Heat Pump AUX Heat: What It Means and When to Worry

AUX heat is the backup heat your system leans on when the heat pump alone can’t keep up. It usually means electric resistance strips inside your air handler, or a paired furnace in a dual-fuel setup, kicking in to close the gap. Short AUX cycles during a cold snap or a defrost cycle are completely normal. What’s not normal:

AUX running continuously for hours in mild, above-freezing weather
AUX activating when the outdoor temperature is well above 40°F
Your energy bill spiking with no obvious cold-weather explanation

If you’re seeing any of those, something’s off with your system, not just the weather.

TL;DR:

AUX heat running constantly in mild weather or above 40°F indicates a system fault rather than normal operation.

Defrost cycle timing, typically every 30 to 90 minutes, should last no more than 15 minutes; longer or more frequent cycles suggest issues.

Using emergency heat manually is costly and should only be a stopgap for severe system failures, not routine operation.

Regular maintenance, such as filter changes and outdoor unit clearance, can reduce unnecessary AUX activation and save on energy costs.

Symptoms like persistent icing, AUX activity above 40°F, or rising utility bills warrant professional diagnosis from a qualified HVAC technician.

How Heat Pump AUX Heat Actually Works
When Does AUX Heat Turn On?
Auxiliary Heat vs. Emergency Heat: What’s the Difference?
How Much Does AUX Heat Cost to Run?
Troubleshooting Checklist: Is Your AUX Heat Normal?
Simple Ways to Reduce Unnecessary AUX Use
When to Call Ricotta for a Heat Pump Diagnostic
Get Your Heat Pump Checked Out by Ricottaheatingandair
Sources

How Heat Pump AUX Heat Actually Works

Your heat pump moves heat from the outdoor air into your home, which works great until the air outside becomes thin on usable warmth. That’s where AUX steps in.

Electric resistance strips. Most homes use heating elements mounted inside the air handler. Think of them like the coils in a toaster, generating heat directly from electricity the moment your thermostat calls for backup.
Dual-fuel systems. Some households pair a heat pump with a gas or oil furnace instead of strips. When temperatures drop, the system switches over to the furnace rather than firing electric coils, which is often cheaper to run in a gas market.
The balance point. Every heat pump has a temperature, typically between 30°F and 40°F, where its heating capacity drops below what your home needs. Below that point, AUX fills the shortfall automatically.

The colder it gets, the harder your compressor has to work and the less heat it can extract from outdoor air, so AUX picks up more of the load as the mercury drops.

When Does AUX Heat Turn On?

AUX heat doesn’t just switch on because your thermostat feels like it. Three triggers explain almost every AUX cycle you’ll see this winter.

Balance-point activation. Once outdoor temperatures fall into that 30°F to 40°F range, your system automatically calls for supplemental heat because the heat pump alone can’t hit your setpoint.
Defrost cycles. When frost builds on the outdoor coil, the system briefly reverses itself to melt it, and during that window your indoor unit can’t produce heat on its own. AUX steps in so you don’t get a blast of cold air from the vents.
Fast thermostat increases. Bump your setpoint up 3°F or 4°F or more in one move, and the system treats it as an emergency, calling AUX to close that gap quickly instead of waiting on the compressor.

Defrost-related AUX use is brief by design. Cycles typically run every 30 to 90 minutes in cold, humid conditions and finish within 15 minutes. If you’re watching AUX kick on far more often than that, or it lingers well past a defrost cycle, that’s worth a closer look.

Auxiliary Heat vs. Emergency Heat: What’s the Difference?

Auxiliary heat and emergency heat get confused constantly, but the distinction matters for both your comfort and your electric bill.

AUX is automatic. Your system decides when to use it, based on outdoor temperature, defrost cycles, or a big thermostat jump. You don’t touch a setting.
Emergency Heat is manual. Flipping that switch on your thermostat shuts the heat pump off entirely and runs the backup source full time, whether that’s strips or a furnace.
Emergency Heat costs more. Since it skips the heat pump completely, you’re paying for 100% backup heat instead of a supplemental boost, and that adds up fast if left on for days.
When to actually use it. Reserve Emergency Heat for a genuinely broken heat pump or a solid-iced outdoor unit that won’t clear. It’s a stopgap, not a comfort setting for a cold week.

How Much Does AUX Heat Cost to Run?

Electric resistance strips are 100% efficient at converting electricity into heat, which sounds great until you compare it to what your heat pump can do on its own. A functioning heat pump can deliver two to four times more heat per dollar of electricity than resistance strips, because it’s moving existing heat rather than manufacturing it from scratch.

Short AUX bursts during a 10 to 15 minute defrost cycle barely register on your bill.
Hours of AUX running because of a stuck thermostat setting or a failing heat pump will show up fast, sometimes within a single billing cycle.
If your backup is a gas furnace rather than electric strips, your relative cost depends on local gas versus electric rates, but strips still tend to run more expensive than a healthy heat pump compressor.

The takeaway: occasional AUX use is a rounding error. Constant AUX use is a budget problem.

Troubleshooting Checklist: Is Your AUX Heat Normal?

Before you call anyone, walk through this in order. Most of it takes five minutes.

Look at the outdoor unit. A light, even coat of frost is normal and should clear on its own during the next defrost cycle. Solid ice that doesn’t melt within 30 to 90 minutes is not normal and points to a fault, often refrigerant, airflow, or a stuck defrost control.
Check your filter. A clogged filter starves the system of airflow, forcing it to work harder and lean on AUX more than it should. Swap it if it’s been more than 60 to 90 days.
Clear the area around the outdoor unit. Snow piled against the coil, debris in the fan, or gutters dumping runoff directly onto the unit can all trigger icing that isn’t a mechanical failure at all, just a placement problem.
Review recent thermostat changes. Did someone bump the setpoint up several degrees, or switch a schedule? That alone can explain a spike in AUX activity.
Note anything unusual. Hissing sounds, defrost cycles that repeat every few minutes instead of every 30 to 90, or AUX running when it’s 45°F outside are all signs to stop troubleshooting and call a technician.

Pro Tip: Keep a mental note of when AUX kicks on. If it’s only during cold snaps or short defrost windows, you’re fine. If you can’t explain why it’s running, that pattern itself is useful information for a technician.

Manufacturer guidance is consistent here: AUX lockout settings and configuration require a technician with access to your system’s dealer settings. Don’t try to disable or reconfigure AUX yourself.

Simple Ways to Reduce Unnecessary AUX Use

You can’t eliminate AUX heat, and you shouldn’t try. But you can stop feeding it unnecessarily.

Skip the deep nighttime setback. Dropping your thermostat more than 3°F overnight forces a big recovery in the morning, and that recovery often triggers exactly the rapid-rise AUX activation we covered earlier. A steady setpoint, or a modest 0°F to 3°F dip, avoids that spike entirely.
Use a heat-pump-compatible smart thermostat. These are built to stage heat gradually instead of slamming AUX on for every temperature change.
Schedule an annual tune-up. A maintenance visit catches low refrigerant, dirty coils, or a sluggish defrost board before they force your system into AUX more than it should need.
Keep the outdoor unit clear. Redirect downspouts away from the pad, and shovel snow back a couple feet in heavy weather.
Consider a cold-climate model if AUX runs constantly. Newer cold-climate heat pumps hold their capacity much further down the temperature scale, which cuts how often AUX has to step in during a typical St. Louis winter.

When to Call Ricotta for a Heat Pump Diagnostic

Some AUX behavior is a system telling you it needs help, not just cold weather doing its thing. Call us when you see solid ice on the outdoor unit that won’t clear, AUX running steadily above 40°F, or a system that just doesn’t feel like it’s keeping up anymore.

We’ve been diagnosing HVAC systems around St. Louis and Jefferson County for 50 years, and we’ve built more than 150 five-star Google reviews doing it honestly, without pushing repairs you don’t need. When you call, a technician checks:

Airflow and filter condition
Defrost control board timing and behavior
Refrigerant charge
Fan motor and compressor function
Thermostat configuration and AUX lockout settings

A note from Alan

Living with a heat pump means making peace with the hum and click of defrost cycles. That’s just how these systems breathe in winter. What actually saves you money is catching the abnormal stuff early, before a small icing issue turns into a compressor problem.

— Alan

Auxiliary heat is a normal backup system, but it should activate briefly and predictably, not run constantly or in mild weather.

Point	Details
Know the balance point	AUX typically activates between 30°F and 40°F when the heat pump alone can’t meet the setpoint.
Defrost cycles are normal	Expect them roughly every 30 to 90 minutes in cold, humid weather, clearing within 15 minutes.
AUX and Emergency Heat differ	AUX is automatic and supplemental; Emergency Heat is manual and runs backup heat exclusively.
Avoid deep setbacks	Keep nighttime setback to 0°F to 3°F to prevent long AUX recovery runs.
Call Ricottaheatingandair for faults	Solid ice, AUX above 40°F, or rising bills mean it’s time for a professional diagnostic.

Get Your Heat Pump Checked Out by Ricottaheatingandair

If you’ve been running through this checklist and still aren’t sure whether your AUX heat is behaving normally, that’s exactly the kind of call worth making. Ricottaheatingandair is the local alternative to guessing your way through a winter with rising bills and an outdoor unit you’re not sure about, honest diagnostics, no upsell pressure, and a technician who actually explains what your defrost board and refrigerant charge are doing.

Ricottaheatingandair

We service heat pumps from every major brand across St. Louis and Jefferson County, and our maintenance plans are built specifically to catch the airflow and refrigerant issues that push AUX into overdrive before they cost you a full winter’s worth of electric strip heat. If your outdoor unit is icing over or AUX won’t quit even on a mild day, schedule a heat pump diagnostic and get a straight answer instead of another guess.

Sources

Manufacturer guidance from Carrier and Trane explains AUX and defrost behavior in depth. For local service, visit our heat pump page.