Your thermostat says AUX, and there is an Emergency Heat option in the menu. AUX is usually a sign that the controls have called for supplemental heat. It is not, by itself, a reason to select Emergency Heat. If the house is warming normally, leave the normal HEAT mode in place while you check what prompted the extra stage. This distinction applies to a ducted heat pump with an installed backup heat source. If the house is losing heat, the outdoor unit has failed, or an alert appears, contact your service company and ask for the temporary operating plan for your equipment. A menu option alone does not tell you what backup heat is installed. AUX is a status; Emergency Heat changes the heating plan In the conventional arrangement described by Carrier's auxiliary-heat guide , AUX is requested automatically when additional heating is needed. Manually selecting Emergency Heat takes the heat pump out of normal heating service and relies on the backup source. It is intended for a malfunctioning heat pump, not as a routine boost button. Read the selected operating mode separately from the current status. “HEAT selected, AUX active” and “Emergency Heat selected” are different observations, even if both can involve the same indoor heater. Write down the actual words or photograph the screen before changing settings. Control labels vary. The Resideo T6 Pro Smart user guide describes an Em Heat mode whose availability depends on the heat-pump and auxiliary/emergency-heat configuration. That is a documented example, not proof that every thermostat has the same modes or equipment behind them. Check the manual for the thermostat model actually on your wall. AUX appears after the morning schedule raises the temperature What to notice: The house cooled overnight, the scheduled setting rises, AUX appears, and the room temperature moves back toward the target. Compare that with operation at a steady setting later in the day. Why it matters: Recovering a temperature setback asks the system to warm the building as well as meet its ongoing heat loss. Natural Resources Canada's heat-pump guidance explains that a heat pump may not supply all the heat needed for rapid recovery, depending on the setback and conditions. Backup heat can therefore run during recovery without proving a breakdown. Next action: Record the old setting, new setting, room temperature and whether the target is reached. Discuss the schedule and the controller's heat-pump recovery features with the installer. Do not keep raising the setting to make the air feel hotter. There is no single setback size that is best for every heat pump, house and rate plan. AUX runs during cold weather What to notice: The controls request supplemental heat as outdoor conditions become colder, but the indoor temperature remains stable. This differs from a room that continues getting colder despite a heating request. Why it matters: The home's heating load and the heat pump's available capacity change with conditions. Carrier describes the thermal balance point as specific to the equipment and home. An outdoor temperature copied from a neighbor's system is not a universal switch point. A brief defrost event can also call for supplemental heat; that is only one possible trigger. Next action: Keep the normal mode steady and log the conditions. Ask whether the amount of supplemental operation is expected for your installation if it seems excessive or has changed markedly. Do not enter installer menus to copy someone else's lockout temperature. Find out whether the backup is electric or fuel-fired Electric resistance strips and a dual-fuel furnace are different backup arrangements. The AUX label does not identify which you own, nor does it establish which heat sources should run together. Use the installation record or ask the contractor to identify the indoor unit, outdoor unit, thermostat and backup source as one system. Trane's auxiliary-heat explanation distinguishes backup sources and discusses the higher operating cost associated with frequent resistance heating. Treat that as a reason to understand the operating pattern, not a fixed savings promise. The cost of a fuel-backed system also depends on the fuel, electricity prices and equipment performance. A useful question for the installer is: “When this screen shows AUX, which equipment is supposed to be running, and what conditions can stop the compressor?” The answer should fit your installed combination. Do not assume a quiet outdoor unit always means failure, or that a furnace and compressor should always operate simultaneously. The status changed after service, or the house is not recovering What to notice: AUX is newly persistent at a familiar setting, an alert accompanies it, or comfort worsened after a thermostat replacement. Record when the change began and whether it follows a schedule, outdoor conditions or recent work. Why it matters: Installed controls can include compressor and supplemental-heat lockouts. Trane Home's control guidance describes configuration-dependent staging and lockouts and assigns their setup to an HVAC technician. A status screen cannot distinguish a deliberate control decision from a configuration or equipment fault on its own. Next action: Ask for a configuration and performance check, with the observation record available. If the heat pump has malfunctioned, ask whether your installed backup can provide temporary heat and how to use its documented user controls. Confirm when to return to normal HEAT after repair. Do not disable backup stages, bypass controls or repeatedly reset the system. If the technician proposes refrigerant work, our equipment and refrigerant identification guide helps organize the model, label and service history. Our questions for reviewing a refrigerant service plan address the subsequent repair discussion. An AUX display does not identify the refrigerant, prove a shortage or specify a factory charge. Choose the next step from the whole pattern What you see What to do HEAT mode, AUX during recovery, target temperature reached Log the schedule change; review recovery settings with the installer if needed. AUX in colder weather with stable indoor comfort Keep normal HEAT; compare the pattern with the installation's expected operation. Backup type or the meaning of a screen label is unknown Identify the installed system and consult its controller manual before choosing another mode. Persistent change, alerts, or falling room temperature Arrange diagnosis and ask for the appropriate temporary heating plan. Emergency Heat was selected during a confirmed failure Keep the repair appointment and confirm when normal heat-pump operation should resume.