The short answer
Auxiliary heat is supplemental heat that the thermostat or heat-pump controls bring on automatically when the heat pump needs help. In many all-electric systems it is electric-resistance heat inside the air handler; in a dual-fuel system it may be a furnace. Brief auxiliary operation can be normal during very cold weather, a large thermostat recovery, or defrost. Long or unexpected use can be expensive and may justify checking settings, airflow, sizing, or equipment operation.
The useful answer
Auxiliary heat is normally automatic
The system decides when to add supplemental heat based on thermostat logic, outdoor conditions, the gap between room temperature and setpoint, and system configuration. A homeowner usually does not switch AUX heat on manually.
AUX does not mean the same thing in every house
For one home it may mean electric heat strips. For another it may mean a propane or gas furnace in a dual-fuel system. Check the equipment and thermostat documentation before assuming how the backup works or what it costs to run.
Look for a pattern, not a single appearance
A short appearance during defrost or severe cold may be expected. Frequent auxiliary operation in mild weather, a system that cannot regain temperature, or a sudden change from prior behavior deserves professional review. Dirty filters and blocked airflow are homeowner-check items; electrical and refrigerant diagnosis are not.
Call for qualified help when
- AUX stays on for long periods during ordinary weather.
- The house loses temperature while auxiliary heat is running.
- The electric bill changes sharply without a clear weather explanation.
- The outdoor unit is not running, repeatedly trips a breaker, or remains encased in ice.
Questions worth asking
- What type of auxiliary heat does this system use?
- At what conditions is it configured to stage on?
- Was the thermostat programmed for this exact heat-pump and backup combination?
- Is the recent runtime consistent with the system design and outdoor conditions?
Source trail
Primary guidance, not contractor marketing.
These sources establish the general answer. A technician still has to look at the actual house and equipment before diagnosing a fault or recommending a system.
- You Installed an Air-Source Heat Pump — Now What? U.S. Department of Energy Building Science Education
- Air-Source Heat Pumps ENERGY STAR
- HVAC Quality Installation ENERGY STAR
Checked against these sources on October 14, 2026.
Every HVAC answer in one place.
Heat pumps, backup heat, maintenance and choosing a system, written for mountain homes.