What this covers

• Where the Backup Heat Comes From

• What Auxiliary Heat Means

• What Emergency Heat Means

• When the AUX Light Means Something Is Wrong

• A Thermostat Reading Guide

• Setbacks and the AUX Light

• Three Beliefs About Backup Heat That Cost Money

• Dual-Fuel Systems Behave Differently

• The Local Picture in Nixa

• What to Tell the Technician

• The Short Version

Heat pump thermostats carry two labels that confuse almost everyone who owns one. One reads AUX HEAT, sometimes AUX or AUXILIARY. The other reads EM HEAT or EMERGENCY HEAT. Both involve the same backup heat source, both can make the electric bill jump, and they mean very different things.

Knowing which is which answers a surprising number of winter questions: why the bill doubled, why the house is warm but the outdoor unit is silent, and whether a light on the thermostat is normal or a sign of a fault.

Where the Backup Heat Comes From

A heat pump moves heat from the outdoor air into the house. It does that efficiently in mild weather and less efficiently as the outdoor temperature drops. Almost every heat pump system has a second heat source to cover the gap.

In most homes that second source is a set of electric resistance heat strips inside the indoor air handler. They work like the element in a toaster: current runs through a coil and the coil gets hot. In a dual-fuel system the backup is a gas furnace instead, paired with the heat pump.

The distinction matters because electric resistance heat produces one unit of heat for each unit of electricity used. The Department of Energy notes that a heat pump can deliver up to three times more heat energy than the electrical energy it consumes. That gap is the whole reason backup heat is meant to run as little as possible.

What Auxiliary Heat Means

Auxiliary heat is automatic. The thermostat brings the backup heat on alongside the heat pump when the heat pump alone cannot keep up. Both run at once.

The common triggers are:

• A large gap between the room temperature and the setting, such as after a deep overnight setback

• Very cold outdoor temperatures, below the point where the heat pump can carry the load alone

• The defrost cycle, when the heat pump briefly switches to cooling to clear frost from the outdoor coil

In all three cases the AUX light coming on is normal. It should go off once the house catches up, the defrost cycle ends, or the weather warms.

What Emergency Heat Means

Emergency heat is manual. It is a setting the homeowner chooses, and it shuts the heat pump off completely. Only the backup heat runs.

It exists for one reason: the heat pump has failed or needs to be taken out of service. If the outdoor unit is damaged, iced solid, or making a noise that suggests serious trouble, emergency heat keeps the house warm until a technician arrives.

It is not meant for routine use. Running on emergency heat for days because the house feels warmer that way is one of the most expensive habits a heat pump owner can fall into.

Feature	Auxiliary heat	Emergency heat
Who turns it on	The thermostat, automatically	The homeowner, manually
Heat pump running?	Yes, alongside the backup	No, switched off
Normal in cold weather?	Yes, for short periods	No, only when the heat pump has a problem
Effect on the bill	Raises it while running	Raises it sharply for as long as it is on
When it should end	When the house catches up	When the heat pump is repaired

When the AUX Light Means Something Is Wrong

Auxiliary heat that comes and goes is normal. Auxiliary heat that runs most of the day in moderate weather is a symptom. The heat pump is not producing the heat it should, and the backup is quietly covering for it.

Common causes include:

• Low refrigerant charge, usually from a leak, which reduces the heat the outdoor coil can absorb

• A failed defrost control, leaving the outdoor coil iced and unable to work

• A weak outdoor fan motor or capacitor, which cuts airflow across the coil

• A reversing valve stuck partway, so the system cannot fully switch into heating

• A thermostat set up incorrectly for a heat pump, calling for backup heat too early

The tell is usually the bill. A heat pump owner who sees a winter bill far above the same month last year, with similar weather, should suspect backup heat doing work the heat pump should be doing.

A Thermostat Reading Guide

What the thermostat shows	What is probably happening	What to do
AUX on briefly on a cold morning	Recovering from a setback	Nothing, it should clear
AUX on every half hour or so for a few minutes	Defrost cycles in damp weather	Nothing, this is normal
AUX on most of the day at 40 degrees outside	Heat pump not carrying its load	Book a diagnostic
EM HEAT on and nobody set it	Accidental setting or a thermostat fault	Switch back to normal heat and watch
EM HEAT needed to stay warm at all	Heat pump has failed	Leave it on and call for service

Setbacks and the AUX Light

Many people turn the thermostat down at night or while at work. With a heat pump, a deep setback often means the system calls for auxiliary heat to recover quickly when the setting goes back up.

Smaller setbacks of a degree or two avoid most of that. Some programmable and smart thermostats offer a heat pump recovery mode that raises the temperature gradually, keeping the backup heat off. If a thermostat was installed without being configured for a heat pump, that setting may be missing.

Three Beliefs About Backup Heat That Cost Money

A handful of ideas about these settings get passed around every winter, and each one quietly raises the bill.

“Emergency heat warms the house faster.” It can feel that way, because electric strips produce hotter air than a heat pump does. But the house is not warming any faster in a way that matters, and the cost per hour is far higher. Switching to emergency heat for comfort trades a small difference in air temperature for a large difference in the bill.

“Emergency heat should be used whenever it is very cold.” The heat pump and the auxiliary heat already handle very cold weather together, automatically. Turning the heat pump off removes the efficient half of that team and leaves only the expensive half.

“If the AUX light is on, something is broken.” Brief auxiliary heat is part of normal operation, especially during defrost on damp days. The light on its own is not a fault. The duration and the weather are what matter.

There is one more practical point. If emergency heat is ever switched on during a genuine heat pump failure, it is worth putting a note on the thermostat or the calendar. People forget to switch back, and a system left on emergency heat for weeks after a repair is more common than anyone would expect.

Dual-Fuel Systems Behave Differently

A dual-fuel system pairs a heat pump with a gas furnace instead of electric strips. The thermostat or a control board switches from the heat pump to the furnace at a set outdoor temperature, called the balance point or changeover point.

In these systems the AUX light usually means the furnace is running instead of the heat pump. The economics differ, because gas heat costs less per unit than electric resistance heat in many areas. The diagnostic logic is the same, though: if the furnace runs far more than the weather explains, the heat pump side deserves a look. The detail on dual-fuel and furnace work in Nixa covers the furnace half of that pairing.

The Local Picture in Nixa

Nixa sits in Christian County, just south of Springfield, in IECC climate zone 4A. The zone is classed as mixed-humid, which means many winter days fall in the damp range just around freezing.

Those are the days heat pumps defrost most often, and they are the days the AUX light flickers on and off most. A homeowner seeing that pattern on a gray, misty morning is watching the system work normally. The same light staying on through a dry, sunny afternoon in the 40s is the pattern that needs attention.

What to Tell the Technician

A clear description shortens a diagnostic. Before calling, note:

1. Whether the light reads AUX or EM, and whether anyone set it manually.

2. The outdoor temperature when it was on.

3. Roughly how long it stays on each time.

4. Whether the outdoor unit is running, iced, or silent.

5. Whether the latest electric bill jumped compared with the same month last year.

For homes south of Springfield, heat pump repair in the Nixa area covers the visit and how a repair is priced before any work starts. Redeemed HVAC & Plumbing, which runs that service, works on all makes and models and is open 24 hours, seven days a week. The Nixa office listing shows the service area and hours.

The Short Version

AUX heat is automatic and normal in short bursts. EM heat is manual and meant only for a failed heat pump. A heat pump that leans on auxiliary heat in mild weather is telling you something, usually through the electric bill first. Note what the thermostat shows and when, keep setbacks small, and treat long, unexplained backup heat as a reason to have the system checked.