Most HVAC companies won't tell you this: auxiliary heat problems rarely need expensive repairs. After tracking runtime data across thousands of systems, we’ve found 70% of high auxiliary heat bills trace to $15 filters or thermostat habits that cost nothing to fix. Aux heat meaning is that your heat pump is using its backup electric heat to help reach or hold the set temperature, so when aux heat runs too often, the first suspects are restricted airflow from a dirty filter or thermostat changes that force backup heat—before you assume something major is broken.
Here's what we learned servicing heat pumps since 2013:
Customers who understand their system's normal auxiliary heat percentage catch problems within days instead of months. Those who don't often run entire winters paying 2-3 times more than necessary—sometimes $200-400 in wasted electricity before they call us.
The difference isn't equipment quality. It's knowledge.
This guide shows you:
When auxiliary heat should activate (below 35-40°F, defrost cycles, large temperature jumps)
When it signals problems (constantly running above 40°F, 50-80% runtime instead of 10-15%)
Why one number—your system's baseline percentage—prevents most service calls
Three habits that reduce auxiliary heat costs more than any equipment upgrade
We've diagnosed hundreds of "my auxiliary heat won't turn off" calls. Most take 20 minutes to fix. Many customers can check the issue themselves before spending money on service.
You don't need a new system. You need to know what's normal for yours versus what's draining your wallet.
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 resistance strips
Activates automatically when heat pump needs help
Every heat pump has this
When it activates:
Outside drops below 35-40°F
You adjust thermostat 3+ degrees at once
System runs defrost cycle
Why it matters:
Costs 2-3X more than heat pump operation
Normal: 10-15% of heating time
Problem: 50-80% of heating time
Bottom line:
Aux heat flashing isn't broken. It's back up doing its job.
But if it runs constantly above 40°F, something's wrong—usually $15 filter or thermostat programming, not expensive equipment.
Track your system's percentage monthly. Catch problems in days, not months. Save $200-400 every winter.
Top Takeaways
1. Auxiliary Heat Is Backup, Not a Malfunction
Auxiliary heat means your backup electric resistance heating is running. Every heat pump has this.
Normal activation:
Below 35-40°F outside
3+ degree thermostat jumps
Defrost cycles
10-15% of total heating time
Problem activation:
Above 40°F outside
Constantly running
Never switches back
50-80% of total heating time
Know the difference before panicking.
2. Auxiliary Heat Costs 2-3X More
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 the fix
Real example: $340 January → $198 February after $15 filter.
3. Most Problems Have Simple Fixes
70% of cases trace to things you can check yourself:
Dirty filters (most common)
Thermostat programmed like furnace
Blocked outdoor vents
8-10 degree temperature jumps
These fixes cost $15-50. Not thousands.
4. One Number Prevents Expensive Problems
Know your baseline aux heat percentage. Track monthly.
Normal baseline:
10-15% for most systems
Varies by weather, insulation, habits
Call service when:
Runtime jumps 20%+ above baseline
Runs constantly above 40°F
Never switches back
Bills spike suddenly
Customers who track catch problems in days. Not months.
5. Understanding Beats New Equipment
Three habits reduce aux heat costs more than any upgrade:
Monthly filter checks
Cost: $15-30
Saves: $80-150 monthly
Gradual adjustments
2-3 degrees maximum
Prevents aux heat triggers
Track baseline
Catch problems early
Before bills spike
Customers who follow these save $500-900 annually. Beats most system upgrades—costs nothing but attention.
Bottom line: Your heat pump probably doesn't need replacing. You probably just need to understand when it's working properly versus when something fixable is forcing expensive backup heating.
What Auxiliary Heating Actually Does
Auxiliary heating is your heat pump's backup electric resistance heating system.
Heat pumps work by pulling heat from outside air and moving it inside. They're incredibly efficient—using 75% less electricity than traditional electric heating. But they have limits.
When outdoor temperatures drop or heating demand spikes, auxiliary heat activates automatically:
Supplements heat pump when it can't keep up alone
Uses electric resistance strips (like giant toaster coils)
Produces heat directly instead of moving it
Costs 2-3 times more per hour than heat pump operation
Think of it like four-wheel drive in your car. You don't use it constantly. You engage it when conditions require extra help.
After installing and servicing heat pumps for over a decade, we've learned most customers don't realize auxiliary heat exists until they see it on their thermostat display or notice a bill spike.
How Auxiliary Heat Works in Your System
Your heat pump has two heating sources working together.
Primary heating (heat pump mode):
Extracts heat from outdoor air
Moves it inside using refrigerant cycle
Operates at 200-300% efficiency
Handles 85-90% of your heating needs in most climates
Secondary heating (auxiliary heat mode):
Electric resistance strips in air handler
Activates when heat pump alone isn't enough
Operates at 100% efficiency (1:1 electricity to heat)
Should handle only 10-15% of heating in typical winters
Both systems can run simultaneously. The heat pump does most of the work. Auxiliary heat fills the gap.
The problem starts when auxiliary heat becomes primary and heat pump becomes backup. That's when we get the service calls.
When Auxiliary Heat Should Activate
Auxiliary heat has three normal activation triggers.
1. Cold outdoor temperatures (below 35-40°F):
Heat pump capacity drops as temperature falls
At some point, it can't pull enough heat from cold air
Auxiliary heat supplements to meet indoor temperature setting
This is by design, not a malfunction
2. Large thermostat adjustments (3+ degrees at once):
Heat pump works gradually
Can't make big temperature jumps quickly
Auxiliary heat speeds recovery time
Shuts off once temperature is reached
3. Defrost cycles (periodic ice removal):
Heat pump temporarily reverses to melt ice off outdoor coil
Takes 5-10 minutes
Auxiliary heat maintains indoor temperature during defrost
Happens automatically every 30-90 minutes in freezing weather
We've tracked hundreds of systems. Those operating normally run auxiliary heat 10-15% of total heating time. Some well-insulated homes in moderate climates run it only 5-8%.
When Auxiliary Heat Signals a Problem
After diagnosing thousands of high-bill complaints, we've identified clear warning patterns.
Your system has a problem if auxiliary heat:
Runs constantly when outdoor temperature is above 40-45°F
Never switches back to heat pump mode
Activates during every single temperature adjustment
Shows 50-80% runtime on your thermostat's data screen
Real example from last month:
A customer called about a $340 January bill. Her outdoor temperature averaged 38°F that month. Auxiliary heat should've run maybe 15-20% of the time.
Her thermostat data showed 68% auxiliary heat runtime. We found a clogged filter restricting airflow. The heat pump couldn't work properly, so the system defaulted to auxiliary heat.
$15 filter replacement. February bill dropped to $198.
The equipment worked fine. The symptom—excessive auxiliary heat—pointed directly to the cause.
Why Auxiliary Heat Costs So Much More
The efficiency difference explains everything.
Heat pump operation:
Moves heat instead of creating it
Delivers 2-3 units of heat per unit of electricity
200-300% efficient
Typical monthly cost: $120-150 for 2,000 sq ft home
Auxiliary heat operation:
Creates heat directly from electricity
Delivers 1 unit of heat per unit of electricity
100% efficient (sounds good but isn't)
Typical monthly cost if running constantly: $240-350 for same home
When auxiliary heat runs 60-70% of the time instead of 10-15%, you're paying resistance heating rates for most of your heating. You bought a heat pump for efficiency. You're getting furnace-level costs, and furnace filters are one of the first places to look because a dirty or restrictive filter chokes airflow, reduces heat delivery to the home, and forces the system to call for auxiliary heat far more often than it should.
We've calculated the difference across our customer base:
Every 10% increase in auxiliary heat runtime costs roughly $20-30 more monthly
Jumping from 15% to 65% auxiliary heat runtime costs $100-150 more monthly
Over a six-month heating season, that's $600-900 in unnecessary costs
Most causes take less than an hour to diagnose. Many take minutes to fix.
Three Most Common Causes of Excessive Auxiliary Heat
After servicing thousands of systems, we find the same issues repeatedly.
1. Dirty Air Filters (70% of Cases)
Clogged filters restrict airflow. A heat pump needs proper airflow to work. When it can't pull air across the coils, it can't extract enough heat from outside or deliver it inside efficiently.
The system defaults to auxiliary heat because it's the only way to maintain temperature.
Fix: Replace filter monthly during heating season. Set phone reminders for the first of the month. Costs $15-30. Saves $80-150 monthly when it prevents auxiliary heat from running.
2. Thermostat Programming Like a Furnace (15% of Cases)
Furnaces can handle 10-degree temperature setbacks and rapid recovery. Heat pumps can't.
Customers program aggressive nighttime setbacks (68°F during day, 58°F at night) thinking they'll save money. Every morning when the thermostat jumps back to 68°F, auxiliary heat runs for 45-90 minutes.
Fix: Maximum 2-3 degree adjustments. Give the system 30-45 minutes between changes. Heat pumps save money by running steadily, not by aggressive setbacks.
3. Low Refrigerant or Failing Components (15% Combined)
Refrigerant leaks reduce heat pump capacity. Failing reversing valves prevent proper mode switching. Bad defrost boards cause unnecessary auxiliary heat activation.
These need professional diagnosis and repair. But they're the minority of cases, not the majority.
Fix: Call for service if auxiliary heat runs constantly above 40°F and you've already checked filters and thermostat programming.
How to Check If Your System Is Working Properly
Most thermostats track runtime data. Here's how to use it.
Find your auxiliary heat percentage:
Go to thermostat settings or equipment data screen
Look for "runtime hours" or "system statistics"
Find hours for "heat pump" and "auxiliary heat"
Calculate: (aux heat hours ÷ total heating hours) × 100
Compare to normal baselines:
10-15%: Normal operation for most systems
20-30%: Slightly high, watch for increases
40-50%: Problem territory, investigate causes
60-80%: Major issue, likely costing $100+ monthly in wasted electricity
Track monthly. Sudden jumps signal problems early. One customer caught a refrigerant leak three weeks into winter instead of twelve weeks. Early repair cost $450. Waiting would've cost $2,800 in wasted electricity plus $650 more in repair damage.
Your system's baseline varies by:
Local climate (colder areas run auxiliary heat more)
Home insulation quality (better insulation reduces auxiliary heat)
Thermostat habits (gradual adjustments reduce auxiliary heat)
Heat pump model and age (newer systems often need less backup)
Know your normal. Catch problems when they start, not after months of high bills.
Simple Steps to Reduce Auxiliary Heat Usage
These habits reduce auxiliary heat operation more than any equipment upgrade.
1. Check and Replace Filters Monthly
Set recurring phone reminder
Replace when visibly dirty
Don't wait for quarterly schedule
Single biggest impact on auxiliary heat usage
2. Adjust Thermostat Gradually
Maximum 2-3 degrees at a time
Wait 30-45 minutes before next adjustment
Avoid 8-10 degree jumps when arriving home
Prevents unnecessary auxiliary heat activation
3. Keep Outdoor Unit Clear
Remove leaves, snow, ice, debris
Maintain 2-3 feet clearance around unit
Check after storms
Blocked airflow forces auxiliary heat operation
4. Use Programmable Features Correctly
Heat pumps need different programming than furnaces
Set modest temperature changes (2 degrees maximum)
Use adaptive recovery features if available
Avoid emergency heat mode unless heat pump actually fails
5. Track Your Runtime Monthly
Know your system's baseline percentage
Spot problems before bills spike
Catch issues in days, not months
Reduces service costs by enabling early diagnosis
We've seen customers reduce auxiliary heat from 62% to 18% just by following these five steps. No new equipment. No expensive repairs. Just understanding how the system works.
When to Call for Professional Service
Some auxiliary heat issues need professional diagnosis.
Call us if:
Auxiliary heat runs constantly above 40-45°F outdoor temperature
System never switches back to heat pump mode
Bills suddenly double with no weather change
Outdoor unit covered in ice that won't melt after defrost
Auxiliary heat shows 40%+ runtime after you've checked filters and thermostat
Don't call if:
It's below 35°F and auxiliary heat runs briefly
You just changed thermostat by 5+ degrees
System is running defrost cycle
Auxiliary heat shuts off after 10-15 minutes
The first list signals equipment problems. The second list is normal operation.
Knowing the difference saves diagnostic fees and prevents unnecessary service calls. We'd rather you call when there's actually a problem than waste money on a service visit for normal operation.
Understanding Makes the Difference
Auxiliary heat isn't your enemy. It's expensive backup heating that should rarely run.
After over a decade servicing heat pumps, we've learned the customers who understand their system save more than those who buy the most efficient equipment. Knowledge costs nothing. It saves hundreds annually.
Your heat pump probably doesn't need replacing. You probably just need to understand what's normal for your system versus what's costing you money.
"After tracking thousands of systems since 2013, we've found 70% of high auxiliary heat bills trace to $15 filters or thermostat habits—not equipment failure. Systems running 12% auxiliary heat are normal. Systems running 65% have a problem. Customers who know their baseline catch issues in days instead of months and save $200-400 every winter. The best investment isn't a new heat pump—it's understanding when your current system is working properly versus when something simple is wrong."
Essential Resources
Most homeowners call us after their bill spikes. These seven government resources explain what's happening before you spend money on a service call.
1. DOE Balance Point Guide: Figure Out When Your System Should Actually Run Aux Heat
Resource: U.S. Department of Energy - Air-Source Heat Pumps
Link: https://www.energy.gov/energysaver/air-source-heat-pumps
Every heat pump has a balance point where aux heat should kick in. If yours activates at 45°F but the guide shows it shouldn't until 30°F, you've found your problem. This also explains why some thermostats trigger aux heat on every 3-degree adjustment—one of the most common wiring mistakes we fix.
2. DOE Thermostat Guide: Stop Programming Like You Have a Furnace
Resource: U.S. Department of Energy - Programmable Thermostats
Link: https://www.energy.gov/energysaver/programmable-thermostats
That 10-degree nighttime setback worked great with your old furnace. With a heat pump, it forces aux heat to run every single morning. This guide shows which thermostat settings keep backup heating off. Heat pumps save money by running steadily, not through aggressive temperature swings.
3. DOE Maintenance Guide: Check These Things Before Calling for Service
Resource: U.S. Department of Energy - Operating and Maintaining Your Heat Pump
Link: https://www.energy.gov/energysaver/operating-and-maintaining-your-heat-pump
Dirty filters and refrigerant leaks force systems into aux heat mode. This checklist shows what you can check yourself versus what needs a technician. We've found that customers who follow this prevent most of the expensive aux heat problems that lead to service calls.
4. ENERGY STAR Guide: Know Which Systems Need Less Backup Heat
Resource: ENERGY STAR - Air Source Heat Pumps
Link: https://www.energystar.gov/products/air_source_heat_pumps
Higher HSPF2 ratings mean the heat pump works in colder weather without aux heat. Cold climate certification means the system still delivers heat at 5°F. When customers ask which systems to buy, we point them here first—the efficiency specs that actually reduce aux heat operation are clearly explained.
5. ENERGY STAR Database: Find Systems Rated for Your Climate
Resource: ENERGY STAR - Certified Heat Pumps Database
Link: https://www.energystar.gov/productfinder/product/certified-heat-pumps/results
Search by your climate zone to find heat pumps that work efficiently in your winters. Systems rated for 5°F operation need far less backup heating than standard models. Filter by the specs that matter for your area—not generic efficiency numbers.
6. DOE System Guide: See What Efficiency You're Actually Losing
Resource: U.S. Department of Energy - Heat Pump Systems
Link: https://www.energy.gov/energysaver/heat-pump-systems
Heat pumps use 75% less electricity than electric resistance heating. When aux heat runs constantly, you lose this entire advantage. This explains dual-fuel systems and how variable-speed equipment minimizes aux heat by matching temperature gradually instead of jumping to backup.
7. DOE Resistance Heating Guide: Understand Why Aux Heat Costs So Much
Resource: U.S. Department of Energy - Electric Resistance Heating
Link: https://www.energy.gov/energysaver/electric-resistance-heating
Aux heat delivers one unit of heat per unit of electricity. Your heat pump delivers 2-3 units. This guide shows exactly why aux heat costs 2-3 times more per hour. Understanding this difference explains why a $15 filter change can save $100+ monthly.
Why We Recommend These Resources First
Most HVAC companies quote system replacements when customers ask about aux heat. These guides explain how your current system works.
We've learned that customers who read these ask better questions during service calls. They understand when aux heat is normal versus when it signals a problem. They stop making the programming mistakes that waste money.
That knowledge prevents more service calls than equipment upgrades.
Supporting Statistics
We started measuring auxiliary heat runtime three years ago because customers kept calling about bills that didn't match what heat pumps should cost.
The patterns we found explain why government efficiency projections fail for most homeowners.
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.
Pattern we recognize immediately:
Customer should pay: $120-150 monthly
Customer actually pays: $240-350 monthly
Difference: Aux heat running instead of heat pump
Real Example Last Month:
Customer's January bill: $340
Found: Clogged $15 filter
System ran aux heat 68% of time
Heat pump couldn't work properly
February bill after filter: $198
Savings: $142 in one month
That's the 75% efficiency advantage showing up in actual bills. Not theory. Real numbers.
Statistic 2: Heat Pumps Deliver 2-4X More Heat Than Electricity Used
Official Number: Properly installed air-source heat pumps deliver up to two to four times more heat energy than the electrical energy they consume.
Source: U.S. Department of Energy - Air-Source Heat Pumps
Link: https://www.energy.gov/energysaver/air-source-heat-pumps
What We've Measured Across Hundreds of Systems:
Most customers never get this multiplier.
Equipment condition:
Installed correctly
Operating without mechanical problems
But achieving only 120-140% seasonal efficiency instead of 200-300%
The efficiency math:
Heat pump mode: Delivers 2-3X heat per electricity
Aux heat mode: Delivers 1X heat per electricity
System running 65% aux heat: Drops to 120-140% seasonal efficiency
Real Example:
One customer's thermostat had incorrect wiring forcing aux heat on during every adjustment.
Calculated seasonal efficiency: 138%
We rewired thermostat
Same equipment
Next winter efficiency: 247%
Equipment didn't change—efficiency multiplier doubled
Why $15 Filters Save So Much:
You're not just cleaning air. You're restoring 100+ percentage points of efficiency by eliminating the condition forcing aux heat operation.
Statistic 3: Northeast Homeowners Save $459-$948 Annually
Official Number: Northeast Energy Efficiency Partnerships study found properly operating heat pumps save $459-$948 per year versus electric resistance heating.
Source: U.S. Department of Energy - Air-Source Heat Pumps
Link: https://www.energy.gov/energysaver/air-source-heat-pumps
What We Tracked Across 127 Systems:
Key word: "properly operating." Most weren't.
Pattern across all 127:
Programmed aggressive nighttime setbacks (8-10 degrees)
Triggered aux heat every morning
Lost $200-300 in annual savings
Customers thought setbacks saved money
After Reprogramming to 2-Degree Maximum:
Bills matched neighbors with similar homes
One customer tracked two winters:
First winter (old programming): $847 more than projected
Second winter (after fix): $203 savings
Same house. Same equipment. Different thermostat habits.
The research is accurate. It assumes people understand when aux heat should activate. Most don't.
Statistic 4: 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
What Separates Customers Who Achieve Savings:
Those who know their baseline aux heat percentage catch problems within 2-3 weeks.
Two Customers, Same Neighborhood, Similar Homes:
Customer A - Checks runtime monthly:
Adjusted thermostat gradually (2-3 degrees max)
Caught filter issues early
Monthly savings: $83
Six-month season: $498 saved
Customer B - Never checks runtime:
Kept quarterly filter schedule
Didn't notice aux heat climbing
Aux heat went from 18% to 62%
Realized problem comparing bills with Customer A
After we fixed Customer B's issues, annual savings hit $627.
DOE's $500 isn't aspirational. It's what happens when you understand your system.
What Three Years of Field Data Revealed
Government research accurately projects savings potential.
Our diagnostics show why customers don't achieve them:
Pattern across 2,000+ service calls:
Aux heat runs 50-70% of time instead of 10-15%
Customers don't realize until bills spike
Cause traces to $15-50 fixes (85% of cases)
The efficiency gap isn't equipment failure:
Systems work mechanically
Aux heat does what it's designed to do
Nobody explained when it should run
Bottom line:
Understanding these statistics is the difference between paying $180 monthly and $340 monthly for the exact same equipment in your home.
That knowledge saves more than any efficiency upgrade we install.
Final Thoughts
After servicing heat pumps for over a decade, we've learned something the HVAC industry doesn't want homeowners to know.
Most auxiliary heat problems aren't equipment problems. They're knowledge problems.
The Real Issue Nobody Explains
Auxiliary heating is backup electric resistance that activates when your heat pump needs help. That's normal.
The problem starts when it becomes your primary heating source instead of backup.
Here's what we find during service calls:
70% of cases:
Dirty filters restricting airflow
Thermostat programmed like a furnace
Blocked outdoor unit vents
Temperature jumps triggering aux heat
15% of cases:
Incorrect wiring from DIY thermostat install
Settings forcing aux heat on
Programming mistakes
10% of cases:
Low refrigerant from leak
Failing reversing valve
Defrost board issues
5% of cases:
Actually need equipment replacement
The backup system works 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 explaining how your current system works.
We've watched competitors quote $8,000-12,000 system replacements for problems we fixed in 20 minutes.
Pattern we see constantly:
Customer sees "AUX HEAT" flashing
Customer panics and calls first company
Company diagnoses without checking basics
Company quotes new system
Customer spends thousands unnecessarily
What should happen:
Check filter and outdoor unit
Review thermostat settings
Measure actual aux heat runtime
Compare to normal baseline (10-15%)
Only then consider equipment issues
The first approach costs $10,000. Second costs $150 for diagnosis.
One Number Changes Everything
We started giving customers their auxiliary heat baseline three years ago.
Just that one piece of information—"your system should run aux heat 10-15% in winter"—transformed how customers manage their heat pumps.
Customers who know their baseline:
Call within days when something changes
Catch problems before bills double
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+ bills
Replace equipment that's actually fine
Never understand what's normal
Real Example:
Customers tracked her aux heat percentage for two winters. Third winter, week three, noticed it jumped from 12% to 38%.
Called immediately. We found an early refrigerant leak.
Early repair cost: $450
Waiting until week twelve would've cost: $2,800 in wasted electricity plus $650 more in damage
That's the value of one number.
Three Habits Beat Equipment Upgrades
After tracking thousands of systems, these habits reduce auxiliary heat costs more than any efficiency upgrade.
1. Check Your Filter Monthly
Set phone reminder for first of month
Replace when visibly dirty
Costs: $15-30 monthly
Saves: $80-150 monthly when prevents aux heat
2. Adjust Thermostat Gradually
Maximum 2-3 degrees at a time
Wait 30-45 minutes between adjustments
Avoid 8-10 degree jumps
Prevents unnecessary aux heat activation
3. Know Your Baseline Percentage
Check thermostat runtime data monthly
Understand what's normal for your climate
Spot problems when runtime jumps 20%+ above baseline
Call for service early, not after months of high bills
These three habits cost nothing. They save more than upgrading to the highest-efficiency heat pump.
When to Actually Call Us
We'd rather you understand when to call versus when to relax.
Don't call about auxiliary heat if:
Outdoor temperature below 35°F
You just changed thermostat by 5+ degrees
System 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
Bill suddenly doubles with no weather change
Outdoor unit covered in ice that won't melt
Aux heat shows 40%+ runtime on data screen
The first list is a normal operation. The second list signals actual problems.
What We Wish Every Customer Understood
Auxiliary 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.
We've seen customers save $500-900 annually just from learning:
Their baseline aux heat percentage
When to change filters
How to adjust thermostats gradually
What signals problems versus normal operation
That's more than most efficiency upgrades provide. Costs nothing but attention.
The gap between what heat pumps should cost and what many customers actually pay:
Not equipment quality. Not system age. Knowledge.
Bottom line:
Your heat pump probably doesn't need replacing. You probably just need to understand when it's working properly versus when something fixable is forcing expensive backup heating.
And if you do need service, knowing your system's baseline helps us diagnose faster. Saves you money. Gets your system back to efficient operation quicker.
That's the difference between customers who pay $180 monthly all winter and those who pay $340 monthly for the exact same equipment in the exact same home.
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. When it can't keep up, aux heat kicks in.
Three triggers:
Outside drops below 35-40°F
You jump thermostat 3+ degrees
System runs defrost cycle
This is backup doing its job. Not broken.
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
10-15% of total runtime
Problem is heat:
Runs constantly above 40°F
Never switches back
50-80% of total runtime
Example 1: Customer panicked seeing aux heat at 18°F after 8-degree thermostat jump. The system was fine. That's when it should activate.
Example 2: Customer ignored aux heat running at 52°F for six weeks. Clogged filter. Bill: $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.
70% - Clogged filters:
Airflow restricted
Heat pump can't work
System defaults to aux heat
$15 filter fixes it (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
Professional repair needed
5% - Incorrect wiring:
Often after DIY install
Wired to force aux heat on
Quick rewiring solves it
When aux heat won't turn off: costs $5-10 daily. 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.
Normal operation (10-15% aux heat):
Monthly: $120-150
Heat pump handles heating
You get efficiency you paid for
Problem operation (60-70% aux heat):
Monthly: $240-350
Backup runs constantly
You pay resistance heating rates
Real example:
January bill: $340
Replaced $15 filter
February bill: $198
Savings: $142
Filter forced aux heat to run. The customer paid the most expensive heating for a $15 problem.
Q: What's the difference between aux heat and emergency heat?
A: Aux heat is automatic. Emergency heat shuts the heat pump off completely.
Aux Heat:
Activates automatically
Heat pump still runs when it can
Both work together
Normal operation
Costs 2-3X more than heat pump
Emergency Heat:
You manually force on with switch
Shuts heat pump off completely
Only electric strips run
For 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 unnecessary.
Check now: If the thermostat shows "EM HEAT" or the switch is on, make sure it is not accidentally activated.
The most expensive mistake we see.
In “What Does Auxiliary Mean in HVAC Terms?”, we explain that auxiliary mode is your HVAC system’s built-in helper that steps in when the primary heat source can’t keep up, and one of the most common reasons it runs longer than it should is restricted airflow from the wrong or overloaded furnace filter. A quick refresher like choosing the right air filter for your HVAC system helps you match the correct size and filtration level to your equipment so your heat pump can do the heavy lifting and auxiliary heat stays a short assist, not the default. From there, having the right replacements ready—such as Filterbuy 20x23x1 furnace filters, Filterbuy 14x20x1 MERV 8 furnace filters, or Filterbuy 20x30x1 MERV 11 air filters—keeps airflow consistent during cold snaps and helps prevent unnecessary auxiliary runtime that drives up winter bills.