Heat Pump Defrost Mode: What's Normal (and What's Not)

If your heat pump periodically makes louder sounds, blows cooler air, or appears to "switch off" for 5-15 minutes, it's likely in defrost mode—which is completely normal. Here's everything Bay Area homeowners need to know.

What Is Defrost Mode?

When a heat pump heats your home, the outdoor coil gets COLD (20-40°F). When outdoor humidity is high (common in Bay Area coastal areas and foggy mornings), frost or ice forms on the outdoor coil.

Defrost mode: Heat pump temporarily reverses to cooling mode, sending hot refrigerant to outdoor coil to melt ice. Takes 5-15 minutes.

What You'll Notice (All Normal)

During Defrost Cycle

✅ Louder outdoor unit - Compressor works harder, fan may speed up
✅ Cooler air from vents - System in cooling mode temporarily
✅ Steam from outdoor unit - Melting ice creates visible vapor
✅ Indoor fan may stop - Prevents blowing cold air into house
✅ Thermostat shows "aux heat" or "defrost" indicator

How Long It Lasts

Normal: 5-15 minutes every 60-90 minutes (when outdoor temp 25-45°F + high humidity)
Bay Area typical: 1-3 defrost cycles per day during winter mornings

When Defrost Mode Activates

Temperature + Humidity conditions:

• Outdoor temp: 25-45°F
• Humidity: 60%+ (coastal fog, morning dew)
• Frost accumulation on outdoor coil

Bay Area frequency:

• Coastal homes (SF, Pacifica): Daily in winter due to fog
• Inland homes (Walnut Creek, San Jose): 2-3 times per week
• Rare above 45°F: Only in extremely humid conditions

How Defrost Mode Works (Technical)

1. Frost detected (sensor or timer-based)
2. System switches to cooling mode (reverses refrigerant flow)
3. Hot refrigerant sent to outdoor coil (melts ice)
4. Indoor fan stops (prevents cold air from entering house)
5. Optional auxiliary heat activates (maintains indoor comfort)
6. Ice melts in 5-15 minutes
7. System switches back to heating mode

What's Normal vs What's a Problem

✅ Normal Defrost Behavior

• Cycles every 60-90 minutes when needed
• Lasts 5-15 minutes
• Ice completely melts
• System resumes heating normally
• Happens primarily in 25-45°F range
• More frequent on foggy mornings

🚨 Call for Service If:

• Defrost cycles every 15-30 minutes (too frequent)
• Lasts 20+ minutes (too long)
• Ice doesn't fully melt
• System won't restart after defrost
• Ice builds up on outdoor coil continuously
• Defrost occurs when outdoor temp above 50°F
• System "stuck" in defrost mode

Bay Area Specific Considerations

Coastal Homes (High Humidity)

Expect: More frequent defrost cycles due to fog and ocean humidity
Normal frequency: 3-5 times per day in winter
Not a problem: As long as cycles complete properly

Inland Homes

Expect: Less frequent defrost cycles (drier air)
Normal frequency: 1-2 times per day on coldest mornings
Rare occurrence: Above 40°F outdoor temps

During Rain

Expect: More defrost cycles (higher humidity)
Normal: Rain itself doesn't trigger defrost, but increased humidity does

Auxiliary Heat During Defrost

Many heat pumps activate auxiliary heat (electric resistance heating) during defrost to maintain indoor temperature.

What you'll notice:

• Indoor temp maintains set point
• Slightly higher electric usage during defrost
• "Aux heat" or "emergency heat" light on thermostat

This is normal and automatic. Your heat pump manages this for you.

Bay Area advantage: Our mild climate means aux heat rarely runs long enough to significantly impact electric bills.

How to Minimize Defrost Cycles

You can't prevent defrost (it's necessary and automatic), but you can reduce frequency:

Keep Outdoor Unit Clear

• Remove leaves, debris blocking airflow
• Maintain 24" clearance around unit
• Trim vegetation that restricts airflow

Improve Drainage

• Ensure water drains away from unit
• Clear condensate drain
• Grade ground away from unit

Annual Maintenance

• Clean outdoor coil (removes dirt/debris)
• Check refrigerant charge (low charge = more frost)
• Verify defrost controls working properly

Defrost Mode Troubleshooting

Problem: Ice Won't Melt

Possible causes:

• Low refrigerant charge
• Defrost sensor malfunction
• Reversing valve stuck
• Blocked outdoor coil

Solution: Call for service—indicates system fault

Problem: Too Frequent Defrost

Possible causes:

• Defrost control faulty (triggering unnecessarily)
• Low refrigerant causing excess frost
• Dirty outdoor coil restricting airflow
• Outdoor fan not working properly

Solution: Schedule service for diagnosis

Problem: House Gets Too Cold During Defrost

Possible causes:

• Auxiliary heat not activating
• Defrost cycle too long
• System undersized for home

Solution: Check thermostat aux heat settings, call for service if persists

Defrost vs Emergency Heat

Defrost mode: Automatic, temporary, resolves itself in 5-15 min
Emergency heat: Manual override, you must turn off when done

Don't confuse them. If your thermostat shows "emergency heat" continuously, you may have accidentally engaged manual emergency heat mode.

The Bottom Line

Defrost mode is NORMAL and necessary for heat pump operation when:

• Outdoor temps 25-45°F
• High humidity (fog, rain, morning dew)
• Cycles last 5-15 minutes
• Ice melts completely
• System resumes heating afterward

Call Galaxy Heating & Air if:

• Defrost cycles too frequent (>10 per day)
• Ice won't melt or keeps returning immediately
• System stuck in defrost mode
• House temperature drops significantly during defrost

Bay Area homeowners: Expect more defrost cycles in coastal/foggy areas. This is normal behavior, not a malfunction.

Need heat pump service or have concerns about defrost cycles? Call (925) 578-3293 for professional diagnosis and service.