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YZE/YZH - Not Defrosting

Article ID: 499
Last updated: 18 Dec, 2020

York Heat Pump: YZE/YZH

Re: Not Defrosting / Outdoor Coil Frozen

These units use a Yorkguard VI Demand Defrost control board.

The board uses an Ambient (outdoor) Sensor and Liquid Line Sensor to determine the proper conditions for defrost to occur.


Qty Possible Causes:
Bad Liquid Line Sensor
 1 Bad Condenser Fan Motor
Bad Fan Capacitor
  Bad Defrost Board
Mis-Located Sensor
Low Refrigerant Charge
Bad Expansion Valve


CHECKOUT:

1. Check Outdoor Fan

  • Make sure the thermostat is calling for heat and the heat pump is not currently in a Lockout Fault.
  • Inspect the outdoor unit and make sure the outdoor fan is operating.
    • Fan Operating → Step 2. Test Defrost Cycle
    • No Fan ⇒ 
      • PSC Models - Check for 230VAC to the fan motor between the  Purple  and  Black  wires.
        • No Power = Bad Control Board or Unit in Defrost
        • Power + No Fan Operation = Bad Fan Motor or Capacitor ⇒ Check Capacitor
      • V/S Models - Check for 24VAC between  Blue  at the compressor contactor low voltage coil &  Yellow  at the Yorkguard "ODF" terminal.
        • No Power = Bad Control Board or Unit in Defrost
        • Power + No Fan Operation = Bad Fan Motor/Controller

2. Check Defrost

  • Yorkguard VI
  • Jump the two "TEST" pins on the outdoor unit control board.
    • If the outdoor (ambient) temperature is below 50 deg. and above -25 deg. the unit should enter a defrost mode.
  • Ensure that the outdoor fan shuts-off and the reversing valve switches to cooling mode.
    • The unit should exit defrost when the liquid line temperature reaches 80 deg.

3. Check Liquid Line Sensor

  • Disconnect high voltage power from the unit by pulling the service disconnect or turning the breaker off.
  • Disconnect low voltage from the defrost board by removing the  Red , "R" wire.
  • Sensor Resistance
  • Disconnect the two  Blue  wires from the defrost board connected to "LL" and "LLG".
  • Set the multi-meter on 200K (not 200) Ohms.
  • Test resistance of the sensor.
  • Use Resistance Chart to determine sensed temperature.
    • If sensed temperature is more than 5 deg. higher or lower than the actual temperature = Bad Liquid Line Sensor
  • Sensor Location
  • It is very common for technicians to put the liquid line sensor in the wrong location (which can cause the unit to terminate defrost before the coil is thawed).
  • The sensor must be located on the condenser (outdoor) coil side of the metering device, as close to the metering device as possible.
    • With an expansion valve, it is possible to mount the sensor between the valve and the coil distributor.
    • With an orifice, the proper location is typically where one of the distributor lines ties into a short 3/8" piece of copper at the coil.

4. Check Ambient Sensor

  • Disconnect high voltage power from the unit by pulling the service disconnect or turning the breaker off.
  • Disconnect low voltage from the defrost board by removing the  Red , "R" wire.
  • Disconnect the two  White  wires from the defrost board connected to "AMB" and "AMBG".
  • Set the multi-meter on 200K (not 200) Ohms.
  • Test resistance of the sensor.
  • Use Resistance Chart to determine sensed temperature.
    • If sensed temperature is more than 5 deg. higher or lower than the actual temperature = Bad Ambient Sensor
      • Ambient Temperature reading less than 50 deg and the board will not initiate defrost = Bad Defrost Board

5. Check Refrigerant Pressures

  • Hook-up refrigerant gauges to the pressure ports on the unit.
    • Blue --> "True Suction" Port (Not the Suction Line)
    • Red --> High Pressure (either refrigerant line)
  • Monitor pressures as the system operates.
  • DO NOT ATTEMP TO ADJUST REFRIGERANT CHARGE WHILE THE COIL IS FROZEN!
Normal Pressure Range
Refrigerant 0-30 Deg O/D 30-50 Deg O/D
R410A
  • 75-100 Suction
  • 250-320 Discharge
  • 100-150 Suction
  • 250-375 Discharge
  • If a Liquid-Line Sight Glass is installed, observe for bubbles in the sight glass (after the system has been operating for at least 5 min.)
    • Bubbles = Low Refrigerant Charge
  • Low Pressure Issues
    • Low Suction + Low Discharge + Bubbles in Sight Glass + O/D Expansion Valve Frosting = Low Refrigerant Charge
      • 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
    • Low Suction + Clear Sight Glass + Expansion Valve Frosting = Bad O/D Expansion Valve 

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Article ID: 499
Last updated: 18 Dec, 2020
Revision: 18
Access: Public
Views: 442
Comments: 0