How to Identify Low Evaporator Heat Load, TXV

Low evaporator heat loading occurs when there is not enough heat entering the refrigerant at the evaporator coil.  This condition can cause the evaporator coil to form a coating of frost or ice on it’s surface.  In extreme cases, superheat could fall to zero degrees and liquid refrigerant can flood back to the compressor.

Causes:

  • Dirty return air filter
  • Dirty indoor fan blower
  • Indoor fan motor not on high speed
  • Undersized duct work
  • Dirty evaporator coil
  • Thermostat set-point too low

Symptoms

Systems running with low evaporator loading will run at pressures below factory required levels.  The TXV may hunt in an attempt to maintain proper superheat levels.

The low heat load condition could cause the evaporator coil temperature to be very cold.  Ice or frost may form on the evaporator surface and suction line surface.  The compressor shell may sweat and the hot gas temperature will be cool.

Repair Procedure

  • Thaw the evaporator coil if frost or ice has formed.
  • Inspect the return air filter and replace if dirty.
  • Inspect the evaporator coil and clean if dirty
  • Inspect the indoor blower and clean if dirty
  • Make sure the indoor blower motor is operating at high speed
  • Make sure the supply and return duct are sized properly
  • Measure indoor CFM for proper flow.  350 to 450 CFM per ton.