When should auxiliary or emergency heat run?

Start by identifying the actual heat sources

Some heat-pump systems have electric resistance backup; others are paired with a fuel-burning furnace. Some have no installed backup source. A thermostat label does not tell you which arrangement you own. Ask the installer to name the equipment and explain when each source is intended to operate.

Trane's auxiliary-heat explainer distinguishes automatically controlled supplemental heat from emergency operation. The exact behavior depends on the equipment and thermostat configuration. Treat AUX as information about operation to interpret, not automatic proof of a fault or a reason to disable a heating stage.

auxiliary-heat explainer · Source checked 2026-09-07

Auxiliary heat and emergency heat are different modes

Auxiliary heat is typically called automatically when the control sequence needs the supplemental source. In a common system, emergency heat is a manually selected mode that relies on backup while locking out the heat-pump compressor. Your equipment's instructions govern the details.

Emergency heat is not a general “extra warm” setting. If it is needed because the heat pump has failed, arrange service and follow the operating instructions for the installed backup system.

There is no universal outdoor-temperature cutoff

The home's heating requirement changes with outdoor conditions, and the heat pump's available output also varies. Equipment selection, backup capacity, controls, and the design assumptions determine the intended sequence. A neighbor's cutoff temperature is not a specification for your house.

NEEP's cold-climate design recommendations address the relationship between loads, equipment selection, and supplemental heat. Ask what condition triggers backup in your system and whether the threshold reflects capacity, an economic control choice, or a manufacturer operating limit. Have those assumptions documented separately.

cold-climate design recommendations · Source checked 2026-09-07

Recovery and defrost can affect the display

Backup operation can be connected to recovery after a temperature setback or to the equipment's defrost sequence. Trane's defrost guidance notes that some systems use supplemental heat during that process. The appearance of AUX during a brief event is different from unexplained continuous operation.

Record the thermostat setting, any recent change, outdoor conditions, and whether the home is reaching temperature. Ask the technician to explain how your thermostat handles recovery. Avoid experimenting with deep setbacks or installer lockouts solely to chase a lower bill; first establish the approved operating strategy and the household's comfort needs.

defrost guidance · Source checked 2026-09-07

Ask for the control sequence in writing

At installation or service, request a short description of normal operation and backup behavior. It should be specific enough that another technician can understand what was intended.

• Which heat sources are installed, and what capacity was selected for each?
• Which temperature sensors and thermostat settings control the changeover?
• Can the heat pump and backup run together, or must they run separately?
• What happens during defrost, schedule recovery, and a detected equipment fault?
• Which settings should the homeowner use, and which require a technician?
• How was the configured sequence tested at handover?

Ask for any unresolved seasonal verification to be recorded. The commissioning checklist provides a place to keep these answers with equipment and airflow results.

Investigate a changed pattern rather than guessing at cost

If AUX appears much more often than it previously did under similar conditions, or the house is not reaching temperature, request an assessment. The cause might involve controls, airflow, equipment performance, or a change in the building. A display alone cannot distinguish them.

Electric resistance backup and a heat pump use electricity differently, while a dual-fuel system adds fuel-price and equipment-performance considerations. Ask for any operating-cost comparison to state rates, weather assumptions, and the heat sources being compared. Do not treat a whole utility bill as a direct measurement of backup energy without supporting data.

Prepare a useful inspection request

Gather model information, thermostat photographs, error messages, and a brief log of AUX operation. Mention recent service, a replacement thermostat, or changes to schedules. If there is a loss of heat in severe weather, make the urgency clear rather than waiting to complete a log.

For the temporary behaviors that can accompany ordinary winter operation, read heat-pump defrost. For unexplained backup use, request a heat-pump inspection and ask the technician to compare the actual sequence with the intended design.