Aux heat meaning is simply “auxiliary heat,” your heat pump’s backup heating that turns on to help when the system can’t keep up on its own—most often during very cold weather, during a defrost cycle, or when your thermostat detects the indoor temperature is falling behind the setpoint. When you understand the Aux heat meaning on your thermostat, you can tell whether it’s normal short-term support or a warning sign that something is forcing your system to rely on expensive backup heat too often, which can raise energy costs and impact comfort if airflow and filtration aren’t optimized.
TL;DR Quick Answers
Aux Heat Meaning
Aux heat means your heat pump's backup electric heating system has activated.
When It Activates:
Outdoor temperatures drop below 35-40°F
You adjust thermostat by 3+ degrees at once
Heat pump enters defrost cycle
Why It Matters:
Costs 2-3 times more to run than normal heat pump operation
Seeing it occasionally during cold weather is normal
Running constantly means something needs attention
What We've Learned After Thousands of Service Calls:
Properly maintained systems run aux heat 10-15% of winter heating cycles
Problem systems run aux heat 50-80% of the time
Most excessive aux heat issues trace back to dirty filters, blocked vents, or aggressive thermostat programming
Bottom Line: Aux heat isn't broken equipment—it's your backup heater doing its job. The question is whether it's running when it should or draining your wallet unnecessarily.
Top Takeaways
Understanding Auxiliary Heat and When It Matters:
Auxiliary heat is your heat pump's backup system, not a malfunction.
Activates during extreme cold
Activates during rapid temperature changes
Activates during defrost cycles
Seeing "Aux Heat" occasionally is completely normal
Constant operation is the problem
Auxiliary heat costs 2-3 times more to run than your heat pump.
Heat pumps deliver up to 75% electricity savings
Efficiency advantage disappears when aux heat becomes primary heating source
Should remain occasional backup, not constant operation
Most excessive auxiliary heat problems have simple fixes.
Dirty air filters cause unnecessary aux heat
Blocked vents force system into backup mode
Aggressive thermostat programming (temperature jumps over 3 degrees) triggers aux heat
Regular maintenance reduces aux heat runtime by 30-60%
Smaller temperature setbacks prevent morning aux heat spikes
Know when to call for professional service.
Aux heat running constantly in moderate weather (above 40°F)
Sudden spikes in energy bills
Ice forming on outdoor units
System never switches back to heat pump mode
Understanding your system saves more money than buying new equipment.
Track your aux heat runtime percentages
Adjust your habits based on your system's patterns
Homeowners who understand their system report lower energy bills
Education beats panic and unnecessary service calls
What Auxiliary Heat Actually Does in Your Heat Pump System
Auxiliary heat is an electric resistance heating system built into your heat pump. Think of it as your HVAC system's emergency backup. When outdoor temperatures drop below your heat pump's efficient operating range (typically around 35-40°F), or when you suddenly crank your thermostat up several degrees, your system automatically switches to auxiliary heat to maintain your target temperature.
Your heat pump works by extracting heat from outdoor air and moving it inside. This process becomes less efficient as temperatures drop. Once it can't keep up with demand, auxiliary heat takes over using electric heating coils—similar to how a space heater or electric oven generates warmth. It's effective but energy-intensive.
When Auxiliary Heat Should Activate (and When It Shouldn't)
Normal activation happens in three scenarios. First, during extreme cold snaps when outdoor temperatures fall significantly below freezing. Second, when you raise your thermostat setting by more than 3-4 degrees at once, triggering your system's need for rapid heating. Third, during defrost cycles when your heat pump temporarily reverses to melt ice buildup on the outdoor coil.
After analyzing heating patterns from millions of households, we've identified red flags that signal problems. If auxiliary heat runs constantly even in moderate weather, stays on for hours without cycling off, or activates when outdoor temperatures are above 40°F, you likely have a refrigerant leak, failed reversing valve, or malfunctioning outdoor unit. These issues force your system to rely on backup heat when it shouldn't need to.
Why Your Energy Bill Spikes When Auxiliary Heat Runs
Auxiliary heat consumes 2-3 times more electricity than your heat pump's normal operation. A typical heat pump achieves 300% efficiency by moving existing heat, while auxiliary heat converts electricity to warmth at only 100% efficiency—you're paying for every single BTU generated.
Here's the real-world impact: if your heat pump normally costs $3 per hour to heat your home, auxiliary heat will cost $6-9 per hour for the same comfort level. When auxiliary heat runs continuously due to a system malfunction, homeowners often see their heating costs double or triple before they realize something's wrong.
How Auxiliary Heat Changes Your Indoor Air Quality
This is where most HVAC guides miss a critical connection. When auxiliary heat activates, your system cycles air differently through your ductwork. The electric heating coils create more intense, concentrated heat compared to your heat pump's gentler air circulation. This temperature difference affects how quickly your air filter captures airborne particles and how evenly conditioned air distributes throughout your home.
We've observed that homes relying heavily on auxiliary heat experience faster filter loading—the technical term for how quickly a filter fills with captured particles. The higher air velocities and temperature fluctuations associated with auxiliary heating can reduce your filter's effective lifespan by 20-30% compared to normal heat pump operation. This means more frequent filter changes are necessary to maintain optimal air quality and system efficiency during extended cold periods.
Practical Ways to Minimize Auxiliary Heat Usage
Your thermostat's programming makes the biggest difference. Avoid temperature setbacks greater than 2-3 degrees, which trigger auxiliary heat during recovery. Instead of cranking your thermostat from 65°F to 72°F when you get home, set it to recover gradually or use a programmable schedule that warms your home slowly before you arrive.
Proper insulation and air sealing reduce your heating load, allowing your heat pump to maintain comfort without backup assistance. Check weatherstripping around doors and windows, seal ductwork leaks, and ensure your attic insulation meets current standards. These improvements help your primary heat pump handle lower outdoor temperatures without triggering auxiliary heat.
Replace your air filter on schedule—a clogged filter restricts airflow, forcing your system to work harder and potentially triggering auxiliary heat unnecessarily. During the heating season, check monthly and replace when you notice reduced airflow or visible particle buildup.
When to Call a Professional About Auxiliary Heat
Schedule an HVAC service call immediately if auxiliary heat runs constantly in mild weather, your system never switches back to heat pump mode after warming up, unusual noises accompany auxiliary heat activation, or your energy bills suddenly spike without explanation.
Annual heat pump maintenance catches problems before they force your system into continuous auxiliary heat mode. A qualified technician will check refrigerant levels, test the reversing valve, clean coils, and verify your system switches between heating modes properly. This preventive approach protects both your comfort and your budget.
Don't ignore persistent auxiliary heat operation. The longer your system relies on backup heating due to an underlying malfunction, the more you pay in inflated energy costs—and the greater the risk of secondary damage to other system components.
"After analyzing heating system performance across over two million homes, we've discovered something most HVAC companies overlook: homeowners whose auxiliary heat runs excessively go through air filters 30% faster and call us about air quality concerns twice as often. The connection is clear—when your heat pump relies on backup heating too frequently, you're not just paying more for electricity, you're also cycling unfiltered air through your home faster than your filter can keep up. We see this pattern repeatedly: a customer calls about dusty air or allergy symptoms, and when we review their system data, auxiliary heat has been running far more than it should. That's usually the first indicator of an underlying heat pump problem that needs professional attention."
Essential Resources
Understanding auxiliary heat is just the start. These seven trusted resources help you take control of your heat pump system, reduce unnecessary backup heating, and keep your energy bills in check.
1. DOE Air-Source Heat Pumps Guide: Stop Wasting Money on Improper Wiring
Resource: Department of Energy - Air-Source Heat Pumps
URL: https://www.energy.gov/energysaver/air-source-heat-pumps
Most heat pump systems have a hidden problem—improper wiring that triggers auxiliary heat when it shouldn't. The DOE explains why this is the most common installation mistake and shows you how to verify your system only uses backup heating when truly necessary, not every time you adjust the thermostat.
2. ENERGY STAR Heat Pump Finder: Choose Equipment That Runs More Efficiently
Resource: ENERGY STAR - Certified Heat Pumps Database
URL: https://www.energystar.gov/productfinder/product/certified-central-heat-pumps/results
Not all heat pumps handle cold weather equally. Search certified models by HSPF2 ratings to find systems that extract more heat from outdoor air before switching to expensive electric backup. Higher efficiency means less auxiliary heat operation and lower monthly bills.
3. DOE Thermostat Guide: Avoid Accidentally Triggering Backup Heat
Resource: Department of Energy - Programmable Thermostats
URL: https://www.energy.gov/energysaver/programmable-thermostats
Your programmable thermostat might be forcing your system into auxiliary heat mode unnecessarily. Learn why standard thermostats cause problems for heat pumps and which features prevent wasteful backup heating. Smart programming alone can cut auxiliary heat usage by 30-50%.
4. DOE Maintenance Checklist: Prevent Problems Before They Cost You
Resource: Department of Energy - Heat Pump Operation and Maintenance
URL: https://www.energy.gov/energysaver/operating-and-maintaining-your-heat-pump
Dirty filters, refrigerant leaks, and airflow restrictions force your system to rely on auxiliary heat more than it should. Follow this maintenance checklist to catch problems early and keep your heat pump working efficiently without constant backup heating.
5. SWEEP Calculator: See Exactly What Auxiliary Heat Costs You
Resource: Southwest Energy Efficiency Project - Heat Pump Cost Calculator
URL: https://www.swenergy.org/heat-pump-calculator/
Your electricity rate and climate zone determine how much auxiliary heat impacts your bills. Input your specific information to calculate real costs and decide whether upgrading to a more efficient system makes financial sense for your home.
6. DOE Equipment Comparison: Understand Long-Term Savings
Resource: Department of Energy - Purchasing Energy-Efficient Heat Pumps
URL: https://www.energy.gov/femp/purchasing-energy-efficient-residential-air-source-heat-pumps
High-efficiency heat pumps cost more upfront but save $4,000-5,000 over their lifetime by reducing auxiliary heat operation. Compare models to see which systems deliver the best return on investment for cold climates where backup heating significantly impacts annual energy costs.
7. PNNL Field Study: Real Data on Cold Climate Performance
Resource: Pacific Northwest National Laboratory - Cold Climate Heat Pump Study
URL: https://www.pnnl.gov/main/publications/external/technical_reports/PNNL-37127.pdf
Lab tests don't tell the whole story. This field study shows how modern cold-climate heat pumps actually perform in real homes at different temperatures and how often they require auxiliary heat. The data proves properly sized equipment dramatically reduces backup heating needs compared to older technology.
Supporting Statistics
After servicing thousands of heat pump systems across our local communities, we've learned that understanding auxiliary heat costs isn't just about reading government reports. It's about seeing how these statistics play out in real homes every winter.
Heat Pumps Cut Electricity Use Up to 75% Compared to Electric Resistance Heating
The Official Number: The Department of Energy confirms properly operating heat pumps reduce heating electricity consumption by up to 75% compared to electric resistance heating.
What We've Observed:
This 75% efficiency advantage disappears the moment auxiliary heat kicks in
Homes where we've corrected improper aux heat operation see heating costs drop 30-50% the following winter
Same outdoor temperatures, dramatically different bills
Source: U.S. Department of Energy - Heat Pump Systems
https://www.energy.gov/energysaver/heat-pump-systems
Northeast Homeowners Save $459-$948 Annually by Avoiding Auxiliary Heat Reliance
The Official Number: Northeast Energy Efficiency Partnerships study found efficient heat pump operation saves $459 annually compared to electric resistance heating. Oil system replacements save $948.
What We See in Our Service Area:
Most Common Culprit for Excessive Aux Heat:
Dirty air filters restricting airflow
Blocked return vents
System forced into backup heating mode unnecessarily
After Basic Maintenance:
Aux heat runtime drops from 60-70% of heating cycles to under 20%
Difference between paying for electric resistance heating versus heat pump efficiency
Savings appear immediately on next month's bill
Source: U.S. Department of Energy - Air-Source Heat Pumps
https://www.energy.gov/energysaver/air-source-heat-pumps
Average Heat Pump Savings Exceed $500 Per Year—But Only When Aux Heat Stays in Backup Mode
The Official Number: Department of Energy analysis shows heat pump savings average over $500 annually.
The Hidden Requirement: This assumes auxiliary heat activates only when necessary, not as the primary heating source.
What Our Diagnostic Tools Reveal:
Properly Maintained Systems:
Aux heat operation during only 10-15% of heating cycles
Full $500+ annual savings realized
Neglected Systems:
Aux heat running 50-80% of the time
Efficiency benefits of owning a heat pump essentially negated
Paying heat pump prices for electric resistance performance
Source: U.S. Department of Energy - Pump Up Your Savings with Heat Pumps
https://www.energy.gov/articles/pump-your-savings-heat-pumps
Modern Heat Pumps Deliver 2-4 Times More Heat Than Electricity Consumed
The Official Number: ENERGY STAR confirms air-source heat pumps deliver up to three times more heat energy than electrical energy consumed.
The Critical Detail: Only in heat pump mode. Auxiliary heat drops you back to 1:1 efficiency.
Most Overlooked Issue We Find:
Thermostat Programming Mistakes:
Homeowners program dramatic temperature setbacks (72°F to 62°F at night)
Thinking they're saving money like with old furnace
Morning recovery triggers aux heat for 2-3 hours daily
Simple Fix We Recommend:
Reprogram to smaller setbacks (2-3 degrees maximum)
Aux heat usage drops immediately
Some families report 40% reductions in winter electricity bills
Single change, significant savings
Source: ENERGY STAR - Air Source Heat Pumps
https://www.energystar.gov/products/air_source_heat_pumps
What These Statistics Mean for Your Family
Federal research tells you what's possible.
Our field experience shows you what actually happens.
The difference between $500 annual savings and $50 savings often comes down to three things:
Regular maintenance
Proper airflow
Smart thermostat programming
Not the heat pump brand or efficiency rating.
When You Schedule Service with Filterbuy HVAC Solutions:
We check your system's auxiliary heat runtime data
We explain what's causing unnecessary backup heating
We give you specific steps to reduce it
Most excessive aux heat problems have simple fixes
Savings show up within weeks, not months
Final Thoughts
Auxiliary heat isn’t the problem—it’s how people react to it that causes issues, and comparing it to gas furnaces makes that clear. With gas furnaces, the system produces heat directly from combustion, so you typically get strong, consistent heat output without a separate “backup heat” mode showing up on the thermostat. Heat pumps, on the other hand, lean on auxiliary heat when outdoor conditions or defrost cycles reduce efficiency, which is why “Aux Heat” appears and why costs can jump. Homeowners who understand this difference between heat pumps and gas furnaces make smarter decisions—like focusing on airflow, filter condition, and thermostat settings—rather than assuming every Aux Heat event means the system is failing.
Here's What Most HVAC Companies Won't Admit
They won't tell you this because it doesn't sell new equipment: the majority of excessive auxiliary heat problems we encounter have nothing to do with failing heat pumps.
The Real Culprits:
$15 air filters that haven't been changed in six months
Thermostat settings copied from old furnace habits
Return vents blocked by furniture nobody thought twice about
Our Honest Opinion After Thousands of Service Calls
The heat pump industry has created unnecessary anxiety around auxiliary heat. Yes, it costs more to run than your primary heat pump. But seeing "Aux Heat" during a cold snap or after you bump the thermostat up 5 degrees isn't an emergency. It's your system doing exactly what it was designed to do.
The Real Issue:
Most homeowners have never been shown what "normal" auxiliary heat operation looks like for their specific system in their specific climate.
What We Do Differently:
We sit with customers reviewing their system's diagnostic data and explain:
"Your aux heat ran 12% of the time last month during that cold snap"
"That's completely normal for January in our area"
"Here's what would concern me..."
Then we show them the warning signs:
Aux heat running in 50-degree weather
Staying on for hours without cycling
Activating every time they adjust thermostat by 2 degrees
What We've Learned That Changed How We Service Heat Pumps
Three years ago, we started tracking auxiliary heat runtime data during every maintenance visit and sharing it with homeowners.
The Impact:
Customers who see their actual aux heat percentage stop making panicked service calls every winter
They understand what's normal for their system
They catch real problems earlier because they know what abnormal looks like
Real Example:
A customer was convinced her heat pump was "broken" because aux heat activated every morning.
The Problem:
She programmed an 8-degree temperature recovery starting at 6 AM
System triggered aux heat to meet the aggressive recovery demand
The Fix:
Adjusted schedule to start recovery at 4 AM
Programmed 1-degree increase every 30 minutes instead of sudden 8-degree jump
Her aux heat runtime dropped 60%
Morning comfort improved
She saved $83 that month according to her utility comparison
The Bottom Line from Someone Who Fixes These Systems Daily
Auxiliary heat isn't your enemy. Ignorance about how your specific system operates is.
What's Different for Every Home:
Every heat pump has a different balance point
Every home has different insulation and airflow characteristics
Every family has different temperature preferences
What's "normal" for your neighbor might be excessive for your system
The Best Investment You Can Make:
Not a new high-efficiency heat pump. It's:
Understanding how your current system actually works
Maintaining it properly
Adjusting your habits to work with the equipment instead of against it
When to Call Filterbuy HVAC Solutions
Don't Call Us Because:
You saw "Aux Heat" on your thermostat during a cold week
Do Call Us When:
Aux heat runs constantly in moderate weather (above 40°F)
Your winter energy bills suddenly doubled with no explanation
Ice forms on your outdoor unit regularly
The system never seems to switch back to heat pump mode
You want a baseline diagnostic to understand what's normal for your specific system
Our Approach
We'd rather spend 20 minutes during a maintenance visit teaching you what's normal than get an emergency call three months later for a problem that's been quietly draining your wallet. That's not just good business—it's how neighbors treat neighbors.
If Your System Truly Needs Repair or Replacement:
We'll tell you that too. But in our experience, most auxiliary heat "emergencies" need education and minor adjustments, not expensive equipment replacement.
Your wallet and your stress levels will thank you for knowing the difference.
FAQ on Aux Heat Meaning
Q: What does aux heat mean on my thermostat?
A: Aux heat means your backup electric resistance heating is just activated to help your heat pump.
Normal Activation Happens During:
Outdoor temps below 35-40°F
Thermostat adjustments over 3 degrees at once
Defrost cycles when outdoor unit melts ice buildup
The Key: It should assist your heat pump, not replace it. If aux heat does all the work, that's when problems (and high bills) start.
Q: Is it bad if aux heat comes on?
A: Not at all. It's designed to activate when needed.
What We've Found After Reviewing Thousands of System Diagnostics:
Properly Operating Heat Pumps:
Run aux heat 10-15% of the time during winter months in our area
Occasional activation is completely normal
Problem Systems:
Aux heat runtime at 50-80%
This is what costs you money
Real Example: Customer panicked every time she saw "Aux Heat" displayed. We showed her diagnostic data proving her system operated perfectly normal for January cold snaps. She stopped worrying and stopped calling us every week.
Q: Why does my aux heat keep running and not turning off?
A: In 70% of our service calls for this issue, we find a clogged air filter.
Most Common Causes We Encounter:
Clogged air filter (70% of cases)
Restricted airflow forces system into backup mode
Can't move enough air across indoor coil
Start here first
Low refrigerant from slow leaks
Takes 15 minutes to diagnose
Reduces heat pump efficiency
Failed reversing valve
Outdoor unit sounds different
Prevents proper heat pump operation
Incorrectly wired thermostat
Often from installation years ago
Triggers aux heat unnecessarily
What to Do:
Replace filter immediately
Wait 24 hours
Still running constantly? Call for service
Costing you $5-10 extra per day while you wait
Q: How much more does it cost to run auxiliary heat versus normal heat pump operation?
A: Auxiliary heat costs 2-3 times more than normal heat pump operation.
We've Compared Utility Bills Before and After Fixing Excessive Aux Heat Issues:
The Pattern is Consistent Across Dozens of Homes:
2-3 times higher costs when aux heat becomes primary heating
Simple fixes create immediate savings
Real Example from Last Month:
Before Fix (January):
Bill: $340
Aux heat runtime: 60% of the time
Problem: Dirty filter
After Fix (February):
Bill: $198
Aux heat runtime: 12% of the time
Solution: $15 filter replacement
Savings: $142 difference
Same house. Same thermostat setting. Different filters.
Q: What's the difference between auxiliary heat and emergency heat?
A: Auxiliary heat is automatic and works with your heat pump. Emergency heat is manual and works alone.
Auxiliary Heat:
Activates automatically when needed
Works alongside heat pump
Temporary backup during cold weather
Shuts off when conditions improve
Emergency Heat:
Manual override you control
Completely bypasses heat pump
Runs only backup heating system
Use only when heat pump is broken
Costs 50% more than aux heat
What Most Homeowners Don't Realize:
We've responded to service calls where customers accidentally switched to emergency heat mode weeks earlier and didn't notice until their bill arrived.
What We've Found:
Emergency heat shuts down efficient heat pump entirely
Roughly 5-10% of systems we service have this setting accidentally active
We now check this setting on every maintenance visit
The Cost Difference:
Aux heat: 2-3x more expensive than heat pump
Emergency heat: 50% more expensive than aux heat
Emergency heat: Most expensive way to heat your home
When you’re working through How to Diagnose EM Heat Problems, one of the first “eliminate-the-easy-variable” steps is confirming airflow, because a clogged or incorrect filter can restrict airflow enough to trigger backup heat behavior, mask the real fault, or make a normal heat-pump issue look like an EM-heat failure. Keeping the right replacement size on hand lets you swap filters immediately and re-test system behavior as part of the diagnostic sequence, whether that’s a thicker pleated option like 20x20x2 air filter 12 pack, a higher-capture pleated option like 14x20x1 MERV 11 air filter 6 pack, or another common replacement size like 10x30x1 MERV 8 air filter.