not that cold. The schedule also plays a part: a large overnight setback can force AUX on in the morning, which is where recovery mode comes in.

Frequently asked questions

Is it bad if my thermostat shows AUX heat?: No. Auxiliary heat is designed to come on by itself in cold weather, during defrost, or when you raise the setpoint a lot at once. It costs more to run than the heat pump alone, so it is worth knowing why it is on. See our page on why a thermostat shows AUX.
When should I use emergency heat?: Only when the heat pump itself has stopped working. Carrier says to use it if the heat pump has broken down or malfunctioned, and Trane says to call for HVAC service right away if you switch to it. Google says you should not need it often.
Why does emergency heat cost more?: It turns the heat pump off and heats with the backup source alone, which on most systems is electric heat or a furnace. Google describes running emergency heat as usually expensive and inefficient compared with letting the heat pump do most of the work.
Are AUX heat and emergency heat the same heater?: Often, yes. Honeywell says it is common for emergency heat and auxiliary heat to be the same source, such as electric heat strips. The difference is how they are used: AUX helps the heat pump automatically, while emergency heat replaces it until you switch it off.