Seeing Aux or Emergency Heat on your thermostat? Both relate to backup heat, but they do not mean the same thing. The first step is to check what your thermostat is displaying and what your equipment manual says.
What backup heat does
A heat pump moves heat from outdoor air into the home. Depending on the system, backup heat may help when the heat pump alone cannot meet demand.
Backup can be electric resistance heat or a compatible furnace. The controls work differently across systems. A dual-fuel setup may switch between the heat pump and furnace rather than run both together.
What Aux Heat means
Aux Heat generally means the controls have called for backup automatically. It may appear during colder weather, recovery after a thermostat change or a defrost cycle, depending on the equipment.
A brief Aux display does not by itself prove a fault. Note how long it stays on, the outdoor conditions and whether the house reaches the set temperature.
Electric resistance backup can use more electricity than heat-pump heating. If Aux runs for long stretches in mild weather, ask for a service assessment. A higher bill alone does not identify the cause.
What Emergency Heat means
Emergency Heat is a setting on some heat-pump thermostats. It generally uses backup heat while keeping the heat pump off. Check the instructions for your exact system.
Ask a technician whether and how to use it if the outdoor unit has failed. Do not use it to keep operating equipment that is smoking, smells of gas or has an electrical hazard. Whether backup heat is available depends on the fault and the system.
Emergency Heat is not a routine comfort setting. Its running cost depends on the backup equipment, energy prices and conditions. Electric resistance backup can increase electricity use.
What to check before calling
Keep checks to user controls and safe observations. Do not open cabinets or touch wiring. Turning the thermostat off does not disconnect electrical power.
- Read the display. Note whether it says Aux, Emergency Heat or an error message. Check the selected mode against the manual.
- Review thermostat changes. Note any recent schedule or temperature change. Follow the manufacturer’s heat-pump settings rather than repeatedly raising the temperature.
- Check the filter instructions. Follow the equipment’s filter schedule and access guidance. Leave access behind equipment panels to a technician.
- Observe the outdoor unit. Note ice, visible damage or blocked airflow from a safe distance. Do not reach through the grille, touch the fan or chip away ice.
- Rinse only as the equipment instructions allow. A gentle rinse with a garden hose, with the power off, can help. Do not use a pressure washer. Before cleaning, disconnect power as the equipment instructions require; thermostat Off does not isolate electrical power. Do not open equipment panels or spray electrical components. If you cannot follow the instructions safely, leave cleaning to a technician.
When to call for service
Call if Aux runs for long periods in mild weather, the house will not reach the set temperature, or the outdoor unit stays covered in heavy ice. Also call if you are relying on Emergency Heat because the heat pump stopped working.
Stop using the system and arrange service for an electrical or burning-plastic smell when there is no smoke, gas odor or immediate danger.
If you see smoke or fire, get everyone out, stay out and call 911 from a safe place. If you smell gas, leave without using switches or a phone indoors. From a safe place outside, call 911 and your gas utility.
Keep carbon monoxide alarms working according to their instructions. If a CO alarm sounds or you suspect CO poisoning, get everyone outside to fresh air and call 911. Do not return until emergency responders say it is safe.