How to Diognose Restricted Metering Device

Restrictions in the metering device will prevent the evaporator coil from receiving the proper amount of refrigerant.  The restriction will cause the suction pressure to be too low.  With low suction pressure present, the saturation temperature of the refrigerant in the evaporator is very low.  Because of low saturation temperature, freezing of the evaporator surface is likely.

Causes

  • Undersized fixed metering device
  • Failed expansion valve
  • Debris in metering device
  • Moisture freezing at metering device

Symptoms

Symptoms running with a metering restriction will have low suction pressure along with high superheat.  The liquid pressure will initially be low which will make the system appear undercharged.  When refrigerant is added to the system, the liquid pressure will rise but the suction pressure will remain low.  At this point you should be able to identify there is a problem in the liquid circuit.  If subcooling is measured, at this time, it will be high.  The high subcooling level is an indication that excess refrigerant is being stored in the condenser coil.

There will be no obvious sign of frosting between the consenser coil and the metering device.

Procedure

  • If the liquid and suction pressure are low, and no obvious sweating or frosting of the liquid line is present, add refrigerant.  If the system is undercharged, both pressures will begin to rise.  If the liquid pressure rises significantly, but the suction pressure remained low, the liquid circuit has a restriction.
  • Check the temperature of the liquid line at the outlet of the condenser coil and at the inlet to the metering device.  If you detect a large temperature drop, there is a pressure drop between the two test points.  Isolate the restriction and make the repair.
  • If there is no temperature drop between the outlet of the condenser and the inlet of the metering device, yet the liquid pressure is elevated, suction pressure is low, superheat and subcooling high, the problem is in the metering device.
  • Pump the system down or recover the refrigerant and inspect the metering device for proper size, operation or debris.  Replace if necessary