When an AXV is used as a metering device, a low refrigerant charge will cause...

• A. Low head pressure
• B. High superheat
• C. Low suction pressure
• D. Short cycling

Answer: (B)

When a metering device like an AXV is used, it tries to keep a desired amount of liquid refrigerant entering the evaporator to maintain a specific superheat at the suction exit. If the system is undercharged, there isn’t enough refrigerant circulating to flood the evaporator. The AXV will still respond to suction temperature to control flow, but with limited liquid available, the evaporator can’t absorb heat efficiently. As a result, much of what vaporizes leaves the evaporator at a higher temperature, producing a higher superheat reading on the suction line. This higher superheat indicates the evaporator is not being adequately cooled by liquid refrigerant because of the low charge.

Note that low head pressure or short cycling aren’t the direct, defining signs of a low-charge condition with an AXV. While suction pressure can drop with undercharging, the characteristic sign tied to a metering device in this context is the rise in superheat.