What does auxiliary heat mean in HVAC?
Auxiliary heat is backup electric strip heat that activates when your heat pump can't keep up alone — typically below 35–40°F outdoors.
Heating
Auxiliary Heat
Auxiliary heat is backup electric strip heat that activates when your heat pump can't keep up alone — typically below 35–40°F outdoors.
What it does
How Auxiliary Heat shows up at home.
Many heat pump systems use electric heat strips or another backup source for very cold weather, large temperature recovery, or heat pump trouble. Auxiliary heat usually engages on its own once outdoor temperatures fall to about 35–40°F, the range where most heat pumps can no longer pull enough heat from outdoor air to keep up alone. It is worth knowing how AUX differs from Emergency Heat, since the thermostat can display either one. AUX turns on automatically when the heat pump needs a hand and still runs the heat pump alongside the strips; EM Heat (sometimes shown as 'EM') bypasses the heat pump entirely and runs on backup heat only, which should be reserved for when the heat pump is broken because it is expensive to run. A short burst of AUX during a cold snap or first-thing-morning recovery is normal. If auxiliary heat runs constantly in mild weather, the heat pump, thermostat settings, airflow, or defrost cycle should be checked — part of a heat pump repair or maintenance visit.
01 Heat source
02 Auxiliary Heat
03 Blower
04 Warm supply air
System relationship
Where Auxiliary Heat fits and why it matters.
The part name is rarely the whole answer. This table connects Auxiliary Heat to the nearby components, the symptoms you might see, and the point where testing beats guessing.
| Relationship | Related item(s) | What this means for a homeowner |
|---|---|---|
| Parent system | the heat pump backup heating system | Auxiliary Heat is part of the heat pump backup heating system. That tells you which side of the system a technician will usually test first. |
| Related components | heat pump, thermostat, electric heat strips, outdoor unit | These are the parts most likely to be checked with auxiliary heat. One weak part can make a nearby part look guilty, especially when airflow, water, heat, or controls are involved. |
| Connected problems | high electric bills, weak heat pump output, thermostat aux alerts | This is what you are likely to notice at home: high electric bills, weak heat pump output, thermostat aux alerts. Those clues are more useful than guessing at the failed part. |
| Maintenance relevance | thermostat setup, heat strip testing, heat pump performance checks | This is where thermostat setup, heat strip testing, heat pump performance checks matters. The goal is to catch dirt, water, electrical weakness, or airflow strain before the next hard-weather day. |
| When to call a technician | aux heat runs constantly, the heat pump cannot keep up, or bills jump suddenly | Schedule service when aux heat runs constantly, the heat pump cannot keep up, or bills jump suddenly. At that point the issue usually needs measurements, not another thermostat setting change. |
Related glossary terms
Words you may hear with Auxiliary Heat.
Fast answers
FAQs about Auxiliary Heat.
These are the practical questions to answer before a technician opens the cabinet or puts gauges on the system.
Can a homeowner fix a auxiliary heat problem?
You can check the thermostat, replace a dirty filter, make sure vents are open, and look for water or ice. Stop before sealed panels, wiring, refrigerant, gas, combustion parts, or safety controls.
When should I call about auxiliary heat?
Call when the problem changes comfort, airflow, safety, water, ice, odor, noise, breakers, or how often the system starts and stops. Tell the technician what changed before you try to name the part.
Need help with Auxiliary Heat?
Tell us what changed in the home: temperature, airflow, water, ice, noise, odor, short cycling, or the message on the thermostat.