Trane Heat Pump: 4TWB, 2TWR, 4TWR

Re: Defrost Problem / Frozen Outdoor Coil

The outdoor unit becomes completely covered in thick, white ice.

CHECKOUT

1. Check Unit Operation

• Turn thermostat to the System "Off" position.
  • Confirm that the compressor stops running.
    • Compressor Continues to Run = Stuck Contactor
• Restart the unit in regular heating mode.
  • Ensure that the compressor and condenser fan are operating normally.
  • No Condenser Fan operation:
    • Make sure unit is not in Defrost Mode
    • Heating Mode & No Outdoor Fan = Bad Fan Motor or Capacitor

2. Test Defrost

• Normal Operation = LED flashing once per second.
• While running in the heating mode, jump TEST_COMMON and FRC_DFT on the defrost board.
  • Fan Shuts Off but Reversing Valve Doesn't Switch = Bad Reversing Valve, Coil or Wiring
  • Goes Into Defrost Mode Normally + Stays In Too Long ⇒ Suspect a Misplaced Coil Sensor → Step 3. Check Sensors
  • Goes Into Defrost Mode but Quickly Goes Back Out = Sensor Problem → Step 3. Check Sensors
  • Does Not Go Into Defrost Mode = Bad Defrost Board or Ambient Sensor → Step 3. Check Sensors

3. Check Sensors

• The coil sensor mounts in a copper sleeve brazed on one of the coil feeder tubes.
• Ensure that the coil sensor is secure inside the sensor sleeve.
  • It is not uncommon for the wire-tie to break and the sensor to come out of the sleeve.
• The Coil and Ambient Sensors are10Kohm sensors (see attached Sensor Resistance chart).
• Compare Coil Sensor resistance to actual coil temperature.
• Compare Ambient Sensor resistance to actual outdoor temperature.

4. Check Refrigerant Charge

1. Check "True Suction" and "Discharge" Pressure
   • Watch for premature freezing of the cap tubes and expansion valve = Low Charge or Underfeeding Valve
   • If a liquid line sight glass is installed, observe for bubble in the sight glass = Low Charge