Quick answer: Aux (auxiliary) heat is backup heat, usually electric heat strips, that the thermostat turns on by itself when your heat pump needs help, such as in a hard freeze or a defrost cycle. Emergency heat is a manual setting that shuts the heat pump off and runs backup heat alone. In Central Texas, use it only if the heat pump has quit.
What is aux heat on a heat pump thermostat?
Aux heat is the backup your thermostat calls on automatically when the heat pump can’t warm the house fast enough on its own. In most Austin-area homes with a heat pump, that backup is a bank of electric heat strips inside the indoor unit, usually an air handler in the attic or a hall closet. The strips run alongside the heat pump, and when the house catches up, the thermostat shuts them back off. Think of them as a helper, not a replacement.
If your home has a dual fuel system, the backup is a gas furnace instead of heat strips. Same idea: the heat pump does most of the work, and the backup covers the coldest hours.
When does aux heat normally come on?
Seeing “AUX” on the thermostat isn’t a problem by itself. It’s normal in these situations:
- Cold snaps and hard freezes. A standard heat pump puts out less heat as the outdoor temperature drops, so the strips pitch in on the coldest nights. Our guide Can a Heat Pump Keep Up With a Texas Freeze? explains how this works.
- Defrost cycles. In cold, damp weather the outdoor coil can frost over. The heat pump briefly reverses to melt it, and many systems run the heat strips for those few minutes so you don’t get cool air from the vents.
- Big jumps in the setpoint. Bump the thermostat up several degrees at once and many thermostats call for aux heat to catch up faster.
- Mornings after a deep overnight setback. Recovering from a big setback is one of the most common reasons people see aux heat at breakfast time.
What is emergency heat?
Emergency heat (often labeled “EM Heat” or “E-Heat”) is a mode you select yourself. It locks out the outdoor heat pump and heats the house with backup heat only. It exists for one job: keeping you warm when the heat pump can’t run, for example when the outdoor unit is damaged or has failed and you’re waiting on a repair.
Emergency heat does not make the house any warmer than normal heat mode. In a home with heat strips, it just swaps an efficient heat source for a less efficient one. Same blanket, bigger bill.
Should I turn on emergency heat when it’s freezing?
Usually no. In normal heat mode the thermostat already brings on backup heat whenever it’s needed, and it still lets the heat pump contribute whatever heat it can. Flipping to emergency heat during a cold snap tends to raise your electric use without adding comfort.
Do use emergency heat if the outdoor unit isn’t running, is making a loud grinding or banging noise, stays encased in ice that doesn’t clear after defrost cycles, or a technician has told you to. Then call for service, because running on emergency heat for days at a time adds up on your bill.
Does aux heat cost more to run?
Yes, per hour of heat it usually does. Heat strips are electric resistance heat: they turn electricity directly into heat, one-to-one, like a giant toaster. A heat pump moves heat from the outdoor air into your home instead, so in mild and moderate weather it delivers more heat for the same electricity. We compare the two in Heat Pumps vs. Electric Resistance Heat for Central Texas Homes.
A little aux heat during freezes, defrost cycles and morning recovery is expected. Aux heat running for long stretches on a mild day is not, and it’s worth having checked.
Why is my aux heat always on?
If the thermostat shows aux heat most of the time, or on days in the 40s and 50s, something is usually making the heat pump fall short. Common causes we find in Austin homes:
- Thermostat settings or wiring that call for backup heat too early, or a thermostat that isn’t set up for a heat pump.
- Large temperature setbacks that force a long recovery every morning.
- A heat pump that’s low on refrigerant because of a leak.
- A reversing valve, defrost control or outdoor sensor that isn’t working correctly.
- A failing compressor or outdoor fan motor.
- A dirty filter or coil that restricts airflow.
- Leaky or poorly insulated attic ductwork losing heat before it reaches your rooms.
- A system that’s undersized for the house, or simply near the end of its life.
Some of these are quick fixes, like a filter or a thermostat setting. Refrigerant, valve and compressor problems need a licensed technician.
How can you keep aux heat use in check?
- Keep setbacks modest. Heat pumps generally do best with small, steady adjustments rather than big overnight drops.
- Use a thermostat designed for heat pumps. Many smart thermostats have settings that limit aux heat and recover gradually. See Smart Thermostats: Are They Worth It?.
- Change or clean the filter on schedule so the system isn’t starved for air.
- Keep leaves, mulch and plants clear of the outdoor unit so it can breathe and defrost properly.
- Book a fall heating tune-up so refrigerant charge, defrost operation and heat strips get checked before the first cold front. Here’s what’s included in an HVAC tune-up.
When does replacement make more sense?
When your heat pump leans on aux heat more every winter, needs repeated repairs, or still uses R-22 refrigerant (production ended in 2020, so only reclaimed supply remains), it’s time to weigh repair against replacement. Our guide Repair or Replace? Deciding What’s Best for Your HVAC System walks through the trade-offs.
Planning a replacement? You can build your system online to compare heat pump, dual fuel and gas furnace options for your home and see an estimate in a few minutes. Our local team reviews every build and confirms the details at an in-home visit.
Frequently asked questions
What does emergency heat mean on my thermostat?
It means the heat pump is off and the house is being heated by backup heat only, usually electric heat strips. It’s meant for when the heat pump can’t run.
Is it bad if the aux heat light comes on during defrost?
No. Many systems run the heat strips during defrost so the air from your vents doesn’t feel cool. The light should go off again once defrost ends.
Why is my aux heat always on?
Common causes are thermostat setup, large setbacks, low refrigerant, a defrost or reversing valve problem, restricted airflow or an aging system. If it runs on mild days, have the system checked.
Does a dual fuel system have emergency heat?
Yes. On most thermostats, emergency heat on a dual fuel system runs the gas furnace alone. If you’re not sure how yours is set up, ask us to walk you through your thermostat’s settings.
The bottom line
Aux heat is your heat pump’s automatic helper, and seeing it on cold mornings or during defrost is normal. Emergency heat is a manual backup for when the heat pump is out of service. Leave the thermostat in normal heat, keep setbacks modest, and call for service if aux heat seems to run all the time or the outdoor unit stops working.
Related guides: Can a Heat Pump Keep Up With a Texas Freeze?, Heat Pumps vs. Electric Resistance Heat for Central Texas Homes, What’s Included in an HVAC Tune-Up? and Smart Thermostats: Are They Worth It?.
About Gold Eagle Services: This guide comes from the team at Gold Eagle Services, a family-run Austin HVAC, indoor air quality and mold remediation company founded in 2017. We’re licensed (TACLA119235 and RCO1783), insured and bonded, and we serve homes across the greater Austin area.
Need help with this in your home? Learn about our heat pump repair, or call 512-489-4653 to schedule a visit.
