Lennox Heat Pump: 13HPX

Re: Defrost Problem/Outdoor Coil Frozen

The Lennox 13HPX Heat Pump uses a Time & Temperature Defrost Board in combination with a Defrost Thermostat (S6).

Defrost Initiation

• Once the S6 Defrost Stat senses a temperature below 30 deg, its contacts should close.
• The Defrost Board will initiate a defrost after 30, 60 or 90 minutes based on the timer jumper.
• Once initiated, the board will remain in defrost until the S6 Defrost Stat contacts open or after 10 minutes of defrost.

CHECKOUT:

1. Check Operating Status

• If the outdoor unit is not already operating, set the thermostat to call for heat.
• Ensure that the compressor and outdoor fan run properly.
  • Compressor & Outdoor Fan Running → Step 3. Check Refrigerant Charge
  • No Outdoor Fan Operation → Step 2. Check Fan Motor

2. Check Fan Motor

• With the compressor operating, use a multi-meter to check for 230VAC power between the Black (at the Defrost Board) and Orange (at the contactor) wires feeding the motor.
  • No Power = Bad Defrost Board or In Defrost Mode
  • Power + No Operation = Bad Fan Motor or Capacitor ⇒ Test Capacitor

3. Check Refrigerant Charge

• If the Outdoor Coil is Frozen → Step 4. Check Defrost Thermostat
  • DO NOT ATTEMPT TO ADD REFRIGERANT WHILE THE COIL IS FROZEN!
• Hook-up refrigerant gauges to the pressure ports on the unit.
  • Blue ⇒ "True Suction" Low Pressure (not the Suction Line)
  • Red ⇒ High Pressure (either refrigerant line)

• Monitor pressures as the system operates.

• Low Pressure Issues
  • Low Suction + Low Discharge = Low Refrigerant Charge or Bad O/D Expansion Valve
    • Try adding refrigerant through the Low Pressure gauge.
      • Suction and Discharge Pressures should gradually increase as refrigerant is added.
      • If the Discharge Pressure increases but the Suction stays low or drops = Bad O/D Expansion Valve

4. Check Defrost Thermostat

• Check for 24VAC between "C" and each "DF" terminal on the Defrost Board.
  • Power on One Terminal Only + Frozen Coil = Bad S6 Defrost Stat or Mis-Placed Defrost Stat ⇒ The stat should be mounted between the Exp Valve & O/D coil.
  • Power on Both Terminals = Defrost Stat Closed → Step 5. Check Defrost Board

5. Check Defrost Board

• Check for 24VAC between "C" and "R" on the defrost board.
  • No Power = Mis-wired Low Voltage
• Check for 24VAC between "C" and "Y" on the defrost board.
  • No Power = No Call for Heat Pump
• Jump "Test" pins to see if unit will go into defrost.
  • No Defrost = Bad Defrost Board
  • Outdoor Fan Stops but Reversing Valve Doesn't Switch ⇒ Check for 24VAC between the two OUT terminals on the Defrost Board.
    • No Power = Bad Defrost Board
    • Power Present + Does Not Switch → Step 6. Check Reversing Valve / Solenoid Coil 

6. Check Reversing Valve / Solenoid Coil

• Ensure that the wires are connected at the reversing valve coil.
• Check for 24VAC between the two wires at the reversing valve coil.
  • No Power = Bad wiring between the Defrost Board and Reversing Valve
  • Power Present + Does Not Switch = Stuck Reversing Valve or Bad Solenoid Coil
    • Confirm that the Reversing Valve solenoid "clicks" each time that 24VAC power is applied
      • Clicks + Doesn't Switch = Stuck Reversing Valve
      • No Click = Bad Solenoid Coil