York Heat Pump: YZF

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.

CHECKOUT:

1. Check Outdoor Fan

• Make sure the thermostat is calling for heat and the heat pump is not currently in a Lockout Fault.
  • Reset Lockout, if necessary, by removing and then re-attaching the low voltage  Red  and  Yellow  wires from the Yorkguard VI.
  • You can bypass the Anti-Short-Cycle Delay by jumping the TEST pins.
• 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  Red  wire (at T2 of the contactor) and the  Black  wire (at the Defrost Board [ODF] terminal).
      • 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 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 and hold 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.
• Remove the "TEST" jumper once the unit enters defrost.
• 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.
    • Enters & Exits Defrost Normally → Step 5. Check Refrigerant Pressures
    • Does Not Enter Defrost → Step 4. Check Ambient Sensor
    • Enters & Exits Defrost too Quickly → Step 3. Check Liquid Line Sensor
    • Reversing Valve Does Not Switch ⇒ Check for 24VAC at the reversing valve solenoid.
      • Power + Doesn't Switch = Bad Reversing Valve
      • No Power = Bad Defrost Control or Loose Wiring

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 (Liquid Line)
• DO NOT ATTEMPT TO ADD FREON WHILE THE COIL IS FROZEN!

DEFROST MODE

• Connect a thermometer to the liquid line near the outdoor unit.
• Initiate a Defrost Cycle by jumping the TEST pins.
• Monitor system pressures and the liquid line temperature while the unit is in a defrost cycle.
• Suction and Discharge pressure should slowly climb as the liquid line warms.
  • Suction Pressure, Discharge Pressure and Liquid Line Temperature Don't Increase ⇒ Check Heating Mode pressures (below)

HEATING MODE

• Monitor system pressures as the unit operates. 
• Allow the unit to run for at least 5-10 min before adjusting charge.

• 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 + 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 + Normal to High Discharge + Clear Sight Glass + Expansion Valve Frosting = Bad O/D Expansion Valve