If you’ve ever walked outside on a frosty morning and noticed your heat pump steaming like it’s on fire, don’t panic. That “smoke” is a normal part of your system’s defrost mode — and it’s actually saving your comfort and your system.
I’m Alex Lane, and I’ve walked more than a few homeowners through the icy mystery of how cold climate heat pumps function in sub-freezing weather. Today, we’re breaking down what defrost mode is, how it works, what’s normal, and when you should be concerned.
Let’s clear the frost on one of the most misunderstood features in modern HVAC.
Why Do Heat Pumps Freeze in the First Place?
Unlike furnaces that generate heat, air-source heat pumps work by transferring heat from the air outside into your home — even when it's cold. But here’s the catch: as the outdoor coil absorbs heat, moisture in the air condenses and can freeze on the coil surface.
That frost isn’t just cosmetic. If left unchecked, it can block airflow, lower efficiency, and even damage your system. That’s where defrost mode steps in.
📘 Why Heat Pumps Freeze Up – HVAC School
How Defrost Mode Actually Works
When the system detects frost buildup on the outdoor coil, it temporarily switches into reverse — literally. The heat pump flips to cooling mode, drawing heat from inside your home and sending it outside to melt the ice on the coil.
To avoid blowing cold air inside during this process, the indoor fan usually shuts off. The entire cycle lasts 5 to 15 minutes, depending on outdoor temperature and humidity.
Types of Defrost Control:
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Timed Defrost: Activates at regular intervals (less efficient)
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Demand Defrost: Uses sensors to activate only when needed
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Inverter-Based Control: Variable-speed compressors reduce frost formation and fine-tune the cycle
📘 Daikin’s Guide to Heat Pump Defrost Cycles (PDF)
What Does Defrost Mode Look and Sound Like?
If you’ve never seen your heat pump enter defrost mode before, it can be a bit alarming. Here’s what you might notice:
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Steam or vapor rising from the outdoor unit (this is not smoke)
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A clicking or hissing sound as the reversing valve activates
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The outdoor fan may stop or reverse direction
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A temporary pause in heating inside the home
These are all normal signs that your heat pump is defrosting properly.
📘 Carrier’s FAQ: Heat Pumps in Cold Weather
When to Worry: Signs of a Defrost Problem
While some frost is expected, excessive ice buildup or constantly running defrost cycles could point to an issue.
🚩 Warning signs to watch for:
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Ice that forms faster than it melts
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System enters defrost mode too frequently or not at all
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Grinding or buzzing noises from the outdoor unit
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Reduced indoor comfort or rising energy bills
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Fan continues running while coil is visibly iced over
These could indicate a failing defrost sensor, stuck reversing valve, low refrigerant, or poor airflow.
📘 Trane’s Guide to Heat Pump Freezing
How to Help Your Heat Pump Handle Ice Better
Your system does the heavy lifting, but you can help it out with a few key maintenance steps:
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Keep 12–24 inches of clearance around the outdoor unit
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Remove snow, leaves, and debris after storms
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Elevate the unit on a pad or wall bracket to prevent snow blockage
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Schedule winter maintenance — especially for systems 5+ years old
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Upgrade to units with inverter tech and demand defrost for smarter performance
📘 The U.S. Department of Energy recommends regular maintenance and proper airflow clearance to help heat pumps run efficiently in cold climates — especially during defrost mode.
How Modern Systems Handle Freezing Weather
Today’s cold climate heat pumps are specifically engineered for sub-freezing temps. Many models use inverter compressors and cold-weather refrigerants that allow efficient operation well below 0°F.
Want to understand how these systems are engineered to handle ice, snow, and extreme temps? Start with our full guide:
👉 Best Heat Pumps for Cold Climates in 2025
When Defrost Mode Isn’t Enough: Consider Backup Options
If you live in a region that regularly sees sub-zero temperatures, even a top-tier heat pump might need help during a deep freeze. That’s where backup heating comes into play.
From electric heat strips to dual-fuel systems that pair a heat pump with a gas furnace, there are options for keeping your home warm no matter the weather.
👉 Read next: Backup Heat Options: What to Pair with a Cold Climate Heat Pump
Final Thoughts from Alex Lane
Defrost mode isn’t a flaw — it’s a feature. It keeps your system efficient, protects critical components, and makes sure your home stays warm even when the temperatures drop.
The trick is knowing what’s normal, when to call for help, and how to support your system with good maintenance. If you’re noticing ice that just won’t melt or rising energy bills this winter, it might be time for a tune-up — or an upgrade.
Stay warm, stay smart — and let your heat pump do what it was built to do.
Alex Lane
Your Home Comfort Advocate
