Automatic defrost
In cold, humid weather an air-source heat pump collects frost on its outdoor coil. Clearing it from time to time is normal operation, not a fault.
Why frost forms
To take heat from the outdoor air, the outdoor coil runs colder than the air. When the coil surface is below 0 °C and the air holds moisture, that moisture freezes onto the fins. Frosting is heaviest at roughly −5 to +5 °C with high humidity, not in the coldest dry weather. As it builds up it blocks airflow and insulates the coil, and output falls.
What defrost does
Every RV model defrosts automatically. The controller detects that frost is limiting performance, briefly warms the outdoor coil to melt it, and then returns to heating. In many air-to-water units the heat for this comes from the heating water, reversing the refrigerant cycle for a short time. While it happens, the fan usually stops, and steam may rise from the unit. Both are normal.
What the system needs
- Enough water volume. Defrost draws energy from the system water. Each model's minimum system water volume, on its product page, must be available during defrost, with a buffer tank if needed.
- Drainage. The meltwater has to drain freely away from the unit without refreezing on a path or under the unit.
- Airflow. Keep the coil clear of snow drifts and leave the clearances the installation manual specifies.
Defrost is not anti-freeze protection
Defrost clears ice from the outdoor air coil. Anti-freeze protection is a different function: it stops the water inside the unit and outdoor pipes from freezing when the heat pump is idle in frost, for example by running the circulation pump or using a heater or glycol. A system needs both, and they are configured separately.
Related: Cold-climate operation · RV series
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