If you’ve owned a heat pump for more than one winter, you’ve probably stared at your thermostat and wondered: “What does auxiliary heat mean, and should I be worried when that light comes on?”
I’ve been around heating and cooling systems for decades, and I can tell you that auxiliary heat is one of the most misunderstood features on a thermostat. Folks call me every winter convinced something is broken, when in reality the system is just doing what it was designed to do. But there are also times when auxiliary heat is a red flag for wasted money and energy. Let me walk you through it the way I would if I were sitting at your kitchen table.
Quick Answer: What Does Auxiliary Heat Mean?
Auxiliary heat sometimes shortened to “aux heat” is the automatic backup heating that a heat pump uses when it can’t keep up with your thermostat’s set temperature. In most homes, that means electric resistance heat strips kick on inside the air handler, though in some setups it could be a gas furnace in a dual-fuel system.
So if you see “AUX” or “Auxiliary Heat” glowing on your thermostat, it simply means the system is adding extra heat to keep your home comfortable. According to the U.S. Department of Energy, auxiliary heat is designed to help maintain set temperatures efficiently without overloading the heat pump.
How Auxiliary Heat Works
To really answer what does auxiliary heat mean, you need to understand how a heat pump works. A heat pump is basically an air conditioner that runs in reverse. It pulls heat from the outdoor air even when it’s cold and moves it indoors.
The trouble is, when the outdoor temperature drops into the 20s or below, or if you suddenly bump your thermostat up by more than three or four degrees, the heat pump may not be able to deliver enough heat quickly. That’s when the thermostat signals the auxiliary heater.
- Electric heat strips: Think of these as giant toaster coils inside your ductwork. They heat up fast but chew through electricity.
- Gas furnace (dual fuel): In some systems, the aux stage is a gas furnace that provides heat when the pump alone can’t.
That’s all auxiliary heat really is a helper system that takes over when the pump struggles. Trane provides an excellent explanation of the different backup types and when they typically engage.
When and Why Auxiliary Heat Turns On
You’ll see the aux light for three main reasons:
- Extremely cold outdoor temperatures: Once the outdoor air gets too chilly for the pump to extract enough heat, the auxiliary heater joins in.
- Large thermostat changes: Crank the thermostat from 65°F to 72°F all at once, and the thermostat often calls for aux to close the gap.
- Defrost cycle: Heat pumps occasionally reverse themselves to melt ice off the outdoor coil. During those few minutes, the indoor auxiliary heat keeps you from blowing cold air inside.
The U.S Department of Energy has even noted that some thermostats are wired to trigger auxiliary strips whenever the setpoint is raised more than about three degrees. That wiring quirk can lead to unnecessary energy use if you aren’t careful.
So the next time you ask yourself “what does auxiliary heat mean when it suddenly pops on?” the answer is: it’s just the system bridging a gap, not necessarily a problem.
Auxiliary Heat vs. Emergency Heat
A common follow-up question is: is auxiliary heat the same as emergency heat? The short answer is no. Here’s a simple comparison:
| Feature | Auxiliary Heat | Emergency Heat |
|---|---|---|
| Who turns it on? | Thermostat (automatic) | Homeowner (manual switch) |
| When it runs | When pump can’t keep up or during defrost | When heat pump fails or you force it |
| Cost | High (electric strips) or moderate (dual fuel) | Highest, since pump is off and only backup runs |
| Purpose | Assists the pump | Bypasses pump completely |
Auxiliary heat is like a helper. Emergency heat is the last resort when the pump itself is disabled. Carrier provides guidance on dual-fuel systems and when emergency heat might be needed.
How to Tell If Auxiliary Heat Is Running
Sometimes the thermostat will flash “AUX.” Other times, you’ll notice different clues:
- A distinct smell of hot metal from the vents when the strips energize (like a toaster warming up).
- A higher than normal electric meter spin or smart monitor alert.
- Warmer but “dryer” air, since strips don’t dehumidify like a pump does.
Knowing these signs helps you recognize when the system is dipping into backup mode. That way you can answer what does auxiliary heat mean in practice without guessing.
Troubleshooting: Why Auxiliary Heat Keeps Coming On
If you’re seeing “Aux Heat” constantly, even when it’s 45°F outside, that’s a red flag. Common culprits include:
- Clogged air filter: Makes the heat pump less efficient, forcing aux to engage.
- Outdoor unit icing or airflow blocked by leaves/debris: System can’t extract enough heat.
- Thermostat programming: Some thermostats are wired or set to call aux too quickly.
- Low refrigerant or compressor trouble: Pump isn’t pulling its weight, so aux takes over.
First, check the simple things: change your filter, clear the outdoor unit, avoid big sudden thermostat jumps. If aux still runs nonstop, it’s time for a technician.
The Cost and Energy Impact of Auxiliary Heat
Now here’s the part nobody likes to hear. Auxiliary heat especially electric strips costs a lot more than running the heat pump.
A typical heat pump might deliver three units of heat for every one unit of electricity (COP ~3). Electric strips are one-for-one. That means they’re about three times more expensive per degree of heating.
A homeowner once asked me “what does auxiliary heat mean for my power bill?” The truth is, if it runs only a few hours here and there, you won’t notice much. But if it’s on every night for weeks, you could see your bill double. That’s why keeping the pump maintained and using smart thermostat habits makes such a difference. ENERGY STAR explains how efficiency ratings translate to real-world savings.
If you’re curious about exactly how much energy your aux heat adds to your home, you can use our Specific Heat Calculator to estimate the energy needed to warm your rooms to your desired temperature. It’s a handy tool for understanding your actual costs and usage patterns.
Tips to Minimize Auxiliary Heat Use
- Don’t bump the thermostat too far at once. Raise it 1–2 degrees at a time if you want to avoid triggering aux.
- Use a programmable or smart thermostat. Many newer models delay or limit aux unless absolutely necessary.
- Maintain your heat pump. Clean filters, service coils, and proper refrigerant charge all help.
- Consider dual fuel. If you have access to natural gas, a hybrid system uses the furnace as auxiliary cheaper than strips in most regions.
Remember: what does auxiliary heat mean for your comfort and wallet? It means you need to respect it, but you don’t have to fear it.
When to Consider Upgrades
If your aux light is on more often than not, and your bills are climbing, it might be time to look at:
- Dual-fuel upgrade: Gas furnace backup can cut costs vs. electric strips.
- Higher efficiency heat pump: Modern variable-speed units perform better in the cold.
- Thermostat replacement: A smarter thermostat can control aux stages more intelligently.
Sometimes the answer to what does auxiliary heat mean in my home? is simply: “your current system isn’t the right match for your climate.”
What Does Auxiliary Heat Mean FAQs
What does aux heat mean on my thermostat?
It means your heat pump has automatically turned on its backup heating (electric strips or gas) to help reach the set temperature.
When should auxiliary heat come on?
It should only run during very cold weather, large thermostat jumps, or defrost cycles.
Is auxiliary heat bad for my system?
No, it’s designed to be there. The problem is cost, it’s far less efficient than the pump.
Does auxiliary heat cost more than a heat pump?
Yes. Running on auxiliary heat is two to three times more expensive than running a heat pump alone. A pump typically delivers three units of heat per unit of electricity, while aux strips are one-for-one, making them much less efficient.
Is auxiliary heat the same as emergency heat?
No. Auxiliary is automatic and works with the pump. Emergency is manual and bypasses the pump entirely.
How can I stop auxiliary heat from running too often?
Raise your thermostat gradually, keep filters and coils clean, and use a programmable or smart thermostat. If aux heat stays on constantly, call a technician to check for low refrigerant, airflow problems, or thermostat wiring issues.
What does auxiliary heat mean for my energy bill?
Auxiliary heat can increase your bill sharply if it runs for long stretches. A few hours here and there won’t matter much, but if aux is on every night, you could see costs double.
Conclusion
So after all this, let’s bring it home: what does auxiliary heat mean? It means your heat pump has a helper. Sometimes that helper is an electric strip heater, sometimes a gas furnace, but its job is the same to keep you warm when the pump can’t do it alone.
Used occasionally, it’s perfectly normal. But if you see it on all the time, it’s a sign something isn’t right, or your system is working harder than it should.
After fifty-plus winters and plenty of cold nights spent talking folks through their “AUX” worries, I can tell you this: don’t panic when you see it, but don’t ignore it either. Understand it, manage it, and your home will stay comfortable without wasting money.
