Your thermostat says AUX again. The heat pump has been running, the house still feels cool, and you are wondering whether the backup heat is doing all the work.
Auxiliary heat is not automatically a problem. It can help during a cold morning, after a large thermostat adjustment, or while the outdoor unit defrosts. But if AUX stays on, comes back repeatedly when the weather is mild, or the house cannot reach the set temperature, the heat pump may not be delivering enough heat. Restricted airflow, a failed outdoor component, a refrigerant problem, or a thermostat setup issue could be responsible.
At Dave’s Air Conditioning Plumbing & Electrical, we would not call AUX a bad part. It is a clue. We check what the thermostat is requesting, whether the outdoor unit is actually heating, and where the heat is getting lost before recommending a repair.
What AUX Heat Actually Means
A heat pump heats your home by moving heat from outside air indoors. Many heat pump installations also have a backup heat source, often electric resistance strips in the indoor equipment. When the heat pump needs help, the thermostat can call for that supplemental heat. That is auxiliary heat.
On systems with electric strips, backup heat generally takes more electricity to provide heat than normal heat pump operation. A brief call for AUX may be part of normal operation. Long periods on AUX can raise your bill, particularly if the outdoor unit has stopped contributing.
AUX heat is not the same setting as emergency heat. AUX can assist while the heat pump runs. Emergency Heat is a thermostat mode that typically relies on the backup source instead of the outdoor heat pump. Thermostats and dual-fuel systems differ, so check your equipment instructions before changing unfamiliar settings. If you switched to Emergency Heat because the outdoor unit failed, arrange for service rather than leaving the cause unresolved.
When Is Switching to Auxiliary Heat Normal?
Start with what happened just before AUX appeared. Timing tells us more than the thermostat label alone.
- A cold morning: As outdoor temperatures fall, the heat pump may need supplemental heat to keep up with the home’s heat loss. The temperature at which that happens depends on the equipment, the house, and its control settings.
- A big thermostat increase: If you raise the setting several degrees at once, the thermostat may request backup heat to recover faster. The same thing can happen after an overnight setback.
- A defrost cycle: Frost can form on an outdoor coil during cool, damp weather. The heat pump periodically defrosts it, and some systems run supplemental heat during that cycle so indoor air does not feel as cool.
What matters is whether AUX shuts off when the system catches up. If the house reaches its setting and the backup heat stops, there may be nothing to repair. If it keeps coming on without an obvious cold-weather or recovery demand, find out why. There is no single outdoor temperature that makes AUX normal or abnormal for every Spartanburg home.
Why AUX Keeps Coming Back
The thermostat is calling for rapid recovery
Many homeowners notice AUX right after they turn up the heat. That does not prove the heat pump is broken. It may mean the thermostat is trying to close a large gap between room temperature and the new setting.
Watch what happens after the room warms up. Does AUX go away, and can the heat pump maintain the temperature on its own? If so, try smaller setting changes. If the temperature barely moves and AUX stays on, look beyond the thermostat adjustment.
The filter or ductwork is choking airflow
A dirty filter can reduce airflow through the indoor unit. Blocked returns, closed registers, a dirty indoor coil, or a blower problem can do the same. The system may run longer while less warm air reaches the rooms. Depending on how the system is controlled, the thermostat may bring on AUX as the home falls behind.
In Upstate South Carolina homes, we also pay attention to ducts in crawlspaces and attics. A loose connection, crushed run, damaged insulation, or leaking return can create a cold-room complaint even when the heat pump itself is running. If several vents feel weak, or one part of the house stays cold, the ducts belong in the diagnosis.
Replacing a filter is a sensible first step. Replacing the heat pump because the house is cold, without checking airflow, is not.
The outdoor unit is not doing its share
The indoor blower can run even when the outdoor equipment is not producing useful heat. A failed capacitor, worn contactor, fan motor problem, control fault, compressor issue, or damaged electrical connection can leave backup heat carrying the load. A tripped breaker is another important clue.
From a safe distance, check whether the outdoor unit runs during a sustained call for heat. Do not assume something is wrong if it pauses briefly; a defrost cycle can change its normal pattern. But if the outdoor unit remains off while the indoor system runs and AUX continues, it needs testing. Repeated clicking, buzzing, or a breaker that trips again are reasons to stop resetting controls and call for help.
Frost is not clearing from the outdoor coil
A light coating of frost can be normal in damp, cool conditions. A thick layer of ice that remains on the coil is different. Ice blocks airflow and interferes with heat transfer, so the thermostat may call for more backup heat while comfort gets worse.
The defrost controls or sensors may be at fault. An outdoor fan or refrigerant problem can also affect coil conditions. Do not scrape or chip ice off the unit. Damaging the coil turns one problem into a larger one. Note what you see and have the system inspected.
A refrigerant problem is reducing heating capacity
A heat pump needs the right refrigerant charge to move heat effectively. If refrigerant has leaked out, the system can run for a long time without warming the house enough. AUX may come on to make up the difference. Ice, poor heating, and unusually long cycles can be clues, but none confirms a leak by itself.
Proper diagnosis includes checking airflow and operating conditions, not just connecting gauges and adding refrigerant. If a leak is confirmed, the next decision is whether the affected part can be repaired at a sensible cost. A recharge without addressing a confirmed leak is not a durable fix.
The thermostat or original setup is wrong
Thermostat wiring, equipment settings, staging, and backup heat controls all affect when AUX starts. A replacement thermostat that was never configured for the installed heat pump can call for heat incorrectly. Poor past installation work can leave blower settings, ducts, or equipment performance mismatched to the house.
These faults are easy to miss if someone only looks for a broken part. The diagnostic question is straightforward: Is backup heat being called because the house truly needs it, or because the controls and equipment are not working together as intended?
Check These Things Before You Call
You can gather useful information without opening the air handler or touching electrical components. Work through these checks in order:
- Read the thermostat display. Is it set to Heat or Emergency Heat? Note the room temperature, set temperature, and whether AUX appears only after you raise the setting.
- Give the system a fair test. Make a small thermostat adjustment rather than a large jump. See whether the house warms and whether AUX eventually shuts off. Do not keep changing settings every few minutes.
- Check the filter. If it is dirty, replace it with the correct size and a type suitable for your system. Make sure the filter fits properly and the access cover is closed.
- Look at the air path. Keep return grilles clear of furniture and make sure supply registers are not blocked. Notice whether weak airflow is limited to one room or present throughout the house.
- Look at the outdoor unit from a safe position. Remove loose leaves or items around it if you can do so without touching the equipment. Note heavy ice, unusual noise, or a fan that does not appear to run.
- Check for a tripped breaker without repeated resets. If a breaker trips again, leave it alone. The underlying fault needs diagnosis.
Tell the technician when AUX starts, whether the outdoor unit runs, and whether the house reaches its set temperature. That information helps separate a control issue from a heat-production or airflow issue. It does not replace electrical testing or refrigerant measurements.
When Is It Urgent?
Frequent AUX operation without other warning signs calls for a service appointment, especially when the house will not get warm. Some symptoms call for a faster response.
- Burning smell, smoke, or visible electrical damage: Stop operating the system if it is safe to do so. Seek immediate help, and use emergency services if there is a fire or immediate danger.
- A breaker that keeps tripping: Do not keep resetting it. The circuit or equipment may have a fault.
- Heavy ice or severe mechanical noise: Stop using the heat pump and arrange an inspection. Do not break ice loose or reach into the unit.
- No usable heat: If indoor temperatures are falling and the home cannot maintain a safe temperature, arrange prompt heating service.
If the heating problem includes repeated electrical faults, our electrical services page covers troubleshooting beyond the heat pump itself. Do not open panels or test live wiring to decide whether a breaker, circuit, or HVAC component is responsible.
What We Would Diagnose Before Naming a Repair
Dave’s Air Conditioning Plumbing & Electrical starts with the sequence of operation. Is the thermostat asking the heat pump to run? Is the outdoor unit responding? Is warm air reaching the rooms? Only then does an AUX indicator become useful evidence.
Depending on the symptoms, an HVAC diagnosis may include:
- Checking thermostat settings and whether a large temperature gap is triggering backup heat.
- Verifying when the thermostat calls for the heat pump and when it calls for auxiliary heat.
- Inspecting the filter, blower operation, indoor coil, and visible airflow restrictions.
- Comparing air temperatures at the system and checking whether heat reaches the living space.
- Inspecting accessible ducts for obvious leaks, damage, or disconnected runs, including crawlspace concerns when applicable.
- Testing relevant capacitors, contactors, motors, and electrical connections rather than replacing them on suspicion.
- Checking outdoor coil condition, fan operation, and whether defrost is functioning correctly.
- Evaluating refrigerant performance and looking for evidence of a leak if readings point that way.
- Reviewing equipment setup or sizing concerns when ordinary component checks do not explain the pattern.
The order matters. A new thermostat will not repair a failed outdoor fan. A capacitor will not fix a disconnected crawlspace duct. And a new heat pump will not correct every control or duct problem left in place. Our job is to show you what we found, explain the options, and price the work upfront.
Repair the Fault, or Consider a Bigger Fix?
Repair first when the evidence supports it
If testing points to a replaceable electrical component, a thermostat setup problem, a sensor, an airflow restriction, or a repairable duct issue, fix that fault and verify that the heat pump heats properly afterward. The goal is not just to make the AUX message disappear. It is to confirm the outdoor equipment and indoor airflow can do their jobs.
A repair can also make sense when a refrigerant leak is located and the condition of the equipment supports fixing it. The decision depends on what failed, what the repair involves, and whether the system is otherwise reliable. No parts-cannon thinking.
Consider replacement or a larger correction when the facts change
Replacement deserves a serious look when the compressor or coil has failed, a refrigerant repair is extensive, or the system has recurring problems and poor overall performance. A system that cannot reasonably meet the home’s needs even after controls and airflow are corrected may need a sizing or installation review.
Sometimes the larger fix is not a new heat pump. Leaky ducts, inadequate airflow, or controls set up incorrectly can continue to waste heat after new equipment goes in. We would rather identify that before you commit to replacement. Dave’s Air Conditioning Plumbing & Electrical can discuss heating and air conditioning services once the cause and scope are clear. Financing options are available for qualifying projects, but the diagnosis comes first.
Keep AUX From Doing Work the Heat Pump Should Do
You cannot eliminate every call for auxiliary heat, and you should not disable it just to keep the thermostat display clear. Backup heat serves a purpose. The practical goal is to avoid unnecessary operation and catch a struggling system early.
- Keep airflow open. Inspect the filter regularly, replace it when dirty, and leave returns and supply registers unobstructed.
- Use thermostat setbacks thoughtfully. Large jumps can prompt backup heat. If you see AUX after a schedule change, test whether smaller adjustments reduce those recovery calls.
- Keep the outdoor unit clear. Remove loose debris around it without reaching into the cabinet or damaging the fins.
- Pay attention to changes. New weak airflow, longer cycles, heavy ice, or a jump in backup heat use can signal a fault even when the system still heats.
- Have performance checked. Maintenance is a chance to find dirty coils, worn electrical components, and airflow issues before a cold spell exposes them.
For homeowners who want planned system checks, ask Dave’s about the RightFirst Comfort Plan. Maintenance cannot prevent every failure, but it is more useful to check performance than to wait until backup heat becomes your main source of warmth.
Questions Homeowners Ask About Auxiliary Heat
Is it bad if AUX heat comes on every morning?
Not necessarily. A cold morning or a scheduled thermostat increase may call for temporary backup heat. If AUX runs for a long time, the home does not reach its setting, or the pattern is new, have the heat pump checked.
Why does AUX run when it does not feel very cold outside?
A large thermostat adjustment can trigger it even in mild weather. If that does not explain it, check the filter and whether the outdoor unit runs. A control, airflow, electrical, defrost, or refrigerant problem may be reducing heat pump output.
Does AUX heat mean my outdoor unit is broken?
No. The thermostat may be requesting extra heat for a normal reason. But if the outdoor unit stays off while AUX carries the heating load, its operation needs testing. The display alone cannot identify which component, if any, has failed.
Will auxiliary heat make my electric bill higher?
Electric resistance backup heat generally uses more electricity to provide the same heat than a properly operating heat pump. How much the bill changes depends on how often AUX runs and other household electricity use. Treat a sudden change as a reason to investigate, not proof of a particular fault.
Should I switch to Emergency Heat to stop the system from struggling?
Not as a routine fix. Emergency Heat typically bypasses the outdoor heat pump and uses the backup source. If the outdoor equipment is malfunctioning, follow your equipment guidance and arrange service. Do not use the setting as a substitute for finding the cause.
Can replacing the air filter solve it?
Yes, if a dirty filter was restricting airflow enough to affect heating. Replace it with the correct filter and observe whether the house warms normally. If AUX still runs excessively, the filter was not the whole story.
Can I turn off auxiliary heat to save money?
Do not disconnect heat strips or defeat controls. That could leave the home without enough heat when the heat pump needs help or during a malfunction. Find out why AUX is running before changing how the system operates.
Get an Answer That Goes Beyond the Thermostat Display
If AUX keeps appearing, you do not need a guess about which part to buy. You need to know whether the call for backup heat is normal, whether the outdoor unit is producing heat, and whether that heat reaches the rooms.
Dave’s Air Conditioning Plumbing & Electrical is a veteran-owned, family-run contractor serving Spartanburg, Boiling Springs, Greer, Duncan, Moore, Inman, and nearby Upstate South Carolina communities. Schedule heating service and we will work toward a clear diagnosis, straight repair or replacement options, and clean workmanship guided by our RightFirst Standard and Done Right the First Time philosophy.