Carrier Heat Pump Aux Heat On Constantly: Causes & Fixes

Last updated Oct 4, 2026 · By fixme.vip Editorial

Carrier heat pump auxiliary heat on constantly: what it means

“Auxiliary heat on constantly” is a symptom, not a numbered fault code. Your thermostat shows AUX (or “Aux Heat On,” or a backup heat indicator on Carrier Infinity/Evolution controls) almost all the time. That means the electric heat strips or dual-fuel furnace are running alongside the heat pump, or in place of it.

Aux heat is meant to supplement the heat pump in three situations: during defrost, when recovering from a large temperature setback, and in very cold weather. When it runs continuously in moderate weather (roughly above 35–40°F outdoors), one of two things is usually happening:

• The thermostat is calling for it because of a setting or wiring issue.
• The heat pump isn’t producing enough heat, so the house never reaches setpoint and the control keeps backup heat on.

How Carrier systems manage aux heat depends on the control. Communicating Infinity/Evolution systems use configurable outdoor lockout temperatures and staging logic. Conventional thermostats bring on aux heat based on temperature droop or timers. There is no Carrier fault code that latches for this condition itself. It clears once the cause is fixed. If the outdoor unit’s control has locked out the compressor, though, that lockout may hold until power is cycled, depending on the board. Check the outdoor unit’s status LED or the Infinity control’s fault history.

If the system runs constantly without enough heat and has a gas furnace, also see the furnace-side page listed under Related codes.

Carrier heat pump auxiliary heat on constantly: common causes, ranked by probability

1. Thermostat set to EM HEAT or misconfigured. Emergency heat was left on, the thermostat is set up for a conventional system instead of a heat pump, or the aux staging and droop settings are too aggressive. Smart thermostats often run aux heat early by default.
2. Large setbacks or frequent setpoint changes. Raising the setpoint more than about 2–3°F at once triggers aux heat on most thermostats. Deep nightly setbacks cause long aux recovery periods every morning.
3. Restricted airflow. A clogged filter, closed registers, or blocked returns reduce heat pump output and can trip the high-pressure limit, so the system leans on backup heat.
4. Outdoor coil iced over or defrost not working. A failed defrost thermostat or sensor, a defrost board fault, or snow and ice packed around the unit keeps the heat pump from producing heat. Aux heat then runs to compensate, and constantly runs during a stuck defrost.
5. Outdoor unit not running or locked out. A tripped outdoor breaker, failed capacitor, bad contactor, or a pressure-switch lockout on the outdoor control means the indoor unit delivers only strip heat.
6. Low refrigerant charge (leak). Undercharge sharply reduces heating capacity. You’ll often see icing patterns, long run times, and lukewarm supply air.
7. Reversing valve stuck or failing. The valve stays partly or fully in cooling position. The heat pump runs, but the house gets little or no heat, so aux carries the load.
8. Wiring faults. On conventional systems, a miswired or shorted W2/E/AUX conductor, or a wrong O/B setting, can energize the strips constantly. Heat strips that stay on even with the thermostat off point to a stuck sequencer or relay in the air handler.
9. Outdoor temperature below the balance point, or an undersized system. In genuinely cold weather, continuous aux heat may be normal. Dual-fuel lockout temperatures set too high on Infinity/Evolution controls also cause this.

How to fix Carrier heat pump auxiliary heat on constantly: safe checks first

• Thermostat mode: Make sure it reads HEAT, not EM HEAT or Emergency. Switch it back if needed, then watch for 30–60 minutes.
• Setpoint habits: Raise the temperature in small steps of 1–2°F. Reduce deep setbacks, or use a heat-pump “smart recovery” setting if your thermostat has one. Don’t change installer-level configuration menus unless you’re following your thermostat’s own manual for heat pump setup.
• Batteries: Replace them if your thermostat uses batteries. Low power can cause erratic behavior.
• Air filter: Replace it if it’s dirty. Open all supply registers and clear return grilles of furniture and rugs.
• Breakers and disconnect: Confirm both the air handler breaker and the outdoor unit breaker (or the outdoor disconnect switch) are on. If a breaker tripped, reset it once. If it trips again, stop and call a technician.
• One power reset: If the outdoor unit appears locked out, turn power off at the thermostat and breakers for 5 minutes, then restore it. Do this only once. Repeated resets can damage the compressor.
• Outdoor unit, visual only: Clear snow, leaves, and debris from around and on top of the unit. Keep at least 12–24 inches of clearance. Don’t chip ice off the coil or pour hot water on it, because the fins are fragile. A thin frost that clears every 30–90 minutes is normal. A solid block of ice is not.
• Condensate: Check that the air handler’s drain line isn’t backed up. Some units shut down on a float switch when it is.
• Panels: Make sure the air handler and outdoor access panels are fully seated.

How a technician will diagnose it

A competent tech will typically:

1. Check thermostat setup: heat pump configuration, O/B orientation, aux and droop settings, and dual-fuel or Infinity lockout temperatures.
2. Read fault history: on the Infinity/Evolution control or the outdoor control board’s LED flash codes, looking for pressure-switch, high-temperature, or compressor lockouts.
3. Measure temperature split: compare supply and return temperatures with strips off. A healthy heat pump typically gives a rise of about 15–30°F depending on outdoor temperature.
4. Verify outdoor unit operation: test the contactor, capacitor, and compressor and fan amperage.
5. Test defrost components: check the defrost sensor and thermostat, the coil sensor, and the board’s defrost initiation and termination.
6. Check refrigerant pressures, superheat, and subcool against Carrier’s charging chart, and leak-search if the system is low.
7. Test the reversing valve: solenoid coil voltage and line temperatures, to confirm the valve shifts.
8. Inspect the air handler: heat strip sequencers and relays, plus W2/E wiring, for strips that stay energized.
9. Measure static pressure and airflow if restrictions are suspected.

Be skeptical of a quote to “add refrigerant” with no leak search, or a compressor or system replacement recommended without documented pressure and amperage readings.

Symptom, cause and what to do

Carrier heat pump auxiliary heat on constantly repair cost

Typical 2026 US ranges, including labor:

Check your warranty. Carrier typically covers parts for 10 years when the system is registered. Labor is usually extra unless you have an extended plan.

Related codes

Carrier Furnace Runs Constantly But Not Enough Heat (for dual-fuel systems with a gas furnace backup). Carrier AC Not Cooling (outdoor unit, refrigerant and capacitor issues shared with heat pump operation). Carrier Furnace Blowing Cold Air. Carrier Furnace Short Cycling. Carrier Furnace Blower Won’t Shut Off.