What is a common sign that refrigerant piping may need insulation?

Visible frost on refrigerant piping is a common sign that insulation may be needed. When refrigerant lines are not properly insulated, they can become cold enough to cause condensation and, ultimately, frost to form on the outside of the piping. This occurs because the temperature of the refrigerant inside the pipe is significantly lower than the ambient temperature, leading to moisture in the air freezing on the surface of the pipe.

Proper insulation helps to maintain the refrigerant's temperature, prevents energy loss, and reduces the risk of moisture entering the system, which can lead to other issues such as corrosion or reduced efficiency. If frost is visible, it is a clear indication that the refrigerant is not being adequately protected from external temperature changes, highlighting the need for insulation.

The other choices indicate different issues or conditions unrelated to insulation. Lack of pressure readings could suggest a different problem such as a leak or a failure in the system that doesn't directly relate to the need for insulation. Unusual noises from the compressor might point to mechanical issues or strain on the system rather than insulation needs. Heating of the refrigerant would typically suggest that the system is functioning under load or encountering a different operational problem, rather than indicating a need for insulation.