Seeing AUX or AUX Heat on your thermostat does not automatically mean your heat pump is broken. Auxiliary heat is a secondary heating source that helps the heat pump when the system needs more heating capacity, such as during colder weather, a large thermostat recovery, or a defrost cycle.
The key is how often it happens and whether your home is still comfortable. Short periods of auxiliary heat can be normal. If AUX heat runs for long stretches, appears frequently in relatively mild weather, or accompanies weak heating performance, the system may need professional diagnosis. AirTeam provides heat pump repair in Houston for homeowners who need to know why their heat pump is relying heavily on backup heat.
What Is Auxiliary Heat on a Heat Pump?
Heat pump auxiliary heat is a backup or supplemental heating source that the system can use automatically when the primary heat pump needs help meeting the thermostat setting. In many all-electric systems, the backup source is electric resistance heat installed in the indoor air handler. In a dual-fuel system, the backup source may instead be a gas furnace.
Most of the time, the heat pump should do the primary heating. The thermostat and controls determine when additional heat is needed based on the system design, indoor temperature, outdoor conditions, and how quickly the home is recovering. Because heat-pump configurations vary, homeowners should not assume that every AUX display represents the same equipment or operating sequence.
If you want the broader system basics, AirTeam also explains how a heat pump works. This guide stays focused on auxiliary and Emergency Heat behavior.
Why Is Auxiliary Heat On?
There are several reasons auxiliary heat may turn on. Some are normal operating conditions; others can point to a heat pump that is no longer carrying the heating load as effectively as it should.
The Outdoor Temperature Is Low
As outdoor temperatures fall, an air-source heat pump may have less heating capacity available. Depending on the equipment and controls, the system can bring on backup heat to help maintain the thermostat setting. There is no single outdoor temperature at which every heat pump should behave the same way.
The Thermostat Temperature Was Raised Quickly
A large increase in the thermostat setting can create a bigger gap between the room temperature and the target temperature. Some controls respond by staging on auxiliary heat so the home reaches the new setting faster.
The Heat Pump Is in a Defrost Cycle
During cold, damp weather, frost can form on the outdoor coil. The system periodically enters a heat pump defrost cycle to warm that coil and melt the frost. Because the heat pump is temporarily directing heat toward the outdoor unit, auxiliary heat may run briefly to help maintain indoor comfort.
The Heat Pump Needs Additional Heating Capacity
Even when the system is working normally, there are conditions in which the heat pump and backup heat are designed to operate together. A short AUX cycle by itself is not proof of a fault.
The Heat Pump May Not Be Performing Correctly
A heat pump that is losing capacity because of an airflow, outdoor-unit, refrigerant, control, sensor, electrical, or other performance problem may rely on auxiliary heat more than usual. The AUX display identifies that backup heat is operating; it does not identify why the primary system needs the help.
Is It Normal for AUX Heat to Come On?
Yes. Seeing AUX Heat occasionally can be normal. It may appear during colder outdoor conditions, after a larger thermostat adjustment, during a defrost cycle, or when the system temporarily needs more heating capacity.
The pattern matters more than one thermostat message. Pay closer attention if AUX heat begins running almost continuously, appears far more often than it did previously, comes on repeatedly during relatively mild weather, or accompanies a home that cannot reach the thermostat setting. A significant increase in heating costs combined with new AUX behavior can also justify a system check.
AUX Heat vs. Emergency Heat: What’s the Difference?
Auxiliary Heat and Emergency Heat both involve the backup heating source, but they are not the same operating mode. Auxiliary heat is generally controlled automatically by the thermostat. Emergency Heat is typically a manual thermostat setting that changes the system to rely on the backup heat source instead of normal heat-pump heating.
| Auxiliary Heat | Emergency Heat | |
| How it turns on | Usually automatic | Typically selected manually |
| Purpose | Supplements normal heat-pump operation | Uses the backup heat source instead of normal heat-pump heating |
| Normal use | Can be normal temporarily | Generally reserved for a heat-pump problem or a situation where normal compressor operation should not be used |
| Typical display | AUX / AUX Heat | EM Heat / Emergency Heat |
Does Emergency Heat Mean There Is an Emergency?
Not necessarily. Emergency Heat is the name of a thermostat operating mode, not a message that a life-safety emergency is occurring. On typical heat-pump controls, selecting Emergency Heat disables normal compressor heating and makes the backup source the primary heat source.
That is different from an actual heating emergency. If your system has stopped heating during severe cold, you have an urgent after-hours problem, or there is another serious heating concern, use AirTeam’s emergency heating repair guide for the appropriate next steps. For serious after-hours heating service, call 713-956-8326.
Should I Turn on Emergency Heat When AUX Heat Appears?
Usually, no. AUX heat is designed to come on automatically when the system calls for supplemental heat. Seeing AUX on the thermostat is not, by itself, a reason to switch the system into Emergency Heat.
Emergency Heat is generally intended for situations in which the normal heat-pump heating operation should not be used, such as a failed outdoor heat pump that needs service. If the system is not heating correctly, the better long-term solution is to diagnose the problem rather than operate indefinitely in Emergency Heat as a workaround.
Does Auxiliary Heat Cost More to Run?
It can. In an all-electric system, electric resistance backup heat generally uses more electricity to deliver the same amount of useful heat than normal heat-pump operation. That is why extended auxiliary-heat runtime can show up as higher energy use.
The actual cost depends on the type of backup heat, runtime, outdoor conditions, thermostat settings, home heating load, equipment efficiency, and utility rates. A dual-fuel system that uses a gas furnace as backup has a different cost relationship than an all-electric heat pump with electric heat strips.
If your utility use rises at the same time that AUX heat begins running much more often, do not focus only on how to disable backup heat. The more useful question is why the heat pump needs so much assistance.
Why Is My Auxiliary Heat Running So Often?
Frequent auxiliary heat can be normal during unusually cold conditions, but it can also result from a change in system performance. The thermostat display alone cannot tell you which condition is responsible.
Colder Outdoor Conditions
The colder it gets, the more likely some systems are to call for supplemental heat. Compare current AUX behavior with the weather and with how the system normally operates.
Thermostat Setpoint Changes
Large or frequent temperature increases can make the thermostat call for additional heat to recover faster.
Defrost Operation
AUX heat may run briefly while the outdoor coil defrosts. If the heat pump is also icing excessively or failing to clear frost, the defrost behavior itself may need attention.
Restricted Airflow
A dirty filter, blocked return, closed or obstructed supply path, or another airflow problem can reduce heating performance. Homeowners can check obvious accessible airflow issues, but persistent problems need diagnosis.
Outdoor-Unit Performance Problems
Fan, electrical, control, reversing-valve, coil, or other outdoor-unit problems can reduce heat-pump capacity and increase reliance on backup heat.
Refrigerant or Heat-Transfer Problems
Refrigerant-system conditions can affect heat transfer and heating output. Refrigerant diagnosis and repair should be handled by a qualified HVAC professional.
Control, Sensor, or Thermostat Problems
Incorrect control signals, failed sensors, thermostat configuration, or other control problems can affect when auxiliary heat stages on and off.
When AUX Heat May Mean Your Heat Pump Needs Repair
The best reason to schedule service is not simply that AUX appeared once. It is a pattern of backup-heat use combined with abnormal heating performance or a clear change from how the system normally behaves.
- AUX Heat stays on for long periods even though conditions are not unusually cold.
- The system increasingly relies on backup heat compared with previous operation.
- The home cannot reach or maintain the thermostat setting.
- Heating output has dropped or the system runs almost continuously.
- Energy use increases sharply at the same time AUX behavior changes.
- The outdoor unit is not running normally or is repeatedly covered in heavy ice.
A professional diagnosis may involve the thermostat and control sequence, electrical components, outdoor-unit operation, airflow, refrigerant performance, defrost behavior, backup heat, and the overall ability of the system to deliver heat. That is why repeated AUX operation is a reason to investigate—not a diagnosis of one specific failed part.
Should You Turn Auxiliary Heat Off?
Usually, homeowners should not disable properly configured auxiliary heat simply to prevent it from appearing on the thermostat. Backup heat is part of the designed heating sequence and may be necessary for comfort, defrost support, or colder-weather operation.
Do not disconnect electric heat strips, bypass safety or staging controls, or change installer-level thermostat settings unless you understand the system configuration. If AUX is running too often, determine why the heat pump needs the extra help instead of removing the backup source from the sequence.
Can AUX Heat Come On During a Defrost Cycle?
Yes. During defrost, the heat pump temporarily changes operation to warm the outdoor coil and melt frost. Depending on the equipment, auxiliary heat can run during that period to help prevent an uncomfortable drop in indoor supply-air temperature.
Brief AUX operation associated with normal defrost is not automatically a problem. For the complete explanation of frost, steam, outdoor-fan behavior, and persistent icing, read What Is a Heat Pump Defrost Cycle? What’s Normal vs. a Problem?.
How Can You Reduce Unnecessary Auxiliary Heat Use?
The goal is not to eliminate every AUX cycle. The goal is to avoid unnecessary backup-heat use while allowing the system to operate as designed.
- Avoid large, repeated thermostat temperature swings when practical.
- Keep accessible filters and airflow paths maintained according to system requirements.
- Keep the outdoor unit free of obvious loose debris and airflow obstructions.
- Do not ignore persistent outdoor icing or a major change in heating performance.
- Keep routine heat-pump maintenance current so developing problems can be found before they become larger failures.
AirTeam’s pre-season HVAC tune-ups can help identify developing heating-system issues before the first significant cold weather reveals them.
Schedule Heat Pump Repair in Houston
Seeing AUX Heat occasionally does not necessarily mean something is wrong. But if your heat pump increasingly relies on backup heat, cannot maintain the thermostat setting, repeatedly ices up, or has changed how it operates, the next step is a professional diagnosis.
AirTeam provides professional heating repair for Houston-area homeowners. A technician can evaluate heat-pump performance, thermostat and controls, outdoor-unit operation, defrost behavior, electrical components, airflow, refrigerant-system performance, and backup heat to determine what is actually happening.
Schedule heat pump service: 713-936-5346.
Auxiliary Heat FAQs
What does auxiliary heat mean on a heat pump?
Auxiliary heat is a secondary heating source that automatically helps the heat pump when additional heating capacity is needed. Depending on the system, it may be electric resistance heat or another backup source such as a gas furnace.
Why is auxiliary heat on?
AUX heat may come on during colder weather, after a larger thermostat increase, during defrost, or when the heat pump needs additional capacity. Frequent AUX combined with poor heating performance can justify service.
Is it normal for AUX heat to come on?
Yes. Occasional auxiliary heat can be part of normal heat-pump operation. The concern is unusually long, frequent, or changed AUX operation along with poor comfort or higher energy use.
What is the difference between AUX heat and Emergency Heat?
AUX heat is generally automatic supplemental heat. Emergency Heat is typically a manual thermostat mode that relies on the backup heat source instead of normal compressor heating.
Does auxiliary heat cost more to run?
Electric resistance auxiliary heat generally uses more electricity than normal heat-pump heating. Actual operating cost depends on backup-heat type, runtime, weather, utility rates, and system configuration.
Should I turn off auxiliary heat?
Usually, no. Properly configured auxiliary heat is part of the system design. If it runs too often, diagnose the reason rather than disabling the backup heat.
Why is my heat pump using auxiliary heat so much?
Possible reasons include very cold conditions, large thermostat changes, defrost operation, airflow restrictions, outdoor-unit performance problems, refrigerant-system issues, or control problems. Persistent heavy reliance on AUX requires diagnosis.
Can AUX heat come on during a heat pump defrost cycle?
Yes. Many systems use auxiliary heat during defrost to help maintain indoor comfort while the outdoor coil warms and clears frost.
Related Heating Guides for Houston Homeowners
Continue with these AirTeam heating guides for related furnace and heat-pump questions:
