A low refrigerant charge in a system with an AXV metering device will result in which condition?

• A. Low suction pressure
• B. High superheat
• C. Low discharge temperature
• D. Cool ambient conditions

Answer: (B)

When an AXV (automatic expansion valve) is regulating, it uses the amount of liquid refrigerant entering the evaporator to control the superheat. If the system is undercharged, there isn’t enough liquid to fully flood the evaporator. The evaporator becomes starved of liquid, so most of what vaporizes is quickly drawn off and then heated as it continues to absorb heat in the coil and piping. By the time the refrigerant vapor reaches the compressor suction, it is significantly warmer than its saturation temperature for the evaporating pressure—that is, the superheat is high. This high superheat is a direct consequence of insufficient liquid absorption in the evaporator, which is what happens with a low charge in an AXV-regulated system. While suction pressure may also drop with low charge, the notable indicator described here is the high superheat. Discharge temperature and ambient conditions aren’t the primary signs of undercharge in this setup.