Before troubleshooting missing auxiliary heat, find out what backup heat was installed. A heat pump may work with electric resistance heaters or with a furnace. The thermostat label won’t tell you what the equipment is or when its controls should bring it on.
Trane’s auxiliary-heat guide distinguishes automatic backup operation from the manually selected Emergency Heat setting. Its equipment examples include more than heat strips. A wiring diagram for an electric air handler should not be applied to a dual-fuel installation.
Start with the heating result
Check whether the room is reaching the normal setpoint. No AUX indication while the heat pump is keeping the house comfortable is a different complaint from a steadily falling room temperature. Keep the ordinary heating setting while you observe; repeatedly raising it by large amounts changes the conditions you are trying to describe.
If AUX appears but the room keeps cooling, record that exact combination. That tells us the request is going out and the backup isn’t answering, which is where a technician starts. A technician needs to confirm the configured stages, the request from the control and the response of the installed equipment.
Check the breaker
Electric strips run on a separate 240V breaker from the heat pump compressor. It trips more often than people expect, especially on older panels or at the start of heating season when the strips haven’t been used in months. Go to your breaker panel and look for a double-pole breaker labeled “Air Handler,” “Heat Strips,” or “AHU.” If it’s tripped, reset it once. If it trips again within a few hours, stop. A breaker that keeps tripping means something is drawing too much current, possibly a shorted element, and continuing to reset it is a fire risk. Call a tech.
Leave installer thresholds alone
Do not copy an outdoor lockout temperature from another thermostat or home. The appropriate setup depends on the system. Don’t change installer settings or move thermostat wires to chase a missing AUX label.
Carrier’s auxiliary-heat explanation also separates automatic supplemental heating from emergency operation. For an outage or failed heat pump, use the backup procedure specified for your installation rather than experimenting with control settings.
An occasional cool-air interval during outdoor defrost has its own explanation in our defrost article. If heating cannot recover, request a diagnostic visit with the thermostat model and indoor equipment label. We confirm the repair scope after testing; the $99 visit fee is credited toward an approved repair.
Key takeaways
- Backup heat may be electric resistance equipment or a fuel-burning furnace.
- AUX on the screen means the thermostat is asking for backup heat. It doesn't show the backup is actually running.
Related questions
Does every heat pump have electric heat strips?
Should I select Emergency Heat to make AUX start?
Need help with your system? Call (925) 999-4095 or book a visit. Repair diagnostics are $99, credited toward the repair. Installation assessments are free.
Further reading
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