Understanding the basics will help your service team
Let’s start with the basics.
Water freezes at 32° F at sea level and atmospheric pressure. When any surface is below that temperature, and the air around it contains moisture, ice/frost will begin to form. In some situations, ice is to be expected, such as in refrigeration evaporators and exposed portions of the refrigeration suction line and the metering device outlet in refrigeration applications. Frost and freezing are also likely in heat pumps operating in heat mode. The outdoor coil on a heat pump becomes the evaporator coil, and during low outdoor ambient conditions, it is expected that the outdoor coil will eventually freeze and require defrost.