What Does Emergency Heat Do on a Heat Pump?

Every ducted heat pump thermostat in the Twin Ports has two backup-heat labels on it, and most owners find out what they mean the first time one lights up on a January night. AUX and EM both use the same backup equipment, but they are not the same thing, and mixing them up is one of the most common and most expensive mistakes after a new cold-climate heat pump installation. One is the system working as designed. The other is a manual override for when something has broken.

What is the difference between AUX heat and emergency heat?

AUX heat is backup heat the heat pump system turns on by itself; emergency heat is backup heat you turn on by hand while the heat pump stays off. The thermostat decides when AUX is needed and turns it off again when it is not. EM, sometimes labeled EM HEAT or EMER, is a system mode you select, and it stays selected until you change it back.

AUX heat versus EM heat on a heat pump thermostat

Question	AUX (auxiliary) heat	EM (emergency) heat
Who turns it on	The thermostat or control, automatically	You, by selecting the mode
Outdoor heat pump	Keeps running on electric strip systems	Locked off completely
When it runs	Defrost cycles, recovery, or when the heat pump cannot keep up	Continuously, until you switch the mode back
Normal to see?	Yes, especially in deep cold	Only while the heat pump is broken
What to do	Nothing	Call for service, then switch back after the repair

What is the backup heat source on your heat pump?

The backup heat on a ducted heat pump is either electric resistance heat strips in the indoor air handler or a gas or propane furnace, and which one you have changes what AUX and EM actually do. It is worth knowing which you own before the first cold night; the label on the indoor unit or the installer's paperwork will tell you.

Electric resistance heat strips

An all-electric heat pump with an air handler usually carries heat strips, which are coils that glow like a toaster element in the airstream. With strips, AUX heat runs alongside the heat pump: the compressor keeps producing heat and the strips top it up. In emergency mode the compressor is locked out and the strips carry the entire house by themselves.

A furnace on a dual-fuel system

On a dual-fuel system the backup is a furnace, and the heat pump and the furnace do not normally run at the same time; the control drops the compressor before the burners light. AUX on a dual-fuel thermostat therefore means the furnace has taken over. Emergency heat means furnace-only heating with the heat pump locked out. When and why the handoff happens is covered in the dual-fuel switchover temperature guide.

Why does AUX heat come on in a cold snap or during defrost?

AUX heat comes on during cold snaps and defrost cycles because those are exactly the moments the backup is designed for, and seeing it is normal. A cold-climate heat pump holds roughly 70–85% of rated capacity at 5°F and keeps producing useful heat to around −13°F to −15°F, but as capacity falls on the coldest nights the house can lose heat faster than the compressor supplies it. The backup fills the gap.

• During a defrost cycle the heat pump briefly reverses to melt frost off the outdoor coil, and backup heat commonly runs so the house does not get a blast of cool air. The vapor you may see outside is explained in why a heat pump steams in winter.
• After a large setback, such as a lowered overnight setting, the thermostat may call AUX to recover faster.
• In a long run of deep cold the backup may run for extended periods, and that is what it is there for.
• In a heating season that commonly stretches from late September into May, a correctly sized and wired backup is a working part of the system here, not a spare.

When should you switch a heat pump to emergency heat?

Switch a heat pump to emergency heat only when the outdoor unit has actually failed or cannot run safely, and then call for service the same day. Cold weather on its own is never the reason. The situations that genuinely call for EM are narrow.

1. The outdoor unit is iced solid, with the coil encased rather than lightly frosted, and it is not clearing through its defrost cycles.
2. The outdoor unit is physically damaged: struck by falling ice or a snowplow, crushed, or with a fan blade hitting something.
3. The outdoor unit is not running at all while the thermostat calls for heat and the house temperature keeps falling.
4. The outdoor unit is making a grinding, screeching or banging noise that was not there before.
5. A technician has told you to run on emergency heat until a part arrives.

Why is leaving emergency heat on so expensive?

Leaving emergency heat on is expensive because it throws away the reason you bought a heat pump. A heat pump moves heat that already exists outdoors into the house, so it delivers more heat than the electricity it consumes. Electric resistance strips turn electricity into heat one for one, with nothing gained. Running strips alone for a week or a month produces a painful electric bill with no change in comfort.

The penalty is gentler on a dual-fuel system, because the furnace is doing the work at furnace fuel prices, but it is still wasteful on mild days when the heat pump was the cheaper source. The fix is simple: once the heat pump is repaired, set the thermostat back to normal HEAT mode. Plenty of surprising winter bills trace back to a mode someone selected on one bad night and forgot.

Why backup heat sizing and wiring matter in the Twin Ports

Backup heat has to be sized and wired correctly in this climate because it does real work, not token work. With an ASHRAE 99% winter design temperature of −17°F to −21°F, there will be hours when the backup carries a meaningful share of the load, and in emergency mode it has to carry all of it. Strips that are undersized, a strip circuit that was never connected, or a thermostat configured so AUX never engages all look fine in October and fail in January.

Fuel matters too. In gas-served Duluth a dual-fuel backup is common. Out in propane and all-electric country, in places like Culver, Knife River and Poplar, the choice between strips and a propane furnace as backup is part of the design, and it decides what emergency heat will cost you if the outdoor unit ever goes down in deep cold.

When to call instead of troubleshooting

Call a technician any time you have had to select emergency heat, because EM is a way to stay warm while something is broken, not a repair. Homeowner checks are limited to the thermostat mode and batteries, the air filter, the breakers for the outdoor unit and the air handler, and brushing snow from around the outside of the unit. Refrigerant, the reversing valve, the defrost control and the strip heater circuits are technician work.

If the heat pump fails at night and the backup is not keeping up either, that is a no-heat situation, and 24/7 emergency HVAC repair is the right call rather than waiting for office hours. If you are planning new equipment, ask how the backup is sized, what fuel it uses, and how the thermostat is set to stage it; those answers belong in any heat pump installation quote for this climate.