If you’ve ever looked at your thermostat on a cold day and suddenly seen AUX HEAT, you’re not alone. This comprehensive, easy-to-understand guide explains exactly what aux heat is, why it turns on, when it’s normal, when it’s a problem, and how to reduce your energy bills by managing it wisely. As a semantic SEO expert, I’ve structured this guide to satisfy user intent, search engine algorithms, and real-world homeowner questions, all without fluff.
What Is Aux Heat?
Aux heat, short for auxiliary heat, is a backup heating system used in homes with heat pumps. When the outdoor temperature is too cold for your heat pump to pull enough warmth from outside air, the system automatically switches to AUX HEAT to help maintain your indoor temperature.
In simple terms: AUX heat = backup heat that helps your heat pump on very cold days. This auxiliary heat is usually powered by:
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Electric heat strips (most common)
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A gas furnace (in dual-fuel systems)
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A hydronic coil (less common)
Aux heat automatically turns on, no action required from you, and turns off once the heat pump catches up.
How Aux Heat Works
A heat pump pulls heat from outside air and transfers it indoors. This works well until the temperature drops too low, usually below 35–40°F (1–4°C). When that happens:
- The heat pump struggles to heat the home.
- The thermostat notices the home is still cold.
- AUX HEAT turns on to help.
This added boost warms your home faster, but it uses much more electricity, which is why many homeowners worry when they see it. Think of aux heat like the “booster engine” for your HVAC system.
When Does Aux Heat Turn On?
Aux heat turns on automatically under these conditions:
Outdoor temperature is very low
Most heat pumps lose efficiency under 35–40°F.
Your system adds auxiliary heat to compensate.
Large temperature differences
If you set your thermostat from 65°F to 74°F, the system may turn on aux heat to reach the target quickly.
Defrost cycle
When a heat pump defrosts, it temporarily switches to cooling mode to melt ice. Aux heat turns on to counteract the cold air entering your home.
Heat pump malfunction
If a component fails (low refrigerant, bad sensor, compressor issue), aux heat may run continuously. This is when you should call a technician.
Is Aux Heat Normal? Or a Problem?
Normal
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Turns on periodically during cold weather
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Runs during defrost
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Turns off after temperature is reached
Problem
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Aux heat stays on constantly
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Aux heat runs when it’s warm outside
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Energy bills suddenly spike
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Home feels colder than usual
If AUX HEAT is always on, the heat pump may need service.
Aux Heat vs Emergency Heat
Many thermostats show both AUX HEAT and EM HEAT, and people often confuse them.
Aux Heat (Automatic)
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System uses backup heat when needed
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Comes on by itself
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Safe and normal
Emergency Heat (Manual)
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You must turn it on manually
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Turns OFF the heat pump
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Uses only the backup heat source
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Extremely expensive to run
Use EM HEAT only when the heat pump is broken or frozen.
What Does Aux Heat Mean on Your Thermostat Brand?
Homeowners often search for brand-specific answers. Here are the common meanings:
Aux Heat on Honeywell Thermostat
Means your heat pump triggered backup heat due to low outdoor temperature or a big temperature change.
Aux Heat on Nest Thermostat
Nest detects insufficient heating and automatically activates auxiliary heat to reach the set temperature.
Aux Heat on Ecobee
Ecobee allows you to set an AUX HEAT temperature threshold. When outdoor temps drop below this threshold, aux heat turns on.
Aux Heat on Sensi
Works similarly to Honeywell, backup heat turns on automatically during heavy heating demand.
Aux Heat on Carrier Thermostat
Indicates that electric heat strips or secondary heating stages are operating. All brands function similarly, they simply label it differently.
Aux Heat Stages
Some thermostats show heating stages:
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Stage 1: Heat pump running
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Stage 2: Heat pump + aux heat
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Stage 3: Maximum backup heat (multiple heat strips or dual fuel mode)
If you see Stage 2 or 3 often, it’s worth checking your settings, or your HVAC system.
Aux Heat in Different Heating Systems
Heat Pump Systems
Most common use of aux heat.
Hybrid/Dual-Fuel Systems
Switch between heat pump and gas furnace automatically.
Geothermal Systems
Aux heat still exists but turns on far less often.
HVAC Units With Electric Strips
Aux heat is the electric resistance heater.
No matter the system, aux heat is always a supplemental heating method.
How Much Does Aux Heat Cost to Run?
Aux heat is expensive, especially electric strip heat.
Approximate usage:
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Heat pump: 1–3 kW
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Aux heat strips: 5–20 kW
This means AUX HEAT can cost 5–10× more than normal heating. Discover wedding terms like What Does Elope Mean and What Hand Does an Engagement Ring Go On.
Why Does Aux Heat Keep Coming On?
Very cold weather
Heat pumps need help in freezing conditions.
Thermostat set too high
Large jumps in temperature trigger auxiliary heat.
Dirty air filters
Airflow problems reduce heat pump efficiency.
Low refrigerant
Heat pump can’t extract enough heat.
Ice on the outdoor unit
Defrost mode triggers aux heat.
Wiring or sensor issues
Thermostat may falsely activate backup heat. If aux heat runs non-stop, schedule an HVAC check.
How to Reduce Aux Heat Usage
Lower your temperature set points
Avoid large jumps. Increase gradually:
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2° increase = heat pump only
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8° increase = triggers aux heat
Use a programmable thermostat
Nest and Ecobee help prevent unnecessary aux usage.
Change air filters regularly
Improves system efficiency.
Keep the outdoor unit clear
Remove snow, ice, and debris.
Seal air leaks
Better insulation reduces heat loss.
Turn off EM HEAT
Never use emergency heat unless the heat pump is broken.
Schedule annual maintenance
A tuned system reduces backup heating needs.
Aux Heat in Specialized Systems
Aux Heat in Geothermal
Used rarely because geothermal draws heat from the ground, not the air.
Aux Heat in AC or HVAC Units
In heat pump mode, aux heat replaces a furnace.
Aux Heater in Vehicles (Not HVAC)
Some cars include “auxiliary heaters” for cold climates, but this is unrelated to home HVAC aux heat.
Common Questions About Aux Heat
What does aux heat mean on my thermostat?
It means your backup heating system is running.
Is aux heat bad?
No, it’s normal during cold weather.
It’s only a problem if it runs constantly.
Should my aux heat come on at 40 degrees?
Yes, many heat pumps trigger aux below 40°F.
Why is my aux heat on when it’s not that cold?
Common reasons:
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Thermostat settings
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Large temperature change
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Malfunction
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Defrost cycle
How do I turn off aux heat?
You can’t directly turn it off, but you can reduce triggers by adjusting your thermostat settings.
Final Summary
Aux heat is a backup heating system used in homes with heat pumps. It turns on when:
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Outdoor temperatures are too low
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Your home needs rapid heating
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The heat pump is defrosting
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The heat pump can’t keep up or is malfunctioning
Aux heat is normal, but expensive. If it runs constantly, it’s time for troubleshooting or professional service.
