Carrier - 38YG Won't Defrost

Carrier Heat Pumps: 38YG

HK32FA006 Time & Temp Defrost Board

Re: Won't Defrost

Carrier 38 Series heat pumps use a Time & Temperature Defrost Control.

• The control uses a defrost thermostat and an adjustable timer to initiate a defrost sequence.

Temperature: When the coil temperature drops below 30 deg. F the coil thermostat should close.

Time: The control board monitors compressor runtime. When the compressor runtime exceeds the time interval set on the board (30, 60 or 90 Minutes), the defrost control checks the status of the defrost thermostat.

If the defrost thermostat is closed at the end of the timer interval, the defrost board will initiate a defrost cycle: 

• Stopping the outdoor fan
• Switching the reversing valve to the cool position
• Energizing the auxiliary heaters.

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 unit in a regular heating mode.
  • Ensure that the compressor and condenser fan are operating normally.
    • Make sure unit is not in Defrost Mode
    • Normal heating Mode & No Outdoor Fan = Bad Fan Motor or Capacitor

2. Check Defrost Stat

• Check for 24VAC from "DFT" to "C" on the defrost board.
  • 24V = Thermostat Closed (ready for defrost)
  • No 24V = Thermostat Open (not ready for defrost)

3. Test Defrost Board

• With heat pump running in the heating mode:
  • Ensure the Defrost Stat is closed.
    • It may be necessary to place a jumper from "R" to "DFT" on the defrost board to simulate a closed defrost stat to initiate a defrost cycle.
  • Jump TP1 & TP2 (Speed-up) on the defrost board with a screw driver or jumper wire for at least 10 seconds or until defrost mode initiates.
    • Fan shuts off but reversing valve doesn't switch = Bad Reversing Valve, Solenoid Coil or Wiring
    • Does not go into defrost mode = Bad Defrost Board

4. Check Refrigerant Charge

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

5. Adjust Timer Interval

• If the defrost cycle works correctly but the unit still freezes-up, it may be necessary to decrease the time interval between defrosts.
  • On the defrost control board, move the yellow jumper wire to either 60 or 30 minutes.