Defrost cycles

Why it happens

In heating mode the outdoor coil is colder than the air. When that air is near or below freezing, moisture freezes on the coil. The frost blocks airflow and the coil stops picking up heat. NRCan describes the usual fix: the reversing valve switches the unit into cooling mode, so hot refrigerant goes to the outdoor coil and melts the frost, and the outdoor fan stops so the heat is not immediately blown away. NRCan

The cloud you see is that meltwater hitting cold air. It looks like the unit is boiling. It is not a refrigerant leak. A leak is not a periodic cloud on a damp cold day.

What you feel indoors

While the cycle runs, the heat pump is not delivering heat to the house. NRCan notes that the indoor fan may keep moving air, and that air can feel cool, which is why supplementary heat is part of a proper defrost strategy. Controls differ. Demand defrost starts when the unit detects frost. Timed defrost, which NRCan describes as roughly every 30 to 90 minutes of heating in frosting conditions, can run when it does not need to, and those extra cycles waste heat.

When it is not normal

• The coil ices into a solid block and stays that way through several attempted cycles.
• Ice builds because the unit is buried, the drain is a dam, or snow from the roof is landing on the fan.
• Defrost runs constantly in dry cold, or never runs in wet cold. Both are control or sensor problems, not weather.

Clear snow. Do not scrape fins. If the ice returns immediately, that is a technician, not another YouTube trick.