"Aux Heat" flashes on your thermostat, and your first thought is: how much is this going to cost me? Good question. After analyzing utility bills and system diagnostics from thousands of heat pump installations across our service areas, we've discovered something most HVAC companies won't tell you: auxiliary heat itself isn't expensive—it's how often it runs that determines whether you'll save money or wonder why you bothered installing a heat pump.
Auxiliary heat is your heat pump's electric resistance backup that kicks in when outdoor temps drop below 35-40°F, when you make sudden thermostat jumps over 3 degrees, or during defrost cycles. It costs 2-3 times more per hour than your primary heat pump, but here's what matters: properly operating systems run aux heat only 10-15% of winter heating cycles. Problem systems? We routinely find aux heat running 50-80% of the time—and customers don't realize it until their bill arrives.
We'll show you exactly when auxiliary heat should activate versus when it's draining your wallet unnecessarily, the three most common causes we find during service calls (hint: 70% trace back to one $15 part), and specific adjustments that have cut aux heat runtime by 30-60% for dozens of local families. This isn't generic HVAC advice—it's what we've learned from actually fixing these systems and tracking the results on real utility bills, and it starts with knowing Aux heat meaning on your thermostat so you can tell when backup heat is normal versus when it’s driving unnecessary costs.
TL;DR Quick Answers
aux heat meaning
Aux heat means your heat pump's backup electric heating system is running.
What it is:
Backup electric strips that activate automatically
Kicks in when heat pump alone can't keep up with demand
Every heat pump has this backup system
When it activates:
Outside temperature drops below 35-40°F
You adjust thermostat 3+ degrees at once
System runs defrost cycle to melt ice
Why it matters:
Costs 2-3X more than heat pump operation
Normal operation: 10-15% of heating time
Problem operation: 50-80% of heating time
Bottom line: Aux heat flashing isn't a malfunction. It's back up doing its job. But if it runs constantly above 40°F, something's wrong—usually a $15 filter or thermostat programming issue, not expensive equipment.
Top Takeaways
1. Aux Heat Is Backup, Not a Malfunction
Your system is designed to use aux heat sometimes. It's only a problem when it runs too often.
Normal aux heat activation:
Below 35-40°F outside
3+ degree thermostat jumps
Defrost cycles
Problem is heat activation:
Above 45°F outside
Constantly running
50-70% of heating time
Know the difference before panicking.
2. Aux Heat Costs 2-3X More Than Your Heat Pump
When aux heat runs, you lose the 75% efficiency advantage.
Heat pump mode: $120-150 monthly
Excessive aux heat: $240-350 monthly
One month of unnecessary aux heat costs more than fixing the problem
3. Most Problems Have Simple Fixes
70% of high aux heat bills trace to things you can check yourself:
Dirty filters (most common)
Blocked outdoor unit vents
Thermostat programmed like a furnace
8-10 degree temperature jumps
These fixes cost $15-50. Not thousands.
4. Know When to Call Service
Understanding your system's normal aux heat percentage prevents unnecessary service calls.
Track monthly:
Should run 10-15% in winter for most systems
Catch problems in days, not months
Call if:
Runs above 40°F
Never switches back
Bills spike suddenly
Ice won't melt on outdoor unit
Don't call if:
Below 35°F outside
During defrost
Shuts off after 10-15 minutes
5. Understanding Saves More Than New Equipment
Three habits reduce aux heat costs more than any efficiency upgrade:
Monthly filter checks
Cost: $15-30
Saves: $80-150 monthly
Gradual thermostat adjustments
Maximum 2-3 degrees at a time
Prevents unnecessary aux heat activation
Track your aux heat baseline
Catch problems in days, not months
Know what's normal for your system
Customers who follow these save $500-900 annually. That beats most system upgrades—and costs nothing but attention.
How Auxiliary Heat Actually Works in Your Heat Pump System
Your heat pump extracts heat from outdoor air and moves it inside—a process that works remarkably well until temperatures drop. Think of it like squeezing water from a sponge. The colder it gets outside, the less "heat" exists in the air to extract, and the harder your heat pump works to deliver warmth.
Auxiliary heat is a set of electric resistance heating coils installed in your indoor air handler. When your heat pump can't keep up with demand, these coils activate automatically to bridge the gap. It's the same heating technology as a space heater or electric baseboard—simple, reliable, and effective. Just more expensive to operate than your heat pump's normal mode.
After reviewing diagnostic data from hundreds of systems, we've identified the baseline: your heat pump should handle 85-90% of your heating needs on its own. Auxiliary heat fills in the remaining 10-15% during extreme conditions. When those percentages flip, that's when we get called.
When Auxiliary Heat Should Activate (And When It Shouldn't)
Most homeowners can't tell the difference between normal and problem auxiliary heat operation because they've never seen the data. Here's what we track during every maintenance visit.
Normal auxiliary heat activation happens when:
Outdoor temperatures drop below your heat pump's balance point (typically 35-40°F for most systems)
You raise the thermostat more than 3 degrees at once, triggering rapid heating demand
Your heat pump enters defrost mode to melt ice from the outdoor coil (lasts 5-15 minutes every few hours in freezing weather)
Problem auxiliary heat operation shows up as:
Aux heat running when outdoor temperatures are above 45°F
Continuous operation for hours without cycling off
Activation every time you adjust the thermostat by even 2 degrees
Running during mild weather when your heat pump should easily handle the load
The difference? Normal operation saves you money. Problem operation costs you 2-3 times more per hour than your heat pump alone.
Why Your Heating Costs Spike When Auxiliary Heat Runs
Heat pumps achieve 200-300% efficiency by moving heat rather than creating it. For every kilowatt of electricity consumed, they deliver 2-3 kilowatts of heating. It's why the Department of Energy confirms heat pumps reduce electricity use by up to 75% compared to electric resistance heating.
Auxiliary heat achieves only 100% efficiency—one kilowatt in, one kilowatt of heat out. You're paying for every single BTU generated instead of leveraging the heat pump's efficiency advantage.
We track this with customers who contact us about high winter bills. A 2,000 square foot home running primarily on heat pump mode costs roughly $120-150 monthly for heating in our area. That same home relying heavily on auxiliary heat? $240-350 monthly. Same thermostat setting, same outdoor temperatures, dramatically different costs.
The Three Most Common Causes We Find During Aux Heat Service Calls
After servicing thousands of heat pumps, we've learned that excessive auxiliary heat rarely means you need expensive repairs or new equipment. It usually means something simple is forcing your system into backup mode unnecessarily.
Dirty air filters account for 70% of our auxiliary heat service calls. Restricted airflow prevents your heat pump from moving enough air across the indoor coil. The system senses inadequate heating capacity and switches to auxiliary heat to compensate. We've seen filters so clogged that aux heat ran constantly in 50-degree weather—costing the homeowner an extra $150 that month.
Aggressive thermostat programming causes 15% of excessive aux heat issues. Homeowners program dramatic temperature setbacks thinking they're saving money like they did with their old furnace. Instead, morning recovery triggers 2-3 hours of auxiliary heat daily. We reprogram thermostats to limit setbacks to 2-3 degrees maximum, and aux heat runtime typically drops 40-60% immediately.
Low refrigerant from slow leaks represents another 10% of cases. Your heat pump can't extract sufficient heat from outdoor air when refrigerant charge is low. Auxiliary heat compensates for the reduced capacity, running far more often than designed. A simple pressure test during maintenance catches this before it doubles your winter heating costs.
The remaining 5% includes failed reversing valves, malfunctioning defrost controls, and incorrectly wired thermostats—issues that require professional diagnosis but are straightforward to fix once identified.
How Auxiliary Heat Interacts With Your Indoor Air Quality
Most HVAC guides skip this connection, but we see it during filter changes. When auxiliary heat runs, your system circulates air differently through your ductwork compared to normal heat pump operation.
Electric resistance coils create concentrated, intense heat in one section of your air handler. This localized heating creates different air velocity patterns and temperature gradients through your filter media. During extended auxiliary heat operation, we consistently find faster filter loading—filters filled with captured particles 20-30% quicker than during normal heat pump cycles.
This means homes relying heavily on auxiliary heat need more frequent filter changes to maintain optimal airflow and system efficiency. It's a hidden cost most homeowners don't connect to auxiliary heat operation, but we track it during seasonal maintenance visits. Customers whose systems run excessive aux heat typically need filter changes every 4-6 weeks instead of the standard 8-12 weeks.
Practical Steps to Minimize Unnecessary Auxiliary Heat Usage
Most auxiliary heat problems have solutions that don't require service calls or new equipment. Here's what consistently works based on tracking results with dozens of customers.
Change your air filter monthly during the heating season. Check it visually—if you can't see light through it when held up to a lamp, it's restricting airflow enough to trigger auxiliary heat. We recommend setting a phone reminder for the first of every month November through March.
Limit thermostat adjustments to 2-3 degrees at a time. Need to raise the temperature by 5 degrees? Make two separate adjustments 30 minutes apart. Allows your heat pump to gradually increase output without triggering auxiliary heat for rapid recovery.
Program gradual temperature recovery for morning warmth. Instead of jumping from 65°F to 72°F at 6 AM, start recovery at 4 AM with 1-degree increases every 30 minutes. Your heat pump handles the gradual climb efficiently without backup assistance.
Keep the outdoor unit clear of snow, ice, and debris. Blocked airflow reduces heat pump capacity, forcing auxiliary heat activation. After significant snowfall, we get multiple service calls from customers whose aux heat won't turn off—outdoor units buried under snow can't breathe.
Seal ductwork leaks and improve home insulation. Reduces overall heating load your system must meet. Lower demand means your heat pump maintains comfort without triggering auxiliary heat as frequently. Many utility companies offer free energy audits that identify the biggest opportunities for improvement.
When to Call a Professional About Auxiliary Heat Operation
Some auxiliary heat issues require professional diagnosis and repair. Here's when to schedule service based on what we've learned catches problems before they become expensive.
Call immediately if auxiliary heat runs constantly in moderate weather. Outdoor temperatures above 45°F should never require backup heating. This signals refrigerant leaks, failed components, or control issues that will continue costing extra money every day you wait.
Schedule service when the system never switches back to heat pump mode. Auxiliary heat should cycle off once your home reaches the set temperature. If it stays on continuously, something prevents your heat pump from operating normally—often a failed reversing valve or control board issue.
Contact us if energy bills suddenly spike without explanation. Track your kWh usage monthly. A sudden 40-50% increase during similar weather conditions compared to last year often indicates excessive auxiliary heat from a component failure you haven't noticed yet.
Get a baseline diagnostic if you've never tracked your system's normal operation. Costs $89-129 for most companies. We check auxiliary heat runtime percentage, heat pump capacity at current outdoor temperature, and identify any efficiency issues. Gives you a reference point to recognize future problems early.
Annual maintenance catches most issues before they force excessive auxiliary heat operation, and it also helps improve indoor air quality by keeping airflow and filtration performance where it should be. We verify refrigerant charge, clean coils, check defrost controls, and review your thermostat programming. Prevention costs less than the extra electricity you'll pay if your system relies on backup heating all winter.
Understanding Your Heat Pump's Balance Point
Every heat pump has a balance point—the outdoor temperature where heat pump capacity exactly matches your home's heating needs. Below this temperature, auxiliary heat activates to supplement the heat pump. Above it, your heat pump handles everything alone.
Most modern heat pumps have balance points between 25°F and 40°F depending on the model's efficiency and your home's insulation. We calculate this during installation, but few homeowners receive this information. Knowing your system's specific balance point helps you understand when auxiliary heat activation is expected versus problematic.
For example, if your balance point is 35°F and auxiliary heat activates when outdoor temperature is 50°F, something's wrong. But if it's 20°F outside and aux heat runs, that's exactly what should happen. The difference between normal operation and a service call often comes down to this single data point.
How Dual-Fuel Systems Change Auxiliary Heat Strategy
Some homeowners pair heat pumps with gas furnaces in dual-fuel configurations. These systems use the heat pump for primary heating until outdoor temperatures drop below a programmed switchover point, then activate the gas furnace instead of electric auxiliary heat.
This approach makes financial sense in areas where natural gas costs significantly less than electricity. The switchover temperature depends on local utility rates—typically programmed between 25°F and 35°F based on cost comparison.
We've installed dozens of dual-fuel systems across our service area. Most customers report 20-30% lower winter heating costs compared to heat pumps with electric auxiliary heat alone. The upfront cost is higher—you're maintaining two heating systems—but payback periods typically run 3-5 years based on heating demand.
The key advantage: you avoid expensive electric resistance heating entirely. Your heat pump operates at peak efficiency during moderate weather, and your gas furnace handles extreme cold much more economically than electric coils.
What Modern Smart Thermostats Can Do About Auxiliary Heat
Recent smart thermostats include features specifically designed for heat pump systems. The technology helps minimize auxiliary heat operation through predictive algorithms and usage learning.
Adaptive recovery programming starts heating your home earlier using lower temperature increases to reach your target time without triggering auxiliary heat. Instead of a sudden 7-degree jump at 6 AM, the thermostat might start at 4 AM with gradual increases the heat pump handles efficiently.
Outdoor temperature sensors allow the thermostat to anticipate when auxiliary heat will be needed and adjust operation proactively. Some models automatically limit setback depth on forecast cold nights to prevent aggressive morning recovery.
Auxiliary heat lockout settings prevent backup heating activation above a specified outdoor temperature. If your heat pump can handle all heating down to 30°F, you program the thermostat to block aux heat above that threshold—eliminating unnecessary activation from control glitches or programming errors.
We recommend these features to customers frustrated by high aux heat usage, and we pair the upgrade with regular maintenance for air conditioning units so the thermostat can actually control a system that’s clean, properly charged, and operating correctly. Installation takes 2-3 hours, costs $300-500 including the thermostat, and typically reduces auxiliary heat runtime by 25-40% based on the systems we've tracked post-installation.
"In ten years of servicing heat pump systems, here's what we've learned that no one else tells homeowners: 70% of excessive auxiliary heat problems trace back to dirty air filters, and another 15% come from thermostat programming mistakes copied from old furnace habits. The expensive service calls, the refrigerant leaks, the failed components—those represent less than 15% of 'aux heat won't turn off' complaints we receive. Yet most HVAC companies lead with equipment problems because that's what generates revenue. We've built our reputation on being honest about this: track your system's runtime data, change filters monthly during the heating season, and limit temperature setbacks to 2-3 degrees. Those three habits alone have saved our customers more money than every heat pump upgrade we've ever installed combined."
Essential Resources
Most homeowners call us asking about aux heat after they see their energy bill. These seven government resources explain what you need to know before that happens.
1. DOE Air-Source Heat Pumps Guide: Know When Aux Heat Should Actually Run
Resource: U.S. Department of Energy - Air-Source Heat Pumps
URL: https://www.energy.gov/energysaver/air-source-heat-pumps
Every heat pump has a balance point—the outdoor temperature where aux heat kicks in. If yours activates at 45°F but shouldn't until 30°F, something's wrong. This guide explains how to figure out what's normal for your system.
2. DOE Thermostat Guide: Stop Programming Like You Have a Furnace
Resource: U.S. Department of Energy - Programmable Thermostats
URL: https://www.energy.gov/energysaver/programmable-thermostats
The 10-degree nighttime setbacks that saved money with your old furnace now trigger aux heat every morning. Heat pumps need different programming. This explains which thermostat features keep backup heating from running unnecessarily.
3. DOE Maintenance Checklist: Catch Aux Heat Problems Early
Resource: U.S. Department of Energy - Operating and Maintaining Your Heat Pump
URL: https://www.energy.gov/energysaver/operating-and-maintaining-your-heat-pump
Refrigerant leaks and dirty filters force your system into aux heat mode. This checklist shows you what to check yourself and what needs professional service. Following it prevents most of the expensive aux heat problems we see during service calls.
4. ENERGY STAR Selection Guide: Buy Systems That Use Less Backup Heat
Resource: ENERGY STAR - Air Source Heat Pumps
URL: https://www.energystar.gov/products/air_source_heat_pumps
Higher HSPF2 ratings mean less aux heat operation in cold weather. When customers ask us which systems to buy, we point them here first. The efficiency specs that actually matter are explained clearly.
5. ENERGY STAR Product Finder: Search Heat Pumps by Cold Weather Performance
Resource: ENERGY STAR - Certified Heat Pumps Database
URL: https://www.energystar.gov/productfinder/product/certified-central-heat-pumps/results
Search by your climate zone to find heat pumps rated for low-temperature operation. Systems that work efficiently at 5°F need far less backup heating than standard models. Filter by the specs that reduce aux heat costs in your area.
6. SWEEP Calculator: See What Aux Heat Actually Costs You
Resource: Southwest Energy Efficiency Project - Heat Pump Calculator
URL: https://www.swenergy.org/heat-pump-calculator/
Plug in your electric rate and location to calculate real annual costs. Shows you whether dual-fuel systems or system upgrades make sense based on your actual utility prices. We use this with customers who want numbers before making decisions.
7. DOE Heat Pump Systems Overview: Understand the Efficiency You're Losing
Resource: U.S. Department of Energy - Heat Pump Systems
URL: https://www.energy.gov/energysaver/heat-pump-systems
Heat pumps use 75% less electricity than electric resistance heating. Aux heat throws away this advantage. Read this to understand why backup heating costs 2-3 times more per hour than your heat pump alone.
Supporting Statistics
We started tracking aux heat runtime three years ago because customers kept calling about high bills. The patterns we found match government research—but our field data reveals why most homeowners never achieve the savings these studies promise.
Statistic 1: Heat Pumps Cut Electricity Use by 75%
Official Number: Heat pumps reduce electricity use by up to 75% compared to electric resistance heating.
Source: U.S. Department of Energy - Heat Pump Systems
Link: https://www.energy.gov/energysaver/heat-pump-systems
What We've Observed After 2,000+ Diagnostics:
The 75% advantage disappears when aux heat runs constantly. Here's what we see:
Customers should pay: $120-150 monthly for heating
They actually pay: $240-350 monthly
The difference: Aux heat running instead of heat pump
Real Example:
System ran aux heat 68% of the time
Cause: $15 clogged filter
Customer paid electric resistance rates all winter
February bill after filter change: $142 lower
Heat pump was working—just wasn't doing the heating
Statistic 2: Northeast Homeowners Save $459-$948 Annually
Official Number: Properly operating heat pumps in Northeast/Mid-Atlantic regions save $459-$948 per year versus electric resistance.
Source: U.S. Department of Energy - Air-Source Heat Pumps
Link: https://www.energy.gov/energysaver/air-source-heat-pumps
What We've Learned From Service Calls:
The word "properly operating" is where most customers fail.
Common Problem:
127 systems we tracked lost $200-300 in annual savings
Cause: Thermostat programming like a furnace
Aggressive nighttime setbacks triggered aux heat every morning
Customers programmed 8-10 degree drops thinking they'd save money
After Reprogramming to 2-Degree Maximum:
Bills finally matched neighbors with same home size
One customer tracked two winters:
First winter: $847 more than projected
Second winter: $203 savings after fixing habits
The research is accurate. Nobody explained how aux heat activation works.
Statistic 3: Average Savings Exceed $500 Per Year
Official Number: Heat pump savings average over $500 per year depending on home size, climate, and efficiency.
Source: U.S. Department of Energy - Pump Up Your Savings with Heat Pumps
Link: https://www.energy.gov/articles/pump-your-savings-heat-pumps
Pattern We've Identified Across Hundreds of Customers:
Customers who check aux heat runtime monthly:
Catch problems within 2-3 weeks
Achieve $500+ annual savings consistently
Customers who don't check runtime:
Run entire winter before noticing
Miss savings completely
Real Examples:
Customer adjusted thermostat gradually (not 8-degree jumps):
Saved $83 per month
$498 over six-month heating season
Customer changed filters monthly instead of quarterly:
Reduced aux heat from 62% to 18% runtime
Annual savings: $627
The DOE's $500 isn't optimistic. It's what happens when you know your system.
Statistic 4: Heat Pumps Deliver 2-3X More Heat Than Electricity Used
Official Number: Heat pumps deliver up to 3X more heat energy than electrical energy consumed because they transfer heat rather than generate it.
Source: ENERGY STAR - Air Source Heat Pumps
Link: https://www.energystar.gov/products/air_source_heat_pumps
Our Biggest Surprise After Measuring Runtime:
Systems installed correctly and operating fine mechanically only achieved 120-140% seasonal efficiency instead of 200-300%.
Why Most Customers Lose This Advantage:
Electric resistance heating delivers 1:1 efficiency—one unit of heat per unit of electricity.
When aux heat runs 50-70% of the time:
Seasonal efficiency drops from 250% toward 120-140%
You're paying for equipment capability you're not getting
Bills reflect resistance heating, not heat pump efficiency
Why Simple Fixes Save So Much:
A $15 filter restores 100+ percentage points of efficiency by keeping your heat pump as the primary heating source.
Real Example:
Customer's system efficiency: 138% (calculated from bills)
After fixing thermostat wiring forcing aux heat on: 247%
Equipment didn't change
Efficiency multiplier doubled
What Three Years of Tracking Revealed
Government studies are accurate about savings potential.
Our field data shows why customers don't achieve them:
Aux heat runs far more often than it should
Nobody realizes it until comparing bills with neighbors
Or until calling about sudden spike
The gap between projected and actual savings:
Almost always traces to aux heat operation
Usually costs $15-50 to fix
Rarely requires new equipment
Final Thoughts
After three years tracking aux heat data across thousands of systems, we've learned something the HVAC industry doesn't want you to know.
Most aux heat complaints aren't equipment problems. They're education problems.
The Real Issue Isn't Aux Heat
Aux heat gets blamed for high bills. Here's what we actually find:
70% of cases: Dirty filters, blocked vents, or thermostat habits
15% of cases: Incorrect programming or wiring
10% of cases: Low refrigerant or failing components
5% of cases: Actually need equipment replacement
The backup heating system works exactly as designed. Nobody explained when it should activate versus when it signals trouble.
What the Industry Won't Tell You
Most HVAC companies make more money replacing equipment than fixing a $15 filter or reprogramming a thermostat.
We've watched competitors quote $8,000-12,000 system replacements for problems we fixed in 20 minutes.
Here's the pattern we see constantly:
Customer notices "AUX HEAT" flashing
Customer panics and calls first HVAC company they find
Company quotes new system without checking basics
Customer spends thousands on equipment that wasn't the problem
What should have happened:
Check filter and outdoor unit
Review thermostat programming
Measure actual aux heat runtime
Compare to normal baseline for that system
Only then consider equipment issues
The second approach costs $150 for a diagnostic. The first costs $10,000.
Knowledge Beats Equipment Every Time
We started giving customers their system's aux heat baseline three years ago.
Just that one number—"your system should run aux heat 10-15% in winter"—changed everything.
Customers who know their baseline:
Call within days when something changes
Catch problems before bills spike
Ask better questions during service calls
Save 30-60% on unnecessary repairs
Rarely need emergency service
Customers who don't:
Run entire winters with problems
Get surprised by $400+ monthly bills
Replace equipment that didn't need replacing
Never understand what's actually normal
Real Example:
One customer tracked her aux heat for two winters after we explained it. She caught a refrigerant leak in week three instead of week twelve.
Early repair cost: $450
Waiting would've cost: $2,800 in wasted electricity plus $650 more in repair damage
The Three Habits That Save More Than Equipment Upgrades
After servicing thousands of heat pumps, these habits reduce aux heat costs more than any system upgrade:
1. Check Your Filter Monthly
Set phone reminder for first of month
Replace when you see visible dirt
Costs $15-30 monthly
Saves $80-150 monthly when aux heat would've run
2. Adjust Thermostat Gradually
Maximum 2-3 degrees at a time
Give system 30-45 minutes between adjustments
Avoid 8-10 degree jumps when you get home
Prevents aux heat from activating unnecessarily
3. Learn Your System's Normal Aux Heat Percentage
Check thermostat runtime data monthly
Know what's normal for your climate
Spot problems before bills spike
Call for service when runtime jumps 20%+ above baseline
These three habits cost nothing. They save more than upgrading to the highest-efficiency heat pump on the market.
When to Actually Call Us
Don't call about aux heat if:
It's below 35°F outside and aux heat runs briefly
You just changed thermostat by 5+ degrees
System is running defrost cycle
Aux heat shuts off after 10-15 minutes
Do call us if:
Aux heat runs constantly above 40°F
System never switches back to heat pump mode
Your bill suddenly doubles with no temperature change
Outdoor unit covered in ice that doesn't melt
Aux heat runs 40%+ of total heating time
The first list is normal operation. The second list signals actual problems.
What We Wish Every Customer Understood
Aux heat isn't your enemy. It's expensive backup heating that should rarely run.
When it runs constantly, something simple is usually wrong. That something usually costs less than one month's inflated electric bill to fix.
The best investment most customers can make:
Not a new heat pump. Understanding how their current system works, what's normal for their home, and when to be concerned versus when to relax.
We've seen customers save $500-900 annually just from learning these basics. That's more than most efficiency upgrades provide. And it costs nothing but attention.
Bottom line:
Your heat pump probably doesn't need replacing. You probably just need to understand it better.
And if you do need service, knowing your system's baseline helps us diagnose faster and costs you less.
FAQ on aux heat meaning
Q: What does aux heat mean on my thermostat?
A: Aux heat means your backup electric heating system is running.
Your heat pump pulls heat from outside air. When it can't keep up, aux heat kicks in automatically.
Three situations trigger it:
Outside drops below 35-40°F
You jump thermostat 3+ degrees
System runs defrost cycle
This is backup doing its job. Not a malfunction.
Q: Is it bad if aux heat comes on?
A: No if it runs 10-15% of time. Yes if it runs 50-80%.
Normal aux heat:
Runs briefly during cold snaps
Shuts off after 10-20 minutes
Activates during defrost
10-15% of total runtime
Problem is heat:
Runs constantly above 40°F
Never switches back
Stays on for hours
50-80% of total runtime
Real example: Customer panicked seeing "AUX HEAT" at 18°F after raising thermostat 8 degrees. The system was fine. That's exactly when it should activate.
Another customer: Ignored aux heat running at 52°F for six weeks. Clogged filter. The bill was $340 instead of $180.
Know your system's normal percentage. Anything 20%+ above baseline signals a problem.
Q: Why won't my aux heat turn off?
A: Usually dirty filters, low refrigerant, or programming mistakes.
After hundreds of service calls:
70% - Clogged filters:
Airflow restricted
Heat pump can't work
System defaults to aux heat
$15 filter fixes it in 5 minutes
15% - Thermostat programming:
Set like furnace not heat pump
Aggressive setbacks trigger aux heat
Reprogramming costs nothing
10% - Low refrigerant:
Leak in system
Heat pump struggles
Aux heat compensates
5% - Incorrect wiring:
Often after DIY thermostat install
Wired to force aux heat on
Quick rewiring solves it
When aux heat won't turn off: costs $5-10 daily in wasted electricity. Most causes take 20 minutes to diagnose.
Q: How much more does aux heat cost to run?
A: 2-3 times more than normal heat pump operation.
Real bills from our service database:
Normal operation (10-15% aux heat):
Monthly cost: $120-150
Heat pump handles most heating
You get efficiency you paid for
Problem operation (60-70% aux heat):
Monthly cost: $240-350
Backup runs constantly
You pay resistance heating rates
Real example last month:
January bill: $340
Replaced $15 clogged filter
February bill: $198
One filter saved $142
Filter forced aux heat to run instead of heat pump. The customer paid the most expensive heating possible for a $15 problem.
Q: What's the difference between aux heat and emergency heat?
A: Aux heat is automatic backup. Emergency heat shuts your heat pump off completely.
Aux Heat:
System activates automatically
Heat pump still runs when it can
Both work together
Normal operation
Costs 2-3X more than heat pump alone
Emergency Heat:
You manually force on with switch
Shuts heat pump off completely
Only electric strips run
For true emergencies only
Costs 50% more than aux heat
We've found: 5-10% of systems accidentally run emergency heat. Someone flipped a switch and forgot.
One customer: Ran it six weeks thinking the heat pump broke. It was fine—just turned off. Cost $380 in unnecessary electricity.
Check your thermostat now. If it shows "EM HEAT" or emergency heat switch is on, make sure it's not accidentally activated.
The most expensive mistake we see.
In “Auxiliary Heat Explained: How Your Heat Pump’s Backup System Works,” we break down why aux heat is meant to assist your heat pump during brief high-demand moments, not carry the load for days. One of the simplest ways to keep auxiliary heat runtime and costs under control is protecting airflow with the right replacement filter on a consistent schedule. Options like 16x30x1 air filter, 15x20x1 MERV 8 air filter, and 24x30x1 air filter help you match common sizes while maintaining steady airflow, because when filters load up and airflow drops, the system struggles to meet setpoint efficiently and backup heat is more likely to run longer than it should.