Frost on the suction line indicates:

• A. That its temperature is above the dew point
• B. That its temperature is below the dew point and below 32°F
• C. That the refrigerant is fully charged
• D. That the compressor is overheating

Answer: (B)

Frost on the suction line happens because the pipe becomes very cold and moisture from the surrounding air condenses on it. The dew point is the temperature at which air becomes saturated with moisture; when the suction line’s surface falls below that point, water vapor condenses on the pipe. If the pipe is also below freezing (32°F), that condensed water freezes, forming frost. So frost indicates the suction line is colder than both the dew point and freezing, not conditions like a hot compressor or being fully charged.