The short answer

• Supply vents may blow slightly cooler air for a few minutes.
• “AUX” appears on the thermostat while the system balances comfort.
• You might hear a quick whoosh as the heat pump switches modes.
• The outdoor fan pauses during defrost.
• Brief puffs of steam outside as frost melts.

Typical defrost cycles: about 3–10 minutes, more frequent on cold, humid days.

What’s normal in defrost season

• Short, cyclical events: the outdoor fan stops, the system reverses to melt frost, then flips back to heating. A whooshing sound is common.
• Slightly cooler supply air indoors: the unit focuses on clearing the outdoor coil. Electric backup heat may kick in to keep rooms comfortable.
• Visible outdoor “steam”: condensation fogs off the warmed coil during and after the cycle.
• More frequent defrost on damp days: North-central Alabama’s winter humidity makes frost form faster.

What’s not normal

• Continuous cold air beyond a few minutes or the home can’t recover temperature
• Thick ice after multiple cycles or a solid ice block around the outdoor unit
• Burning/electrical odors or new grinding/metal-on-metal noises
• Rapid repeat defrosting every few minutes all day
• Tripped breakers or “AUX” stuck on for hours

Safe homeowner checks before you call

• Thermostat sanity check: Heat mode, fan on Auto, setpoint above room temp.
• Filter: replace a dirty filter to restore airflow.
• Outdoor clearance: gently clear leaves/debris around the unit; don’t chip ice off the coil.
• Registers: make sure supply and return vents are open and unblocked.

If problems stick around, schedule professional service. Ethridge HVAC handles heat pump repair, service, and replacement across Greater Birmingham and nearby cities.

How a heat pump defrost works

1. Frost happens: in cool, humid weather, moisture freezes on the outdoor coil and insulates it, hurting efficiency.
2. System reverses: the heat pump briefly runs in cooling mode to warm the outdoor coil and melt frost. The outdoor fan often stops to speed the melt.
3. AUX heat offsets comfort: electric heat strips or backup heat temper indoor air during the cycle.
4. Back to heat: once clear, the unit switches right back to heating.

Birmingham-area tips for fewer defrost headaches

• Check the filter every 1–3 months in winter.
• Maintain 18–24 inches of clearance around the outdoor unit for airflow and drainage.
• Set it and forget it: avoid frequent mode changes; let the heat pump manage itself.
• Book maintenance before cold snaps. A tuned system defrosts faster and runs cheaper. See Ethridge’s preventative plans.

When to use Emergency Heat

Use Emergency Heat only if the outdoor unit is iced solid, won’t run, or is faulting. Emergency mode bypasses the heat pump and uses backup heat only, which costs more. Call us to diagnose why the outdoor unit is struggling.

FAQ

Why does my thermostat say “AUX”?

Auxiliary heat is helping during defrost or on very cold mornings. It shouldn’t stay on for hours.

Why is there “smoke” from the outdoor unit?

It’s steam. The coil warms to melt frost, and water vapor briefly fogs off the unit.

Why does the outdoor fan stop during defrost?

That’s intentional. Pausing the fan helps the coil heat up faster and shed ice.

Do I need a new system if defrosts are frequent?

Not necessarily. Humid weather in the 30s–40s can trigger more cycles. If comfort drops, bills spike, or icing worsens, schedule service.

Need help with a stubborn defrost or “cold air” issue?

Contact Ethridge HVAC for heat pump repair, service, or replacement throughout Greater Birmingham.

Related resources: Preventative HVAC Maintenance | Heating Services | Efficient Heat Pump Installation in Hoover, AL | Importance of Regular HVAC Maintenance