When “AUX” lights up for auxiliary heat, don’t panic. It’s your heat pump asking for a brief assist—not sounding an alarm. Emergency heat sits beside it on the thermostat, but it’s a different tool entirely. AUX is automatic support, while emergency heat is a manual backup. At Pointer Cooling and Heating, we walk homeowners through this difference every winter. Let’s set expectations now to keep comfort high—and the bill in check.
Auxiliary Heat: What It Is and Why You See It
When your thermostat calls for auxiliary heat, the system briefly runs electric heat strips to help the heat pump reach the set temperature. AUX should kick on briefly, then drop out once the set point is nearly reached. You’ll notice it during deep cold snaps or when recovering from a big setback (like warming the house several degrees at once). That behavior is normal. But if AUX hangs around for long stretches, something is limiting airflow or capacity. We start with the basics: airflow, thermostat settings, and outdoor coil condition. For heat-pump basics, see the DOE Energy Saver heat pump guide.
When Auxiliary Heat Should (and Shouldn’t) Turn On
Auxiliary heat should appear during rapid warm-ups, sub-freezing weather, or while the system finishes a defrost. It shouldn’t run for hours. Constant AUX usually points to restricted airflow, a dirty coil, bad staging, undersized equipment, or leaky ductwork. If AUX shows up outside those moments—or lingers—start with a quick field check below.
Quick Field Check You Can Do
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Look for closed or blocked supply vents.
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Inspect the return grille; it should be clean and unobstructed.
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Verify the filter size and orientation (arrow toward the blower).
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Note when AUX appears: only on cold mornings and during cold snaps, or all day?
Emergency Heat: The Manual Backup, Explained
Emergency heat is the “big hammer.” You switch it on when the outdoor unit shouldn’t run—like a failed fan, heavy icing beyond normal defrost, or a known mechanical issue. It relies on electric resistance heat, which is effective but expensive. Use it sparingly. If you’re unsure, call Pointer Cooling and Heating before you flip that switch; a five-minute conversation can save a day of high-rate heat. (Your wallet will thank you—so will your breaker panel.)
For a deeper dive, read our blog: What Is EM Heat and When Should You Use It?
Auxiliary Heat vs Emergency Heat: Quick Differences
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Who triggers it? Auxiliary heat is automatic; emergency heat is manual.
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What does it replace? AUX supplements the heat pump; emergency heat bypasses it.
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Cost profile: Both can be costly, but emergency heat is usually the highest draw.
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When to see it: AUX during cold snaps or big setbacks; emergency heat during faults.
Quick Do/Don’ts (Tech-Approved)
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Do allow auxiliary heat briefly in very cold weather or big morning warm-ups.
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Don’t leave emergency heat on unless the outdoor unit shouldn’t run.
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Do keep set-backs modest in winter (about 2–3°F).
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Don’t ignore constant AUX—check airflow, filter, and coil.
Red Flags That Need a Call
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AUX shows for hours, not minutes.
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Outdoor unit iced over beyond normal defrost.
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Breakers trip when the thermostat calls for heat.
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Supply air feels cool even with AUX on.
Troubleshooting: If AUX Stays On Too Long
If auxiliary heat keeps running continuously, it’s time for professional diagnostics. When you schedule a heat pump diagnostic, here’s what we’ll do:
Technician Diagnostics We Perform
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Airflow testing: Measure static pressure, verify blower speed, and confirm clear return/supply paths.
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Outdoor performance: Clean the coil, check the fan, confirm defrost timing, and verify refrigerant charge.
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Controls and staging: Check thermostat programming, heat-strip staging, and sensor calibration.
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Duct evaluation: Test for leakage or restrictions; poor duct design can force AUX to do the heavy lifting.
We’ll pinpoint the bottleneck and get AUX back to short, normal assists.
When It’s Not the Equipment
Sometimes the issue is routine, not equipment. Large nightly setbacks, closed interior doors, and blocked returns can stretch heat-up times. We’ll help you adjust habits so the heat pump does the work—and auxiliary heat only steps in briefly.
Smart Settings to Reduce Auxiliary Heat Run Time
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Use modest winter setbacks (about 2–3°F).
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Enable “adaptive recovery” on compatible thermostats.
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Schedule gradual warm-ups before you wake.
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Keep the outdoor unit clear of debris and snow.
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Replace filters every 1–3 months, depending on conditions.
For practical, room-by-room efficiency tips that help cut AUX time, check ENERGY STAR — Energy Savings at Home.
If you’re unsure about settings, we can help you dial them in for fewer AUX runs.
Defrost Cycles, Steam, and Auxiliary Heat
Seeing steam from the outdoor unit on cold mornings? That’s defrost. It’s normal. The system may use auxiliary heat briefly during defrost to keep supply air comfortable. Odd grinding or repeated hard restarts are not normal. If you hear anything alarming, shut it down and give us a call. We’d rather catch a small problem than meet a burned-out contactor later. If you’re planning around cold snaps, the National Weather Service winter safety pages cover storms, freezing temps, and safe operation.
FAQ: Auxiliary Heat & Emergency Heat
Auxiliary heat is an automatic assist your heat pump uses when it can’t reach the set temperature fast enough—like during deep cold or big set-back recoveries. It briefly adds electric heat strips, then turns off once the set point is met.
Yes—auxiliary heat is normal during cold snaps, morning warm-ups, or defrost cycles. It’s a problem if AUX stays on for hours. Start with airflow (clean/replace the filter, open supply vents, clear the return). If AUX persists, schedule a diagnostic with Pointer Cooling and Heating.
Use emergency heat only when the outdoor unit shouldn’t run—examples: a failed fan motor, heavy ice beyond normal defrost, or known mechanical issues. Emergency heat relies on electric resistance heat and costs more to operate, so it’s a manual backup for true faults, not everyday heating. What Is Auxiliary Heat on a Thermostat?
Is Auxiliary Heat Normal, and When Is It a Problem?
When Should I Use Emergency Heat Instead of Auxiliary Heat?
Maintenance & Upgrades: Keep AUX in Its Lane
Seasonal maintenance keeps auxiliary heat in support mode, not star mode. Our tune-ups include coil cleaning, sensor checks, staging verification, and duct evaluations. If your system is near the end of its service life or undersized, request a quote for heat pump replacement. At Pointer Cooling and Heating, we lay out clear options with real numbers—no mystery pricing, no pressure.
Contact Pointer Cooling and Heating
Have questions about your settings, a spike in bills, or an AUX light that won’t quit? Contact us at Pointer Cooling and Heating. We’ll walk you through quick checks, schedule a diagnostic if needed, and get your system running the way it should. We proudly serve our neighbors in Moncks Corner, SC, and nearby communities—but our guidance applies anywhere that winter bites. Most importantly, we’ll help you decide when auxiliary heat is normal and when it’s not.
