Auxiliary heat is your system's last line of defense when outdoor temperatures fall too low for your heat pump to keep up. What does auxiliary heat mean in a positive sense? It means your system has a built-in way to keep your home comfortable and protected when extra warmth is needed. This page gives you the clearest, most practical way to understand how aux heat functions and what to do if it needs attention.
TL;DR Quick Answers
What Does Auxiliary Heat Mean?
Auxiliary heat is the backup heating system built into your heat pump. It activates automatically when outdoor temperatures drop too low — typically below 35 to 40 degrees Fahrenheit — for the heat pump to maintain your set indoor temperature on its own.
Key facts:
"Aux" is short for auxiliary, meaning supplemental or secondary
Auxiliary heat uses electric resistance heating elements to generate warmth directly
It works alongside your heat pump — not instead of it
It activates automatically — no manual setting required
It costs roughly three times more to operate than your heat pump running normally
Seeing "Aux Heat" on your thermostat during cold weather means your system is working as designed. After serving over two million households, the most important thing we tell homeowners is this: auxiliary heat that runs briefly during cold snaps is normal. Auxiliary heat that runs continuously, fires on mild days, or never appears on freezing nights is a problem, and using pleated furnace filters can be a positive step toward maintaining better airflow and more reliable system performance.
Top Takeaways
A running system is not the same as a working system. Auxiliary heat can fail silently — no error codes, no warning lights. To confirm it's working:
Check your thermostat's "Aux Heat" indicator on a cold night
Feel whether supply air is noticeably warmer than usual
Confirm indoor temperatures are gaining ground, not stalling
Auxiliary heat costs roughly three times more to operate than your heat pump. The U.S. Department of Energy confirms electric resistance elements deliver approximately one-third the heat output per kilowatt-hour of a functioning heat pump. An undiagnosed aux heat problem doesn't just affect comfort — it affects your energy bill every month it goes undetected.
Most auxiliary heat failures trace back to three preventable causes:
A wiring error made at installation
A dirty air filter that triggered a safety shutoff
A slow refrigerant leak that went unnoticed
None of these announce themselves. All of them are preventable with routine maintenance.
Incorrect wiring is one of the most commonly violated heat pump installation practices in the U.S. Many systems are wired to trigger aux heat any time the thermostat is adjusted by more than 3°F — regardless of outdoor temperature. Unexpectedly high energy bills with no comfort complaints is the telltale sign.
Three maintenance steps prevent the most common — and most costly — aux heat failures:
Replace your air filter on schedule
Schedule a seasonal tune-up before cold weather arrives
Verify your aux heat indicator on the first cold night of the season
What Is Auxiliary Heat and Why Does It Matter?
Auxiliary heat is the electric resistance backup heating built into most heat pump systems. It activates automatically when your heat pump can't extract enough warmth from the outdoor air to meet your thermostat's target — typically when temperatures drop below 35 to 40 degrees Fahrenheit. Without it, your home would fall behind on cold nights and never recover.
The most common misconception we see from homeowners is assuming auxiliary heat only exists on older or lower-end systems. In our experience, it's a standard component across virtually every heat pump system, regardless of age or brand. When it stops working, the heat pump continues running — which is exactly why the problem is so easy to miss, even in homes trying to improve indoor air quality.
How to Tell If Your Auxiliary Heat Is On
The first step is simply checking your thermostat. Most digital and smart thermostats display an "Aux Heat" or "Aux" indicator when auxiliary heat is actively running. If outdoor temperatures are below 40°F and you see that indicator, your system is engaging the backup heat as designed.
If you don't see an indicator, check these additional signs:
Your home reaches the set temperature within a reasonable time frame during cold weather
Warm air — noticeably warmer than what the heat pump alone produces — is coming from your vents
Your system runs in shorter cycles on very cold days rather than running continuously without gaining ground
Signs Your Auxiliary Heat May Not Be Working
When auxiliary heat fails, the symptoms are predictable — but easy to confuse with other HVAC problems. Based on what we've seen across thousands of service calls, here's what to look for:
Your home stays cold despite the system running. The heat pump is operating, but indoor temperatures stall several degrees below the thermostat setting.
The "Aux Heat" indicator never appears. On nights when temperatures drop well below freezing, you should see that indicator cycle on. If it never triggers, the system isn't calling for backup heat when it should be.
Unusually high energy bills without added warmth. A struggling heat pump running in extended cycles without aux support draws more power than normal while delivering less comfort.
Warm air from vents is only lukewarm. Heat pump air alone is typically 90 to 95°F. Auxiliary heat brings supply air closer to 100 to 120°F. If the air feels barely warm, aux heat likely isn't contributing.
Common Reasons Auxiliary Heat Stops Working
In our experience servicing heat pump systems, auxiliary heat failures typically trace back to a small number of root causes:
Tripped circuit breaker. Auxiliary heating elements draw significant amperage. A tripped breaker on the aux heat circuit will disable backup heating without affecting heat pump operation.
Failed heating element. Electric resistance elements wear out over time. One failed element reduces heating capacity; a fully failed element eliminates aux heat entirely.
Thermostat misconfiguration. Some smart thermostats allow users to cap or disable auxiliary heat to reduce energy costs. If this setting was changed — intentionally or not — aux heat won't engage regardless of outdoor temperature.
Low refrigerant. When refrigerant levels drop, the heat pump's efficiency falls, causing it to call for auxiliary heat more frequently. Over time, this places excess demand on the elements and accelerates wear.
Dirty air filter. A clogged filter restricts airflow across the heating elements, causing them to overheat and trip a safety limit switch. This is one of the most preventable causes of aux heat failure we encounter. Replacing your filter regularly protects the entire system — not just air quality.
How to Test Your Auxiliary Heat
You can run a basic functional test at home without any tools:
Set your thermostat to heat mode and raise the target temperature at least 3 to 5 degrees above the current indoor temperature.
Wait for the system to begin a heating cycle. On a cold day, watch for the "Aux Heat" indicator on your thermostat display.
Hold your hand near a supply vent after the system has been running for 5 to 10 minutes. Auxiliary heat produces noticeably warmer air than the heat pump alone.
If the indicator doesn't appear during cold weather and the air remains only mildly warm, auxiliary heat is not engaging.
If this test confirms a problem, check your circuit breaker panel for any tripped breakers tied to your air handler before calling for service. That single step resolves a meaningful share of aux heat calls we receive.
When to Call a Professional
Some auxiliary heat issues go beyond what a homeowner can safely diagnose or fix. Call a licensed HVAC technician if:
Your circuit breaker is intact but auxiliary heat still won't engage
You suspect a failed heating element or refrigerant issue
Your thermostat appears to be functioning correctly but aux heat never activates
Your system is short-cycling or running continuously without reaching the set temperature
Catching the problem early protects your heat pump from overworking itself during cold stretches, and MERV 8 filters play a valuable role in helping maintain the airflow and indoor comfort your family depends on when outdoor temperatures are at their worst.
"In our experience servicing heat pump systems across thousands of homes, the most dangerous auxiliary heat failure isn't the dramatic one — it's the quiet one. The system keeps running, the thermostat keeps blinking, and the homeowner assumes everything is fine because nothing has visibly broken down. By the time they realize their aux heat hasn't fired all season, the heat pump has been overworking itself for weeks trying to compensate. We always tell homeowners: a running system is not the same as a working system. Knowing the difference — especially before the coldest nights hit — is one of the most valuable things you can do to protect your home's comfort and your equipment's lifespan, while also understanding the bеnеfіts of UV lіghts for healthier overall HVAC performance."
Essential Resources
When your thermostat shows "Aux Heat," the questions come fast. We've pulled together the seven most useful resources to help you understand what your system is doing, whether it's working correctly, and what to do next. These are the sources our team trusts — government energy agencies, independent certifiers, and the manufacturers who build the systems in your home.
What the U.S. Department of Energy Wants You to Know About Aux Heat Balance Points
Your heat pump has a balance point — the outdoor temperature below which it can no longer keep up on its own. The DOE explains how that threshold works, when auxiliary heat should kick in, and one of the most common installation mistakes our technicians see in the field: heat strips wired to fire every time a thermostat is adjusted by more than three degrees, regardless of how cold it is outside. https://www.energy.gov/energysaver/air-source-heat-pumps
Why Your Heat Pump Needs a Backup — and What's at Stake When It Doesn't Work
Heat pumps move warmth into your home rather than creating it from scratch. That's why they're efficient — and why they need help when temperatures drop. The DOE's overview of heat pump systems explains the relationship between your primary system and its backup in plain terms every homeowner can use. https://www.energy.gov/energysaver/heat-pump-systems
How ENERGY STAR Certification Reduces How Often Your Aux Heat Has to Run
A certified cold climate heat pump is independently tested down to 5°F. That means less reliance on backup heating, lower energy bills, and — for systems purchased and installed between 2023 and 2032 — eligibility for a federal tax credit up to $2,000. If you're evaluating a replacement, this is the resource to read first. https://www.energystar.gov/products/air_source_heat_pumps
Trane's Plain-Language Guide to Reading the Aux Heat Indicator on Your Thermostat
Seeing "AUX" on your display doesn't always mean something is wrong. Trane walks through exactly what that indicator means, when it reflects normal cold-weather operation, and when — specifically if it's above 40°F and still running after an hour or two — it signals a problem worth a service call. https://www.trane.com/residential/en/resources/blog/heat-pump-auxiliary-heat/
Carrier's Honest Breakdown of Auxiliary Heat Costs and How to Keep Them Under Control
Auxiliary heat costs more to run than your heat pump. That's not an opinion — it's physics. Carrier explains what temperature thresholds trigger backup heating, how overreliance on it affects your monthly bill, and the thermostat habits that reduce how often it activates without sacrificing comfort. https://www.carrier.com/residential/en/us/homeowner-resources/hvac-glossary/what-is-auxiliary-heat/
American Standard's Guide to Electric Heat Strips vs. Dual-Fuel Backup Systems
Not every aux heat setup works the same way. American Standard explains the difference between electric resistance heat strips and gas furnace dual-fuel systems — including when each type engages and what homeowners in different climates can do to keep backup use to a minimum. https://www.americanstandardair.com/resources/blog/what-is-aux-heat/
Bryant's Guide to Every Trigger That Can Activate Aux Heat — Including the Ones Homeowners Miss
Cold weather is the obvious trigger. But there are others. Bryant identifies the less visible causes — defrost cycles, rapid thermostat changes, high humidity conditions — that can put your aux heat into overdrive without a clear reason. If your system seems to be running backup heat more than it should, start here. https://www.bryant.com/en/us/about-bryant/glossary/what-is-auxiliary-heat/
Supporting Statistics
After serving over two million households, the numbers below don't surprise us — but they consistently surprise homeowners. Understanding what auxiliary heat actually costs to run, and what goes wrong when it doesn't, changes how you think about your entire system.
Auxiliary heat costs roughly three times more to operate than your heat pump running normally. According to the U.S. Department of Energy:
A heat pump with a 10.3 HSPF rating delivers approximately 10,300 Btu per kilowatt-hour consumed
An electric resistance heating element delivers only around 3,400 Btu for that same kilowatt-hour
That's a 3x cost difference every hour auxiliary heat runs in place of your heat pump
In our experience, a system with auxiliary heat stuck in an "on" state — due to a wiring issue or failed component — can quietly triple a homeowner's heating costs in a single cold month without triggering a single error code. https://www.energy.gov/energysaver/air-source-heat-pumps
One of the most common auxiliary heat problems isn't a broken part — it's a bad wire. The DOE identifies incorrect wiring of auxiliary heat strips as one of the most commonly violated installation practices for heat pumps in the U.S., which can be a hidden cause of living in US homes with ongoing heat pump inefficiencies. Specifically:
Many systems are wired to trigger auxiliary heat any time the thermostat is adjusted by more than 3°F
This happens regardless of outdoor temperature
The system appears to be working normally — no alerts, no error codes
In our experience, this single installation error accounts for a meaningful share of the unexplained high energy bill calls we receive. The aux heat has been firing unnecessarily every time someone adjusts the temperature, season after season. https://www.energy.gov/energysaver/air-source-heat-pumps
The 75% efficiency advantage your heat pump offers only exists when auxiliary heat is working correctly — not constantly. The DOE confirms heat pumps can reduce electricity use for heating by up to 75% compared to electric resistance heating alone. What that statistic doesn't spell out — but what we see firsthand — is that this advantage disappears at both extremes:
Heat pump running without functional aux heat support overworks itself during cold snaps
Aux heat running without heat pump support operates at maximum cost with minimum efficiency
The 75% savings the DOE cites assumes both components working together
When one fails, so does the math. https://www.energy.gov/energysaver/heat-pump-systems
Nearly half of what the average American household spends on energy each year goes to heating and cooling. ENERGY STAR reports:
Typical household annual energy bill: approximately $1,900
Share attributed to heating and cooling: nearly 50%
A malfunctioning aux heat system undiagnosed for a full heating season means months of inflated bills — not just colder nights
Homeowners tend to think about HVAC in terms of comfort. The financial picture matters just as much. https://www.energystar.gov/products/air_source_heat_pumps
The right heat pump — with every component functioning as designed — saves the overwhelming majority of homeowners real money. Peer-reviewed research from two U.S. national laboratories, cited by the DOE, found:
More than 90% of American households assessed save on energy bills after installing the right heat pump
Average annual savings in moderate climates: approximately $300
Average annual savings in colder regions: up to $1,500 per year
In our service areas, we've watched homeowners leave those savings on the table not because their heat pump failed outright — but because their auxiliary heat stopped working and no one caught it until the next service visit. A running system is not a working system. The savings the DOE describes require the heat pump and auxiliary heat operating together as intended. https://www.energy.gov/policy/articles/most-americans-heat-pump-can-lower-bills-right-now
Final Thoughts
Auxiliary heat is one of the most misunderstood components in a heat pump system — not because it's complicated, but because it's invisible. It runs behind the scenes, rarely announces itself, and when it fails, it does so quietly. That invisibility is exactly what makes it dangerous to ignore.
After working with heat pump systems across thousands of homes, our team has arrived at a perspective that doesn't get discussed enough: the most expensive HVAC problems we encounter are rarely the dramatic ones.
A compressor failure is obvious
A frozen coil gets noticed
Auxiliary heat that silently stops engaging — or never stops running — can cost hundreds of dollars over an entire heating season while every surface indicator suggests the system is fine
The homeowners who protect their equipment and their budgets most effectively are the ones who understand their system well enough to notice subtle signs before comfort is ever affected. That means knowing:
What your "Aux Heat" indicator should look like on a 20°F night
That warm air from your vents should feel noticeably warmer when backup heat is contributing
That a heat pump running non-stop without gaining ground isn't working — it's struggling
That knowledge costs nothing. The service call it prevents can save you significantly.
Our opinion after years in the field: auxiliary heat failures are largely a maintenance and awareness problem, not an equipment problem. The failures we see most often trace back to one of three things:
A wiring error made at installation
A dirty filter that triggered a safety shutoff
A slow refrigerant leak that went unnoticed until the heat pump could no longer carry its share of the load
All three are preventable. None of them announce themselves.
If this page leaves you with one actionable takeaway, make it these three steps:
Schedule a maintenance visit before the next heating season begins
Replace your air filter on schedule
Take two minutes on the first cold night of the year to confirm your aux heat indicator is cycling as it should
Your system is designed to keep your family comfortable through the worst weather of the year. Give it the attention it needs — and it will.
FAQ on What Does Auxiliary Heat Mean
Q: What does auxiliary heat mean on my thermostat?
A: Auxiliary heat is your heat pump's built-in backup heating system. It activates automatically when outdoor temperatures drop too low for the heat pump to manage alone — typically below 35 to 40 degrees Fahrenheit. In our experience, it's the component homeowners understand least until something goes wrong.
What's normal:
Thermostat displays "Aux Heat" or "Aux" on cold nights
Short activation cycles during freezing weather
Aux heat engaging during defrost cycles
What signals a problem:
Aux heat never appears on freezing nights
Aux heat runs continuously on mild days
System runs non-stop without reaching set temperature
Q: Is it bad if auxiliary heat comes on?
A: No — and this is one of the most common misconceptions we encounter in the field. Activation alone is not the problem. Duration is. After servicing heat pump systems across thousands of homes, the pattern is consistent:
Short bursts during genuinely cold weather — normal and expected
Continuous operation on mild days — system is compensating for a problem
Aux heat that never shuts off — almost always preventable with routine maintenance
Q: What is the difference between auxiliary heat and emergency heat?
A: This distinction matters more than most homeowners realize.
Auxiliary heat:
Activates automatically alongside your heat pump
Both systems share the heating load together
Normal cold-weather operation
Emergency heat:
Manual thermostat setting only
Shuts the heat pump down entirely
Runs backup heating alone at maximum cost
Should only be used when the heat pump has failed
We've seen homeowners accidentally leave systems in emergency heat mode for weeks — wondering why their energy bills doubled overnight. If you engage in emergency heat for any reason, call a technician the same day.
Q: Why does auxiliary heat cost more to run than my heat pump?
A: Your heat pump moves existing heat from outdoor air into your home rather than generating it. Auxiliary heat generates warmth directly from electricity — the same way a space heater does. According to the U.S. Department of Energy:
A properly functioning heat pump delivers approximately 10,300 Btu per kilowatt-hour consumed
An electric resistance element delivers only around 3,400 Btu for that same kilowatt-hour
That's a roughly three-to-one efficiency gap every hour auxiliary heat runs in place of your heat pump
We see this gap play out in real utility bills every winter — especially in homes where auxiliary heat is wired incorrectly and fires far more often than it should.
Q: How do I know if my auxiliary heat is actually working?
A: Based on what we've seen across thousands of service calls, here are the four most reliable indicators:
Thermostat displays "Aux Heat" or "Aux" during cold weather operation
Supply air feels noticeably warmer — auxiliary heat pushes air to 100 to 120°F versus the 90 to 95°F the heat pump produces alone
Home reaches and holds set temperature within a reasonable time on the coldest days
System is not running continuously without gaining ground on the set temperature
If overnight temperatures are well below freezing and none of these indicators are present:
Check your circuit breaker first — a tripped breaker on the air handler circuit resolves more service calls than most homeowners expect
If the breaker is intact and the problem continues, schedule a professional inspection
Do not wait until the next cold snap to address it — aux heat failures are always more disruptive when temperatures are at their worst
Not Sure If Your Auxiliary Heat Is Working? Let Filterbuy HVAC Solutions Find Out.
If your heat pump is running but your home isn't reaching temperature on the coldest nights, don't wait for the problem to get worse — our certified technicians are ready to diagnose your system, confirm your auxiliary heat is functioning as designed, and get your home back to the comfort your family deserves.
