Heat Pump Defrost Cycle
If you've ever seen your outdoor heat pump steaming, hissing, or temporarily blowing cool air in winter, that's the defrost cycle — and it's almost always normal. Here's how it works, how long it should last, and when it's actually a problem.
What the Defrost Cycle Does
When a heat pump is in heating mode, the outdoor coil pulls heat out of the outside air. As it does, moisture in the air condenses on the coil and — in cold enough weather — freezes into a layer of frost. Frost insulates the coil, blocking heat transfer and tanking efficiency.
The defrost cycle is the heat pump's automatic solution. The reversing valve flips the system into cooling mode for a few minutes, sending hot refrigerant through the outdoor coil to melt the frost. The outdoor fan stops so it doesn't blow cold air across the freshly warmed coil. You'll usually see steam rising and hear a soft 'whoosh' as the system reverses.
Some heat pumps run electric backup heat (also called auxiliary or emergency heat) during defrost so the indoor air stays warm. Without backup, you'll feel the supply air cool down briefly until defrost completes.
What a Normal Defrost Cycle Looks Like
If your heat pump does all of these in sequence, it's working correctly.
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Triggers on cold + humid days
Most common when outdoor temperature is between 25°F and 40°F with high humidity. Above ~45°F, frost rarely forms.
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Outdoor fan stops
The fan stays off so it doesn't waste heat. This is normal — not a fan failure.
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Steam rises from the outdoor unit
That's water vapor from the melting frost. Looks dramatic but it's just steam.
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Lasts 5–15 minutes
Most cycles complete in under 10 minutes. Anything over 15 minutes suggests a problem.
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Indoor air briefly feels cool
If your system doesn't run aux heat during defrost. The supply vents may blow 60–65°F air for a few minutes.
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Happens every 30–90 minutes in cold weather
Frequency depends on outdoor temp and humidity. Once per heating cycle is typical.
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System returns to heating automatically
When the coil temperature sensor confirms the frost is gone, the reversing valve flips back and warm air resumes.
Signs of a Defrost Problem
Any of the following means it's time to call us. In Coastal Georgia we don't get extreme cold often, but when these symptoms show up they tend to worsen quickly.
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Defrost lasts more than 15 minutes
Indicates a stuck reversing valve, low refrigerant, or a failing defrost board.
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Outdoor unit covered in solid ice
Normal frost is thin and white. Solid ice means defrost isn't completing — usually a sensor or control board issue.
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Defrost cycle every 10–15 minutes
Defrost should run when needed, not constantly. Frequent cycling wastes energy and points to refrigerant or control problems.
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System never defrosts even with visible frost
Defrost initiator failure. Frost will keep building until the system overheats or trips out.
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Loud bang, hissing, or sustained whooshing
Brief sounds are normal; sustained noises suggest a refrigerant or valve problem.
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Indoor air stays cold for a long time
If aux heat isn't engaging during defrost, you'll feel a sustained cold draft. Heat strips may have failed.
Coastal Georgia Notes
Coastal Georgia winters are mild — we typically see only 10–30 days a year cold enough to trigger frequent defrost. That's good news: defrost wear is one of the most common causes of heat pump component failure in colder climates, and we mostly avoid it here.
What we DO see locally is salt-air corrosion on the defrost sensor (a small thermistor clipped to the outdoor coil). When this corrodes, the system may either over-defrost or fail to defrost at all. Annual maintenance catches this before it leaves you without heat.
Heat Pump Acting Strange in Cold Weather?
Most defrost behavior is normal — but if something feels off, our techs can diagnose it in one visit. Same-day service available across Coastal Georgia.
Heat Pump Defrost FAQs
How long should a heat pump defrost cycle last?
Typically 5–15 minutes. If your system spends more than 15 minutes in defrost mode, or defrosts more often than every 30 minutes, call a technician.
Why is my heat pump steaming?
That's the defrost cycle melting frost off the outdoor coil. The steam is water vapor — totally normal.
Why does my heat pump blow cold air during defrost?
During defrost, the system temporarily reverses to cooling mode to melt frost outside. If your system doesn't have auxiliary heat strips engaged during defrost, the indoor air feels cool for a few minutes. This is normal.
How often should a heat pump go into defrost mode?
It depends on weather. In cold, humid conditions, every 30–90 minutes is normal. In milder Coastal Georgia weather, defrost may not happen at all during a given heating cycle.
Can I run my heat pump without defrost?
No. Disabling defrost will allow ice to build up, dramatically reduce efficiency, and eventually damage the compressor. If defrost is misbehaving, the right fix is to repair it.
Why is my heat pump covered in ice?
Thin frost is normal in cold weather. Solid ice covering large portions of the outdoor unit means defrost isn't completing — usually a sensor or reversing-valve issue requiring service.