HeatingCodesAux heat vs emergency heat: what each one means on your heat pump thermostat
The short version
| Auxiliary (AUX) heat | Emergency (EM) heat | |
|---|---|---|
| Who turns it on | The thermostat, automatically. | You, by choosing the setting. |
| What the heat pump does | Keeps running alongside the backup heat. | Is shut off entirely. |
| Normal use | Yes, in cold weather, during defrost and for large temperature changes. | No. Only if the heat pump has broken down or is not operating. |
| What it uses | Electric heat strips in the air handler, or a gas or oil furnace in a dual-fuel system. | The same backup source, alone. |
Both manufacturers describe it the same way. Carrier writes that auxiliary heat is automatic and "runs alongside your heat pump to boost heating power," while emergency heat "shuts off the heat pump entirely and uses only the backup heating source." Trane says emergency heat is a setting you should only use "in a true emergency," and that if you engage it you should call for HVAC service right away.
When AUX heat is normal
Both companies list the same situations:
- Very cold outdoor temperatures. Carrier says there is no single outdoor temperature at which every heat pump turns on auxiliary heat, because it depends on the heat pump's capacity and the home's heating load (the balance point). Variable-speed models can operate at much lower temperatures than single-stage ones.
- Defrost cycles. When the outdoor unit frosts up in winter it enters a defrost cycle, and the backup heat can help avoid cold air blowing from the vents. Trane says that if AUX shows while the heat pump is running, the heat pump may be defrosting, and you may see frost or ice on the outdoor unit.
- A large thermostat increase. Carrier cites raising the thermostat by more than 2 or 3 degrees at once, and Trane says 3 or 4 degrees. The system calls for backup heat to reach the new temperature faster.
When AUX heat suggests a problem
Trane says it is only a bad thing if aux heat comes on and stays on when outdoor temperatures are above 40 degrees F, because that can indicate a problem with the heat pump or thermostat. If it is above 40 degrees F and the thermostat shows AUX HEAT for more than an hour or two, Trane says you might want to investigate. Carrier lists a dirty air filter, a refrigerant leak, a malfunctioning sensor or a thermostat that was raised too quickly as possible reasons for AUX in mild weather above 40 degrees F.
Trane also suggests two homeowner checks: make sure the outdoor heat pump is not covered with or surrounded by snow, and, if you lower the set temperature by several degrees, check that the AUX indicator disappears while the heat pump keeps running. If the heat pump will not turn on at all, call for service.
Why AUX heat can raise your bill
Carrier says electric resistance auxiliary heat is much more expensive to operate than a heat pump, which it describes as potentially 300 percent efficient versus 100 percent for resistance heat. Trane agrees that electric strips use more energy than the heat pump, while noting that a gas furnace as the backup may or may not cost more depending on gas and electricity prices.
The tips both companies give are about the same: avoid deep nighttime setbacks that force a big morning recovery, raise the thermostat only a degree or two at a time, use a programmable or smart thermostat that ramps up gradually, keep the filter clean and schedule maintenance. Trane suggests keeping the temperature around 68 degrees. You can estimate what a given setback saves with the heating cost calculator.
What to check if AUX heat runs a lot in mild weather
- Check the outdoor temperatureConfirm it is above 40 degrees F. If it is colder, AUX may be normal.
- Check the air filterReplace a dirty filter. Carrier lists it as a possible reason for AUX in mild weather.
- Look at the outdoor unitMake sure it is not buried in snow. Frost or ice with AUX showing may just be defrost. If the unit is not running, call for service.
- Lower the setpoint a few degreesTrane says the AUX indicator should disappear while the heat pump keeps running. If AUX stays on for long periods, call for service.
Frequently asked questions
Is AUX heat the same as backup heat?
Yes. Carrier says auxiliary heat and backup heat generally refer to the same concept: a supplemental heat source that supports or replaces the heat pump when needed.
At what outdoor temperature does aux heat come on?
It varies. Carrier says there is no single temperature for every system. Trane says the thermostat triggers aux heat when it is below 40 degrees F or a set trigger temperature, during defrost, or after a large setpoint increase.
Should I use emergency heat to save money in a cold snap?
No. Carrier and Trane both say emergency heat is for a heat pump that has broken down. Using it shuts off the heat pump and relies only on the backup source, which is usually electric resistance and costs more.
Why does cold air blow from the vents sometimes in winter?
During defrost the heat pump runs in a way that can briefly cool the supply air. Carrier says supplemental heat can help prevent cold air from blowing from the vents during that time.
Sources
Safety note: this page is general information drawn from the sources above. Gas, combustion and line-voltage repairs belong to a licensed professional. Your own equipment's label and manual take priority over anything here. Read the full disclaimer.