Which symptom most reliably indicates an undercharged refrigerant system?

• A. Frost on the suction line
• B. Normal operation
• C. Frost on the discharge line
• D. High suction pressure

Answer: (A)

Frost on the suction line signals an undercharged refrigerant system because when there isn’t enough refrigerant, the evaporator pressure and temperature drop. The suction line carries low-pressure gas returning to the compressor, and that line becomes cold enough to freeze moisture from the surrounding air, forming frost. This frost directly reflects the evaporator operating at a lower temperature due to insufficient refrigerant flow, making it the most reliable indicator of undercharge.

Normal operation isn’t a sign of undercharge, and frost on the discharge line isn’t typical for a low charge—it points to other issues affecting the compressor or hot-side components. High suction pressure suggests something other than undercharge, such as overcharge, restricted refrigerant flow elsewhere, or heat load conditions.