Understanding Auxiliary Heat in HVAC Systems

“Auxiliary heat” (sometimes shown as “Aux Heat”) means your heat pump’s backup heat source has kicked in automatically to help it keep up. It uses electric resistance heating — less efficient than the heat pump alone, but reliable regardless of how cold it gets outside. Seeing this on your display is normal, not a malfunction.

When It Comes On

• Cold weather — as outdoor temperatures drop, a heat pump extracts less heat from the outside air, so the system brings in auxiliary heat to make up the difference. This is the most common trigger.
• Defrost cycles — periodically, the system briefly reverses to melt frost off the outdoor coil. Auxiliary heat runs during this short window so indoor temperature doesn’t dip while the heat pump is busy defrosting.
• A fast temperature demand — raising the thermostat several degrees at once, or recovering quickly after it was set back, can trigger auxiliary heat to close the gap faster than the heat pump alone would.
• Poor insulation or heat loss — a drafty home loses heat fast enough that auxiliary heat engages more often than it would in a well-sealed one, even at the same outdoor temperature.

Auxiliary Heat vs. Emergency Heat

These get confused constantly, and mixing them up costs people money:

If your display says “Aux,” that’s the system working correctly. If you’ve manually switched to “EM Heat,” that’s a different thing entirely — see what EM heat means and when to actually use it.

Does It Cost More to Run?

Yes — electric resistance heat is less efficient per unit of energy than a heat pump, so your bill reflects more auxiliary heat use during a cold stretch. This isn’t a flaw; it’s the tradeoff for keeping your home warm when the heat pump alone can’t fully keep up. Occasional use during genuinely cold weather is expected and not worth worrying about.

It’s worth a closer look if:

• Auxiliary heat is running constantly, even in mild weather
• Your energy bill has jumped noticeably beyond what the weather would explain
• It’s engaging far more often than it used to for the same outdoor temperature

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If It’s Running More Than It Should

1. Check the thermostat setting — an unusually high target temperature, or a large jump after a setback, triggers auxiliary heat more than a steady, moderate setting would.
2. Replace the air filter — restricted airflow makes the heat pump work harder and lean on backup heat sooner.
3. Check for drafts and insulation gaps — heat loss forces the system to compensate constantly, even in moderate cold.
4. Have the heat pump itself inspected — if auxiliary heat is doing more of the work than it should, the heat pump may not be performing at full capacity (low refrigerant, a dirty outdoor coil, or another fault), and a technician can confirm it rather than you guessing.

Reducing Reliance on It

• Keep air filters clean and replace them every 60-90 days
• Address drafts and insulation gaps, which reduce how hard the system has to work overall
• Use a programmable or smart thermostat with gradual setbacks rather than large, sudden temperature jumps
• Schedule annual maintenance — a well-tuned heat pump needs less help from backup heat to begin with

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Also Read: Optimal Winter Thermostat Settings

Common Questions

Is auxiliary heat the same as emergency heat?

No. Auxiliary heat is automatic and runs alongside the heat pump; emergency heat is manually selected and replaces the heat pump entirely. See the comparison table above.

How do I know if aux heat is on?

Check the thermostat display — it typically shows “Aux Heat,” “Aux,” or a similar indicator when it’s active.

Is it bad if auxiliary heat comes on a lot?

Occasional use in cold weather is normal. Frequent use in mild weather, or a noticeable jump in your energy bill, is worth having the heat pump inspected rather than assuming it’s just the season.