What Is a Heat Pump Defrost Cycle?
Key takeaways:
• A heat pump defrost cycle, or defrost mode, is necessary when cold, humid conditions cause a buildup of frost or ice on a heat pump’s outdoor coil.
• It’s a normal process that may cause a cloud of steam to rise from the outdoor unit.
• American Standard heat pumps are equipped with demand defrost technology for comfort and efficiency.
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. Your heat pump system has two coils – an indoor coil and an outdoor coil. In the late fall and winter, the outdoor coil is generally 10-20°F colder than the outdoor air temperature.
Because it’s colder than the surrounding air, condensation forms, just like it does on a cold beverage can. When the outdoor temperature is low enough, the condensation on the coil freezes. This can block airflow.
“Your heat pump will periodically run a defrost cycle to melt 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 for the heat pump to enter the defrost cycle. Remember, the coil is usually 10-20°F colder than the ambient air temperature. This means your heat pump could easily enter defrost mode when the outdoor temperature is in the low 40s. However, it’s more common when the temperature is in the 20s or 30s.
How does the heat pump defrost cycle work?
To melt the ice or frost on the outdoor coil, the heat pump’s reversing valve reverses the flow of refrigerant to heat up the outdoor coil, essentially putting the heat pump into cooling mode for a few minutes. After the ice melts, the heat pump reenters an energy-efficient heat mode.
How do I know if the heat pump defrost cycle is active?
You may not even know that your heat pump has entered the defrost cycle, 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 covers the experience of 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 for a few minutes, but again, that is perfectly normal.”
What does the 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 caused by the ice melting and evaporating, and is perfectly normal.”
- Mark Woodruff, Senior Product Manager Ducted Outdoor
You have to be outside or looking at your heat pump from inside to notice this. Also, you may see AUX or AUX heat on your thermostat screen when the heat pump is in defrost mode. This tells you that the electric heat strips are keeping your home warm during the defrost cycle.
What does the heat pump defrost cycle sound like?
If you’re inside your home, you probably won’t hear anything different. However, if you are outside near the heat pump, you may hear a swooshing sound as the refrigerant reverses direction. The outdoor fan may stop. The noise level of the compressor may increase or decrease during the defrost cycle. Again, this is all completely normal and necessary.
American Standard’s innovative approach to the heat pump defrost cycle
“American Standard heat pumps use sensors, so we only defrost when necessary - this is called demand defrost. The cycle ends as soon as any ice is cleared, which means 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 a schedule. 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, no matter what.”
- Mark Woodruff, Senior Product Manager Ducted Outdoor
The bottom line
The bottom line is that the heat pump defrost cycle is part of normal cold-weather operation. That’s not smoke coming from your heat pump. It’s a cloud of steam rising during the defrost cycle. It can be alarming the first time you see it, 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 your HVAC service provider
If your heat pump seems to be constantly switching into defrost mode or the defrost cycles last longer and longer, and ice and frost start covering the unit, call your local American Standard dealer for help.
Anne Fonda, Content Writer
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
