Aux heat not working on a heat pump usually points to thermostat settings, tripped breakers, or failed heat strips that need pro repair.
What Aux Heat Does On A Heat Pump
When outdoor air turns cold, a heat pump starts to struggle. The system still moves heat from outside to inside, but it moves less of it with each pass. Aux heat steps in as backup so the house does not slip into a long chill while the heat pump works hard.
Aux heat uses electric resistance strips or a gas furnace to add extra warmth. The thermostat calls for this backup stage when the room temperature drops several degrees below the set point or when the defrost cycle runs. Without working aux heat, the home may hover a few degrees below the target temperature on freezing nights and the system may run almost nonstop.
On most thermostats you can see a small indicator when aux heat turns on. It may show as AUX, EM HEAT, or a small icon near the current mode. If that light never appears during a cold snap, you likely have aux heat not working on heat pump conditions even if the main heat pump still runs.
Aux Heat Not Working On Heat Pump – Quick Checks
Before you jump to worst case repairs, start with simple checks that do not involve tools. These steps help rule out settings and basic power issues that stop aux heat from kicking in when the thermostat calls for it.
- Confirm heat mode — Make sure the thermostat sits in Heat mode, not Cool or Off, and that any Schedule holds the set point you expect.
- Raise the set point — Bump the temperature up by three to five degrees so the thermostat has a strong call for heat and aux heat has a chance to engage.
- Check fan setting — Use Auto instead of On so the blower does not mask the moment when aux heat should start warming the air stream.
- Look for an aux indicator — Watch the screen for AUX, EM HEAT, or a similar label while the system runs to see whether the control board even tries to start backup heat.
- Inspect breaker panel — Find the breakers labeled Heat, Air Handler, or Furnace and reset any handle that sits between On and Off.
- Check any air handler switch — Many indoor units have a toggle beside the cabinet that can shut off power to the heat strips.
Quick check — If the thermostat shows aux heat active but the air from the vents still feels cool or just mildly warm, the issue likely sits with the heat strips, fuses, or wiring instead of the thermostat itself.
On mild winter days you may not see AUX on the screen at all, and that can be normal. Aux heat tends to appear when outdoor air drops near freezing or when you make big jumps on the thermostat. That pattern shows that the system leans on backup heat only when the load spikes.
Aux Heat Not Working On Your Heat Pump – Main Causes
Aux heat depends on several parts working in sequence. The thermostat has to send a call, the control board has to pass it along, the heat strips or gas burners have to energize, and the blower has to move air across them. A failure at any stage leaves the backup heat offline.
In many homes the thermostat is the first suspect. Wrong system type, miswired terminals, or a weak battery can keep the aux call from leaving the wall. Smart thermostats sometimes reset after power glitches and shift the system type away from heat pump with aux heat, which stops backup stages from running while the main compressor still responds.
Inside the air handler or furnace cabinet, heat strips sit on their own circuits. If a high limit safety switch trips because of poor airflow, or if a strip burns out with age, you lose a chunk of heating power. Some units also use small cartridge fuses on the control board; if one blows, the board cannot send power to the strips even when the thermostat calls for them.
Airflow problems also hurt aux heat output. A clogged filter, closed supply registers, or blocked return grilles keep warm air from moving through the ducts. That can trigger safety switches and can make the air feel lukewarm at the vents even when the strips run.
Some systems also rely on outdoor sensors and defrost logic that change how aux heat behaves. If a sensor reads the wrong temperature, the control board may either bring aux heat on too late or lock it out when the house needs help. A technician can read sensor values through the thermostat or board menu and compare them with real outdoor readings from a trusted thermometer.
| Symptom | Likely Cause | DIY Or Pro? |
|---|---|---|
| AUX light never appears | Thermostat mode or wiring issue | Start DIY, call tech if wiring is wrong |
| AUX light on, air still cool | Failed heat strips or tripped safety | Needs licensed HVAC technician |
| Breaker trips whenever aux runs | Shorted strip, loose wire, or overload | Pro repair only for safety |
| Some rooms warm, others cold | Airflow restriction in ducts or vents | DIY filter and vent checks first |
Step-By-Step Fixes You Can Try Yourself
Safety first — Any work near live wiring or inside metal cabinets carries shock risk. Stay on the side of caution and stop at the cabinet door if you are not fully comfortable around electrical parts.
Reset Power To The Heat Pump System
- Turn the thermostat off — Slide the mode to Off so the system does not try to run while you work.
- Flip the indoor breaker off — Turn off the breaker labeled Air Handler or Furnace, then switch off any outdoor unit breaker if it has a separate handle.
- Wait one minute — Give the control board time to fully discharge before you restore power.
- Turn breakers back on — Restore indoor and outdoor power, then set the thermostat back to Heat and raise the set point.
Quick test — After the reset, watch the thermostat for an AUX or EM HEAT label while the house warms. If it appears and the air now feels warmer, the issue may have been a control board glitch.
Check And Replace A Dirty Filter
- Locate the return grille or cabinet slot — Many filters sit behind a large grille near the ceiling or floor, while others slide into a slot beside the indoor unit.
- Slide the filter out — Note the air flow arrow and size printed on the frame so you can match it later.
- Inspect the surface — If you see a gray mat of dust that hides the original color, the filter chokes airflow and can trigger high limit switches.
- Install a clean filter — Match the size and point the arrow toward the blower so air flows through the media correctly.
Open Vents And Clear Returns
- Walk each room — Make sure supply vents are open and not hidden behind furniture, rugs, or long curtains.
- Inspect return grilles — Remove any objects leaned against the large grilles so the blower can pull air freely.
- Feel the airflow — With the system running, place a hand near several vents to check for a steady, warm stream of air.
If airflow improves yet the house still struggles on cold nights, aux heat not working on heat pump conditions likely remain in the strips or controls instead of the ducts.
When To Call An HVAC Technician
Some aux heat problems sit squarely in the professional zone. Heat strips pull amounts of current, and any repair that alters wiring or overcurrent protection needs a trained eye. A licensed HVAC technician also has the meter, clamp ammeter, and training needed to test staged heat banks without damage.
Call for service when the thermostat shows AUX or EM HEAT but the supply air stays cool. That pattern points toward failed strips, open safety switches, or damaged sequencers that no longer energize each heat stage. An electrician or HVAC technician can test each leg for continuity and replace burned components with matching parts.
Repeated breaker trips are another red flag. Breakers that flip each time aux heat starts may hide shorted elements, loose lugs, or mis-sized wiring. Resetting the breaker again and again without a proper repair increases fire risk and can weaken the breaker over time.
Thermostat replacements also benefit from a pro visit when your system uses dual fuel. These setups pair a heat pump with a gas furnace for aux heat. The thermostat needs correct configuration so the furnace and heat pump never run in heating mode at the same time. A technician can confirm outdoor sensor readings, balance points, and staging so aux heat comes on only when needed.
Preventing Aux Heat Problems In The Next Cold Snap
Once aux heat works again, a few habits can keep it ready for the next stretch of freezing weather. Small bits of care reduce wear on heat strips, keep safety switches happy, and lower power bills while still keeping the house comfortable.
- Change filters on a schedule — Swap filters each one to three months based on dust levels and pets in the home.
- Keep supply and return paths clear — Leave space around vents and grilles so air moves without strain.
- Set steady temperatures — Use modest night setbacks instead of large swings that force aux heat to run hard at sunrise.
- Book yearly maintenance — Have a technician open the cabinet, tighten connections, and measure strip amperage before the main heating season.
- Watch your energy bills — Sudden winter spikes can hint that aux heat runs more than needed or that the main heat pump lost efficiency.
Aux heat gives a heat pump system a safety net when outdoor temperatures dip. When that safety net fails, the house feels chilly and the electric meter spins faster than usual. With careful checks, a few basic DIY steps, and timely help from a qualified technician, you can restore steady warmth and keep aux stages ready for the next cold spell.
Once repairs are complete, pay attention during the first cold stretch of the season. Glance at the thermostat now and then to see when AUX appears, where the indoor temperature settles, and how long each heating cycle lasts. That quick feedback helps you spot new issues early, plan filter changes, and decide when a maintenance visit makes sense instead of waiting for a no-heat night.