Heat Pump Aux Heat in Alabama: Normal vs Problem

If your heat pump's thermostat suddenly reads "Aux Heat" or "Emergency Heat" and you didn't put it there, your first reaction is usually one of two things: panic (is this broken?) or confusion (was it always doing this?). Both reactions are reasonable. Auxiliary heat is one of the most misunderstood features on Alabama heat pumps, and most homeowners only learn about it when their electric bill jumps $80 in a single month.

Auxiliary heat is normal in some conditions and a sign of a system problem in others. Knowing the difference saves you money and helps you catch real issues before they cascade.

By Chad Wiswall, Owner & Lead HVAC Technician, Alabama HVAC License #92244 | Learn more about Chad

This guide is the Alabama-specific complement to our complete Alabama HVAC homeowner's guide. Let's break down when aux heat is doing its job and when it's wasting your money.

What Is Auxiliary Heat and Why Does Your Heat Pump Have It

A heat pump moves heat from outside air into your home. That sounds backwards in winter, but even at 30°F there's heat energy in the air the system can extract. The colder it gets outside, the less heat is available to extract, and the heat pump becomes less efficient.

Auxiliary heat (also called secondary heat, strip heat, or emergency heat depending on your thermostat) is a backup heating source that kicks in when the heat pump alone can't keep up. In most Alabama heat pump installations, aux heat means electric resistance heating elements installed in the air handler.

Electric resistance heat is roughly 3 times more expensive to run than a heat pump in compression mode. So every minute aux heat is running, your bill is rising faster than necessary. The goal is to use aux heat only when you actually need it.

When Aux Heat Is NORMAL (Don't Panic)

Scenario 1: Outdoor Temperature Below 25 to 30°F

This is the classic case. As outdoor temperatures drop into the 20s, your heat pump's capacity to extract heat from outdoor air drops sharply. The system can't satisfy your thermostat setpoint with compression heat alone, so aux heat engages to make up the difference.

In Alabama, we see this most often on those few really cold January nights when temperatures dip into the upper teens or low 20s. Your system running aux heat during those nights is exactly what it's designed to do.

Scenario 2: Defrost Cycle

When outdoor temperatures hover in the 30s with high humidity, frost builds up on the outdoor coil. Your system periodically reverses (sending hot refrigerant outdoors) to melt that frost. During defrost (which lasts 5 to 15 minutes), the indoor system would blow cold air, so aux heat fires up to keep your house warm.

If you walk outside and see steam rising off the outdoor unit while aux heat is running on your thermostat, that's a defrost cycle. Completely normal. Happens every 30 to 90 minutes depending on conditions.

Scenario 3: Big Setback Recovery

If you set your thermostat back to 62°F overnight and program it to 70°F at 6 AM, the system needs to raise indoor temperature 8°F quickly. The thermostat may engage aux heat to speed that recovery. This is called "smart recovery" or "intelligent recovery" on most modern thermostats.

Setting your thermostat back more than 4°F at a time in winter often triggers aux heat for the recovery, which can offset the savings from the setback. Some homeowners actually use less energy keeping the heat pump at a constant temperature than they would using deep setbacks that force aux heat.

Scenario 4: First Few Days of Heating Season

If you turn your heat pump on for the first time after a long cooling season, the system runs through self-test cycles. Some thermostats display "Aux Heat" briefly during this verification. This usually clears within 24 hours of normal operation.

When Aux Heat Is a PROBLEM (Real Money Being Wasted)

Problem 1: Aux Heat Running Above 35 to 40°F Outdoor Temperature

Heat pumps are designed to run efficiently down to about 25 to 30°F outdoor temperature. If aux heat is engaging when it's 45°F or 50°F outside, something is wrong. Possible causes:

Low refrigerant charge in the heat pump (system not producing full capacity)
Failed compressor or compressor on time delay protection
Reversing valve stuck or failing (system stuck in cooling mode)
Defrost board failure (incorrectly forcing aux heat)
Outdoor temperature sensor failed (system thinks it's colder than it is)
Thermostat misconfigured (set point too high for outdoor conditions)

Each of these costs money to repair, but ignoring them costs more in inflated electric bills.

Problem 2: Aux Heat Always On, Compressor Never Runs

If you hear the indoor blower running and feel warm air, but the outdoor unit is silent (no fan spinning, no compressor humming), your system is running on aux heat only. The heat pump portion has failed.

This is sometimes triggered manually when a homeowner sees an issue and switches the thermostat to "Emergency Heat" mode and forgets to switch it back. Check your thermostat for an "EM Heat" mode and switch it back to "Heat" if applicable.

If you're already in "Heat" mode and the outdoor unit isn't running, you need a service call. Common causes: failed compressor capacitor ($150-$350 fix), tripped breaker on outdoor disconnect (free fix once identified), failed contactor ($200-$400), failed compressor ($1,800-$3,500), or refrigerant leak.

Problem 3: Aux Heat Cycling On and Off Repeatedly

If you watch your thermostat and see aux heat indicator flicker on and off every few minutes, your thermostat staging is misconfigured or your heat pump is borderline-undersized for your home. A heat pump tech can adjust thermostat staging parameters to delay aux heat engagement.

How to Verify Aux Heat Status on Common Thermostats

Honeywell T6 / T9 / T10

Aux heat shows as a small "Aux Heat" indicator near the temperature reading. To check current status, swipe down to the main system display.

Nest Learning Thermostat

Look for a red flame icon plus an "AUX" label in the lower left corner. The Nest app shows "Heating with auxiliary heat" in the activity log. You can also see the percentage of heating time that used aux heat in the monthly Energy Report.

Ecobee SmartThermostat

The home screen shows "Aux Heat 1" or "Aux Heat 2" under the current temperature when aux heat is engaged. The HomeIQ feature in the Ecobee app tracks aux heat runtime per day.

Trane / American Standard ComfortLink

Aux heat appears as "AUX" in the upper right corner. Some models show a yellow LED indicator.

Energy Cost Impact: What Aux Heat Costs You Per Hour

Most Alabama heat pumps have 10kW or 15kW electric resistance heat elements (some have 20kW). Here's the per-hour cost at Alabama Power's typical residential rate (around $0.14/kWh in 2026, varies by tier):

10kW aux heat strips: ~$1.40/hour ($33.60/day if running continuously)
15kW aux heat strips: ~$2.10/hour ($50.40/day if running continuously)
20kW aux heat strips: ~$2.80/hour ($67.20/day if running continuously)

Compare that to compression-mode heat pump operation, which typically draws 2-4kW depending on size, so $0.28 to $0.56/hour. Aux heat is roughly 3 to 5 times more expensive to run.

If you're seeing a $150 winter electric bill jump to $300 in January, aux heat running excessively is almost certainly part of the cause. Pair this with our guide on cost to run HVAC year-round in Alabama for the full picture.

When to Call Us About Aux Heat

Call us for diagnostics if:

Aux heat runs above 40°F outdoor temperature on a regular basis
Your electric bill has spiked $80+ in a month with no other changes
Your outdoor unit isn't running at all but the system is producing heat
Your heat pump is over 10 years old and aux heat usage has increased gradually
You hear unusual noises from the outdoor unit during cold weather
The thermostat shows aux heat indicator constantly, even after warming up

A heat pump diagnostic runs $89 to $150 in our service area. If we identify the cause is a simple fix (low refrigerant, bad capacitor, sensor issue), we can typically resolve it same-day.

Frequently Asked Questions

Should I switch to Emergency Heat manually to "help" my heat pump in cold weather?

Almost never. Emergency Heat mode disables the heat pump compressor and runs only on resistance strips, which is much more expensive than letting the heat pump and aux heat work together as designed. Only use Emergency Heat if the heat pump compressor has failed and you can't get a tech same-day.

Is "Aux Heat" and "Emergency Heat" the same thing?

Same heating elements, different control logic. Aux heat is automatic supplemental heating that engages when the heat pump needs help. Emergency Heat is manual, full-time resistance heating with the heat pump disabled. Aux heat is what you want most of the time.

How cold does it have to get before aux heat is normal in Alabama?

For most modern heat pumps installed in Alabama, aux heat engaging below 25-30°F outdoor temperature is normal. Older single-stage heat pumps may engage aux heat below 35°F. Premium variable-capacity systems may avoid aux heat down to 15°F or lower.

Will a new heat pump reduce my aux heat usage?

Often dramatically. Newer multi-stage and variable-capacity heat pumps from Goodman, Trane, Bryant, and Mitsubishi maintain heating capacity at lower outdoor temperatures than 15-year-old single-stage systems. If you're replacing for other reasons, expect 30-60% less aux heat runtime.

Can I disable aux heat to save money?

Don't. Without aux heat backup, your home will fail to maintain setpoint during defrost cycles or cold snaps. The "savings" would come at the cost of comfort, frozen pipes, and a system trying to run beyond its capabilities. Better to fix the underlying issue forcing excess aux heat use.

Aux Heat Stuck On? Bill Way Higher Than Normal? Let's Diagnose It.

Chad's AC Direct services heat pumps across Montgomery, Auburn, Prattville, Wetumpka, Dadeville, Lake Martin, and 11 other Alabama cities. We carry diagnostic tools that pinpoint heat pump problems in under an hour and most repairs are same-day.

Call (334) 264-6464 for Montgomery or (334) 478-1438 for Dadeville and Lake Martin areas. No commission-based upselling, no scare tactics, just an honest diagnostic and a written quote. Schedule online through our contact page if that works better.

Heat Pump Auxiliary Heat: When It's Normal, When It's a Problem (Alabama Homes)