What Is a Heat Pump Defrost Cycle?
Summary: The heat pump defrost cycle, or defrost mode, occurs when cold, humid conditions cause icing or frost buildup on the heat pump’s outdoor coil. It’s a totally normal process that may cause a cloud of steam to rise from the outdoor unit.
By Anne Fonda in collaboration with Mark Woodruff, Senior Product Manager, Ducted Outdoor
Why does my heat pump have a defrost cycle?
An air source heat pump extracts heat from the air during the refrigeration cycle to either heat or cool your home. A heat pump system has two coils – an indoor coil and an outdoor coil. In the winter, the outdoor coil is typically 10-20°F colder than the outdoor air temperature.
Because it’s colder, the coil collects condensation like a glass of ice water or iced tea would. When the outdoor temperature is low enough, the condensation freezes. This can block airflow.
“Your heat pump will periodically run a defrost cycle to clear away any ice that may have formed on the outdoor coil. This keeps your system running at peak efficiency during winter.”
- Mark Woodruff, Senior Product Manager, Ducted Outdoor
The outdoor temperature DOES NOT need to be freezing – remember, the coil is typically 10-20°F colder than the ambient air temperature. So, your heat pump could easily enter the defrost cycle when it’s in the low 40s outside. However, it’s more common when the temperature is in the 20s or 30s.
What happens during the heat pump defrost cycle?
To melt the ice or frost during the defrost cycle, the reversing valve reverses the flow of refrigerant to heat up the outdoor coil, essentially putting the heat pump into the cooling cycle for a few minutes. After the ice is melted, the heat pump reenters an energy-efficient heat mode.
What does the heat pump defrost cycle feel like?
You may not even know that your heat pump is in defrost mode, Woodruff explains. “The system will still run during defrost mode, but the homeowner likely will never know anything different is happening. Backup heat comes on (assuming the homeowner has backup electric heat strips, and most do), and so the homeowner's experience is just “hey, my system is heating like normal”. That fits 99% of all homeowners.
There is a small 1% of folks who may not have backup heat (either because they chose not to at the time of purchase, or they have a dual fuel system and we do not fire the furnace + heat pump together), and those folks may notice cool air coming from their air vents instead of warm air, but again, that is perfectly normal.”
What does heat pump defrost mode look like?
“As your heat pump is defrosting, you may notice a cloud of steam coming from your outdoor unit, which is the ice melting and evaporating, and is perfectly normal.”
- Mark Woodruff, Senior Product Manager Ducted Outdoor
Of note, you may see AUX or AUX heat on your thermostat when the heat pump is in defrost mode. This tells you the electric heat strips are keeping your home warm during the defrost cycle.
What does the defrost mode sound like?
If you’re inside your home, you may not hear anything. However, if you are outside near the heat pump, you may hear a swooshing sound as the refrigerant reverses course. The outdoor fan may stop. The noise level of the compressor may rise or fall during defrost mode. Again, this is all completely normal.
Trane’s innovative approach to the heat pump defrost cycle
“Trane uses sensors, so we only defrost when necessary - this is called demand defrost. We end the cycle as soon as any ice is cleared, which means we run fewer defrost cycles. This also makes our cycles shorter (usually 3-5 minutes), so our systems are more friendly for the homeowner and more efficient.
Many of our competitors’ heat pumps defrost based on timing. Timed defrost means their heat pumps defrost after so many run hours, whether the system needs it or not. And those defrost cycles last for a set period of time, no matter what.”
- Mark Woodruff, Senior Product Manager Ducted Outdoor
The bottom line
The bottom line is the heat pump defrost cycle is part of normal cold-weather operation. The first time you see a cloud of steam rising from your heat pump, it can be alarming, but if you know what to expect, you won’t panic. Your heat pump will reenter heating mode in a few minutes.
When to call for HVAC service
If your heat pump seems to be constantly switching into defrost mode or the defrost cycles last longer and longer with frost and ice starting to cover the unit, call your local HVAC dealer for help.
Anne Fonda
Content Writer, Trane Technologies
A Content Writer with Trane Technologies, Anne Fonda researches topics and writes for Trane® and associated residential HVAC brands. She works in collaboration with Trane Technologies subject matter experts, offering easy-to-understand, informative content on complex topics. Her goal is to help consumers make informed decisions on the products and services they need.
She has written for HVAC and other service provider websites for over 16 years. Before transitioning to web content writing, Anne had a 14-year stint as an award-winning journalist. She graduated cum laude from the University of Missouri-Columbia School of Journalism.
When she’s not working, Anne enjoys playing word games, reading, gardening, spending time with family, and visiting gardens and museums.
Expert review by Mark Woodruff, Senior Product Manager, Ducted Outdoor
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