Diognosing Liquid Line Restrictions

Restrictions in the liquid line will prevent the evaporator coil from receiving the proper amount of refrigerant.  The restriction will cause a large pressure drop between the condenser and the metering device.  This large pressure drop can be detected by checking the temperature of the liquid line at the outlet of the condenser coil and at the inlet to the metering device.

Causes

  • Restricted liquid line dryer
  • Undersized liquid line
  • Excessive liquid line pipe fittings
  • Kinked liquid line.

Symptoms

Systems running with a liquid line 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 measures, at this time, it will be high.  The high subcooling level is an indication that excess refrigerant is being stored in the condenser coil.

Look for frosting or sweating of the liquid line or liquid line dryer.

Procedure

  • If the liquid and suction pressure are low, and no obvious sweating or frosting of the liquid line is present, begin to add refrigerant.
  • If the system is undercharged, both pressures will begin to rise.  If the liquid pressure rises significantly, but the suction pressure remains 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.  No significant drop in temperature should be detected.  If you detect a large temperature drop, there is a pressure drop between the two test points.  Isolate the restriction to 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 is high, perform diagnostics for a restricted metering device.