How to Diognose an Over-Feeding Metering Device

An evaporator coil that is receiving a higher than normal flow of refrigerant through the metering device will operate in a flooded condition.  The system may experience high humidity complaints, compressor starting problems, or compressor failure.

Causes

  • Piston too large in fixed metering device
  • Dirty piston seat not allowing metering device to seat properly
  • TXV sensing bulb not insulated or in good contact with the suction line

Symptoms

The metering device isolates the high side from the low side pressure.  When an overfeeding condition is present, the suction side of the system operates in a flooded state while the high side of the system operates in a starved state.  The suction pressure will be high and the superheat low.  The liquid pressure will be low and the subcooling will be low.

If the system is using a reciprocating type compressor, it may apear that the compressor valve plate is bad.

Procedure

  • Close the liquid service valve and pump the system down.  if the system does not hold a pump down and the liquid line service valve is properly closed.  Replace the compressor. (Recip)
  • If the system holds a pump down cycle, inspect the metering device to determine if it is seated properly or too large.
  • If the system has a TXV, make sure the sensing bulb is properly insulated and in good contact with the suction line.  If it is not, correct the problem.  If it is, replace the TXV.