So your thermostat suddenly shows "AUX HEAT" and your energy bill spikes. Been there! Let me explain exactly what's happening with your heating system when that mysterious light comes on. Honestly, I wish someone had broken this down for me when I first noticed auxiliary heat kicking in during that brutal winter storm last year.
The Heat Pump Basics You Need First
To really get what is auxiliary heat, you've got to understand heat pumps. Unlike old-school furnaces that burn fuel to create heat, heat pumps move existing heat from outside to inside your home. Clever, right? But here's the catch – when temperatures drop below freezing (around 35°F/2°C), these systems struggle to extract enough heat from the frigid air.
Why Heat Pumps Need Help in Winter
Ever notice how your fridge feels warm at the back? That's the same principle. But when it's 20°F outside, there's just not much heat to "steal" from the air. That's when your auxiliary heat (sometimes called "aux heat" or "backup heat") jumps into action. Honestly, without it, you'd be shivering on cold mornings.
Here's a quick energy comparison to show why heat pumps need backup:
| Temperature (°F) | Heat Pump Efficiency | Heat Output |
|---|---|---|
| > 40°F | Excellent (300-400% efficient) | Full capacity |
| 30-40°F | Good (200-300% efficient) | Slightly reduced |
| 20-30°F | Fair (150-200% efficient) | Significantly reduced |
| < 20°F | Poor (100-150% efficient) | Insufficient alone |
Auxiliary Heat Explained in Plain English
So what is auxiliary heat really? It's your system's built-in backup plan – usually electric resistance heating coils (like giant toaster wires) or sometimes a gas furnace. When your heat pump can't keep up, these secondary heaters turn on automatically to boost indoor temperatures.
The thermostat decides when to activate auxiliary heat based on:
- Outdoor temperature (typically below 35°F)
- Temperature gap between current and desired setting
- How long the heat pump has been running without reaching the target
I made the mistake once of cranking up my thermostat 5 degrees during a cold snap. Bad idea! The auxiliary heat ran for hours and added $50 to my bill. Lesson learned – small adjustments only when it's freezing out.
AUX HEAT vs EM HEAT: Critical Difference
This trips up so many homeowners. Your thermostat has two modes:
| Feature | Auxiliary Heat (AUX) | Emergency Heat (EM) |
|---|---|---|
| Activation | Automatic | Manual selection |
| Function | Supplements heat pump | Replaces heat pump |
| When to use | Normal cold weather operation | Heat pump failure or extreme conditions |
| Cost impact | Moderate increase | Significant increase |
If you manually switch to EM heat, you're bypassing the heat pump entirely and running ONLY the expensive backup heat. I only did this once when my heat pump froze solid during an ice storm. Saved my pipes but cost me $300 that month!
When and Why Auxiliary Heat Activates
Understanding when auxiliary heat engages helps you manage costs. From my HVAC buddy's experience, these are the most common triggers:
- Rapid temperature recovery: When you raise thermostat more than 2-3°F
- Defrost cycles: Heat pumps periodically melt ice buildup (aux heat maintains warmth indoors during this)
- Extended cold spells: Sustained temperatures below freezing
- Morning warm-up: Bringing home from overnight setback temperature
Pro tip: Avoid drastic thermostat changes in winter. Instead of jumping from 62°F to 70°F, increase gradually in 2-degree increments every 30 minutes.
Cost Implications of Auxiliary Heat
Here's where things get painful. Auxiliary heat uses 2-3 times more electricity than your heat pump. How painful? Check these real-world scenarios:
| Scenario | Heat Pump Only | With Auxiliary Heat | Cost Increase |
|---|---|---|---|
| Mild day (45°F) | $1.20/hour | $1.20/hour | 0% |
| Cold day (28°F) | $1.80/hour | $3.60/hour | 100% |
| Extreme cold (10°F) | $2.50/hour | $7.50/hour | 200% |
See why that AUX light gives homeowners nightmares? Last February when we had that polar vortex, my neighbor didn't understand why his bill doubled. Turns out his auxiliary heat was running 8 hours daily!
Troubleshooting Auxiliary Heat Problems
Sometimes auxiliary heat misbehaves. Here's what I've learned from fixing my own system and helping friends:
Common AUX Heat Issues and Fixes
- Constantly running: Usually caused by dirty air filters (replace monthly in winter) or thermostat malfunctions
- Not activating when needed: Could be blown fuses, tripped breakers, or faulty sensors
- Short cycling: Turns on/off rapidly - often indicates oversized unit or thermostat placement issue
- Strange burning smell: Normal during first seasonal use (dust burning off), but persistent smell indicates electrical issues
Warning: If you smell burning for more than an hour or see smoke, shut off system immediately and call a pro. Those electric coils get crazy hot!
Smart Ways to Manage Auxiliary Heat Usage
You don't have to choose between comfort and bankruptcy. After that $300 energy bill fiasco, I developed these practical strategies:
- Install a smart thermostat: Models like Nest or Ecobee minimize auxiliary heat use with adaptive recovery
- Maintain moderate setbacks: Don't lower more than 5-7°F overnight
- Improve insulation: Focus on attics and basements where 25% of heat escapes
- Schedule professional maintenance: Dirty coils force auxiliary heat to work harder
- Check thermostat settings: Confirm AUX lockout temperature is set correctly (usually 35-40°F)
When Should You Worry About Auxiliary Heat?
Based on HVAC techs I've interviewed, call a pro if:
- Aux heat runs continuously for >4 hours
- Your bill increases >30% without temperature changes
- System can't maintain temperature within 2°F of setting
- You hear unusual buzzing or clicking from air handler
Seriously, don't ignore these. I put off servicing once and ended up replacing a $500 control board that failed because of an overloaded auxiliary heat circuit.
Auxiliary Heat Alternatives Worth Considering
If auxiliary heat costs are killing you, consider these options:
- Dual-fuel systems: Heat pump + gas furnace combo (uses cheaper gas for backup)
- Cold-climate heat pumps: Newer models work efficiently down to -15°F
- Geothermal systems: Ground-source heat pumps avoid auxiliary heat needs
- Supplemental zone heating: Ductless minisplits or space heaters for frequently used rooms
The upfront costs might sting, but my cousin installed a cold-climate heat pump in Vermont and eliminated auxiliary heat entirely. His payback period was about 5 years with current energy prices.
Your Auxiliary Heat Questions Answered
Why is my auxiliary heat running when it's not that cold outside?
Usually three culprits: 1) Thermostat set too high too quickly, 2) Heat pump in defrost cycle, or 3) Malfunctioning outdoor sensor. Check your thermostat manual for temperature differential settings.
Can I permanently disable my auxiliary heat?
Technically yes, but I wouldn't recommend it. You risk frozen pipes during extreme cold. Instead, adjust the lockout temperature setting to only activate when absolutely necessary.
Does auxiliary heat use more electricity than regular heat?
Significantly more! Electric resistance heating (what most auxiliary systems use) is 100% efficient at turning electricity to heat, but heat pumps can be 300% efficient by moving existing heat.
How often should auxiliary heat turn on?
During normal winter operation? Maybe 10-20% of heating time in moderate climates. If it's kicking on constantly during 40°F weather, you've probably got an issue. Mine runs about 15 minutes per hour when it's below freezing.
Is auxiliary heat dangerous?
Not if properly maintained. The main risks are electrical fires from faulty wiring (get annual checkups) and carbon monoxide if you have a gas backup system (install detectors!).
Why does my thermostat say "auxiliary heat" but I have a gas furnace?
Some systems mislabel gas backup as auxiliary heat. True auxiliary heat refers specifically to electric resistance systems. Gas backup is typically called "emergency heat" or "second stage." Confusing, right?
Final Thoughts on Managing Your System
Getting a handle on auxiliary heat transformed how I approach winter heating. Now when I see that AUX light, I don't panic – I check the outdoor temperature, verify my thermostat settings, and maybe throw on a sweater before cranking up the heat. Understanding what is auxiliary heat and how it works might just save you hundreds this winter.
The key takeaway? Auxiliary heat is your friend in extreme cold, but an expensive friend who shouldn't overstay their welcome. With smart habits and proper maintenance, you can keep that relationship healthy and affordable.
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