If your thermostat says AUX Heat, your heat pump has called on a secondary source of heat to help warm your home. In many cases, that is normal. Auxiliary heat may come on during colder weather, after a large thermostat adjustment, or while the heat pump is completing certain operating cycles. The bigger concern is when AUX Heat runs for long periods, shows up frequently in mild weather, or your home still cannot reach the set temperature.
Quick answer: AUX Heat does not automatically mean your heat pump is broken. Think of it as backup help. Occasional use can be normal; persistent or unexpected use is the reason to look more closely.
What Does Aux Heat Mean on a Thermostat?
“Aux” is short for auxiliary. In a heat-pump system, auxiliary heat is a secondary heating source that can supplement the heat pump when the system needs more heating capacity. The exact backup source depends on how your HVAC system is designed. Many all-electric systems use electric resistance heat strips. A dual-fuel setup may pair the heat pump with a gas furnace or another secondary heat source.
The controls decide when that secondary heat should operate. U.S. Department of Energy Building Science Education guidance describes control strategies that prioritize the heat pump and bring in secondary heating when additional heat is needed.
That is why seeing AUX on the thermostat is not, by itself, a diagnosis. The useful question is why the system needed backup heat at that moment.
Why Does My Thermostat Say Aux Heat?
It Is Cold Outside
As outdoor conditions get colder, your home loses heat faster and the heat pump has to work harder. Depending on the equipment, system sizing, thermostat controls, and the home itself, the controls may call for auxiliary heat to help maintain the indoor temperature.
There is no single outdoor temperature at which every heat pump should switch on AUX Heat. Different systems are designed and configured differently, so a generic temperature rule can be misleading.
You Raised the Thermostat Several Degrees
A large setpoint change creates a bigger heating demand. If you raise the thermostat several degrees at once, some systems will use auxiliary heat to help the home reach the new target more quickly. That does not necessarily mean something is wrong.
The System Is Recovering From a Temperature Setback
The same idea applies after a deep overnight or away setback. If the thermostat asks the system to make up a large temperature difference, the controls may call for backup heat during recovery. Heat-pump thermostat strategies are designed to balance comfort with limiting unnecessary use of secondary heat.
The Heat Pump Is in a Defrost Cycle
During heating season, moisture can freeze on the outdoor heat-pump coil. The system periodically runs a defrost cycle to clear that frost. On some systems, auxiliary heat operates during defrost so the air coming from the vents does not feel as cold while the outdoor unit is temporarily reversing operation.
The Heat Pump Is Struggling to Keep Up
Auxiliary heat can also run because the primary heat pump is not satisfying the thermostat as expected. Restricted airflow, thermostat or control problems, outdoor-unit issues, equipment performance problems, or overdue maintenance can all contribute to poor heating performance. The thermostat display tells you that backup heat is being used; it does not tell you the underlying cause.
Is It Normal for Aux Heat to Come On?
Yes. Temporary auxiliary-heat operation can be a normal part of a heat-pump system. The pattern matters more than the fact that AUX appeared once.
| Usually Normal | Worth Investigating |
| AUX appears temporarily during a cold snap, large temperature recovery, or defrost cycle, then turns off as the home reaches the setpoint. | AUX runs almost continuously, appears often during mild weather, the home cannot maintain temperature, or energy use rises sharply without an obvious weather explanation. |
If your system reaches the set temperature and AUX turns off again, that is generally less concerning than a system that depends on backup heat for long periods and still cannot keep the home comfortable.
Aux Heat vs. Emergency Heat: What Is the Difference?
These thermostat labels sound similar, but they serve different purposes.
Auxiliary Heat: The system generally activates it automatically when additional heating is needed.
Emergency Heat: This is typically a manually selected mode that relies on the system’s backup heating source rather than normal heat-pump operation. Exact behavior varies by equipment and thermostat configuration.
Do not switch to Emergency Heat simply to make the house warm faster. If the heat pump is not operating normally, the cause should be diagnosed rather than masked by leaving the system in a backup-only mode.
Does Aux Heat Use More Electricity?
It can. In an all-electric system, auxiliary heat is often provided by electric resistance elements. Heat pumps move heat rather than creating all of it with resistance heating, so heavy reliance on electric backup heat can increase electricity use compared with normal heat-pump operation.
A dual-fuel system is different because the secondary source may be a gas furnace. That is why there is no universal percentage or dollar amount for the cost of AUX Heat. Outdoor temperature, utility rates, equipment efficiency, thermostat settings, home heat loss, and the type of backup heat all affect operating cost.
The practical takeaway: do not worry about a brief AUX cycle, but do pay attention if backup heat seems to run far more often than it used to.
How Long Should Auxiliary Heat Stay On?
There is no universal number of minutes that applies to every heat-pump system. AUX runtime depends on outdoor temperature, the thermostat setpoint, equipment capacity, home heat loss, airflow, control strategy, and whether the system is recovering from setback or defrost.
Consider scheduling service when AUX Heat:
- Runs almost continuously instead of cycling off as the home reaches temperature.
- Frequently appears during relatively mild Central Texas weather.
- Stays on while the home continues to lose temperature.
- Accompanies unusually long heat-pump runtime or weak airflow.
- Shows up along with thermostat errors, unusual equipment behavior, or a sudden unexplained increase in energy use.
Should I Turn Aux Heat Off?
Usually, no. If your system is designed to use automatic auxiliary heat, the controls may be calling for it for a legitimate reason. Disabling backup heat can reduce comfort or keep the home from reaching temperature when the heat pump needs help.
Instead of trying to bypass heat strips, breakers, relays, or installer-level thermostat settings, look at the operating pattern. If AUX appears briefly and the house warms normally, monitor it. If AUX is persistent or the system is not heating correctly, have the system evaluated.
What Can You Check Before Calling an HVAC Technician?
A few safe homeowner checks can help you describe the problem more clearly without opening equipment or changing technical controls.
Check the Thermostat Setting
Confirm the thermostat is set to Heat, the desired temperature is reasonable, and Emergency Heat was not selected accidentally. If you recently raised the thermostat several degrees, give the system time to recover and watch whether AUX eventually turns off.
Check the Air Filter
A heavily loaded filter can restrict airflow and reduce HVAC performance. Inspect or replace the filter according to the equipment and filter manufacturer’s normal maintenance instructions.
Make Sure Supply and Return Airflow Is Not Blocked
Keep supply registers open and avoid blocking return grilles with furniture, rugs, or stored items.
Watch Whether the Home Reaches the Set Temperature
The most useful clue is the overall pattern: does the house reach the thermostat setting, does AUX turn off, and does the system maintain comfort afterward?
Observe the Outdoor Unit From a Safe Distance
You can note whether the outdoor heat pump appears to be operating and whether you see an obvious problem. Do not remove panels, test electrical components, handle refrigerant parts, or use tools or hot water to remove ice.
When Should You Call an HVAC Technician About Aux Heat?
AUX Heat is a reason to schedule professional service when the backup heat appears to be compensating for a system that is not working normally.
- Auxiliary heat runs nearly all the time.
- AUX appears frequently during mild weather.
- The thermostat calls for heat but the home does not warm up.
- The outdoor heat pump does not appear to be operating normally.
- Airflow is weak or the system runs for unusually long periods.
- The thermostat shows errors or inconsistent temperature readings.
- Energy use rises unexpectedly without a comparable change in weather.
- The system is overdue for heating maintenance.
Oasis Services provides heating and furnace service in Central Texas, including heat-pump service, heating repair, maintenance, and thermostat-related troubleshooting. A technician can evaluate the heat pump, thermostat, airflow, controls, and backup-heating operation together instead of guessing from the thermostat display alone.
What Does Aux Heat Mean for Central Texas Homes?
Central Texas winters are generally milder than winters farther north, but cold fronts can still arrive quickly. During a sharp temperature drop, it may be normal to see AUX Heat appear while a heat pump responds to increased heating demand.
The context changes when AUX Heat shows up repeatedly during relatively mild weather or the system still cannot hold temperature. In that situation, the goal is not simply to turn AUX off. It is to find out why the heat pump or controls are relying on backup heat more than expected.
If you are deciding whether a heat pump is the right long-term heating option for your home, see Oasis Services’ Heat Pump vs. Furnace in Central Texas guide for a broader comparison of system types, efficiency, comfort, and backup-heat strategies.
Aux Heat Running More Than Expected?
If your thermostat frequently shows AUX Heat, your home is not reaching temperature, or you are unsure whether your heat pump is operating normally, Oasis Services can inspect the thermostat, heat pump, airflow, controls, and heating system to identify the cause.
Schedule Heating Service: Request heating and heat-pump service from Oasis Services
For broader repair, installation, thermostat, and HVAC options, visit Oasis Services HVAC Services.
Frequently Asked Questions About Aux Heat
What does AUX Heat mean on my thermostat?
It means the system has activated a secondary heat source to supplement the primary heat pump. Depending on the system, that backup heat may come from electric resistance elements, a furnace, or another configured source.
Is it normal for auxiliary heat to come on?
Yes. Temporary AUX operation can be normal during colder weather, large thermostat recovery, defrost cycles, or other periods of high heating demand.
Why does my thermostat say AUX Heat when it is not very cold?
AUX can appear because the thermostat is recovering from a large setpoint change, because of control settings, or because the heat pump is struggling to satisfy the thermostat. Frequent AUX operation in mild weather is worth evaluating.
Is AUX Heat the same as Emergency Heat?
No. AUX Heat is generally activated automatically when the system needs backup heating. Emergency Heat is typically a manually selected mode that relies on the backup source rather than normal heat-pump operation.
Does auxiliary heat cost more?
Electric resistance auxiliary heat can use more electricity than normal heat-pump operation. Cost depends on the type of backup heat, weather, system efficiency, utility rates, and how long the backup heat runs.
How long should AUX Heat stay on?
There is no universal time limit. The important question is whether AUX turns off as the home reaches temperature or continues running unexpectedly for long periods.
Should I turn AUX Heat off?
Normally, no. Automatic auxiliary heat may be operating for a valid reason. Persistent or unexpected operation should be diagnosed rather than simply disabled.
Does AUX Heat mean my heat pump is broken?
Not necessarily. Backup heat is part of many normal heat-pump systems. A problem is more likely when AUX operates continuously, appears often in mild weather, or the home cannot maintain the thermostat setting.
Technical reference: DOE Building Science Education – Controls for a Heat Pump With Secondary Heating.
