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 Auxiliary Heat means 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.
What it connects to
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.
Questions homeowners ask about Auxiliary Heat
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.
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.