York Heat Pump: YZE/YZF

Re: High Pressure Fault (Defrost)

The heat pump was running in defrost mode and the high pressure switch tripped.

A Defrost High Pressure Lockout is indicated by the following LED pattern on the outdoor unit control board:

• Green = Off
• Red = 3 Flashes

CHECKOUT:

1. Check Liquid Line Sensor

• If the unit is operating, turn the unit "Off" at the thermostat.
• Pull the outdoor unit disconnect.
• Remove 24VAC power from the Yorkguard VI board by removing the "R" wire from the board.
• Unplug the Liquid Line Sensor from the control board.
• Check the resistance of the sensor (200K Ohm scale)
• Use the York Resistance Chart to compare the sensed temperature to the actual temperature. 
  • Sensor resistance At or Near Actual Temperature → Step 2. Check Defrost
  • Sensor Resistance +/- 10 Deg from Actual = Bad Liquid Line Sensor

2. Check Indoor Airflow

• Set the thermostat to call for heat.
• Confirm that the indoor fan is operating and air is blowing out of the supply registers
  • No Airflow → Step 3. Check Blower Motor
  • Adequate Airflow → Step 4. Check Refrigerant Pressures

3. Check Blower Motor

• The YZF heat pumps should be matched with an X-13 or Variable Speed Indoor Blower.
• At the air handler, check for 240VAC to the motor.
• Check for 24VAC between "C" and "G", "Y1" or "Y2".
  • No 24VAC power = No signal from thermostat
  • 240V Power Present + 24V Power Present + No Blower Operation = Bad Blower Motor or Bad Control Board
    • X-13 Motor ⇒ Check for 24VAC between "C" and "Red" and "C" and "White" going to the motor.
      • Power Present + No Blower Operation = Bad Blower Motor
      • No Power = Bad Control Board

4. Check Refrigerant Pressures

• If the heat pump is in lockout, reset by cycling 24VAC power to the outdoor control board (pull the "R" wire for several seconds then re-attach).
• Hook-up refrigerant gauges to the pressure ports on the unit.
  • Blue --> "True Suction" Low Pressure
  • Red --> High Pressure (either refrigerant line)

• Place an insulated temperature probe on the liquid line between the evaporator coil & O/D expansion valve.
• Initiate a call for heat from the thermostat.
• Allow the unit to run for at least 10 minutes.
• Monitor high-side pressures as the system operates.
  • Pressure exceeds Normal Range or kicks-off on Hi Pressure safety in the first 5 minutes = Bad Overcharge or No Indoor Airflow 
  • Compressor never kicks on & the control locks-out on Hi Pressure ⇒ Suspect a Bad Pressure Switch

5. Check Sub-Cooling

• Record "Saturation Temperature" from the discharge pressure gauge.
• Subtract the Actual liquid line temperature reading.
  • Saturation Temp - Liquid Line Temp = Sub-Cooling
    • Sub-Cooling > 13 deg. + High Discharge + High Suction = Overcharged
    • Sub-Cooling > 13 deg. + High Discharge + Low Suction = Bad Expansion Valve
    • Sub-Cooling < 2 deg. + High Discharge = Low Indoor Heat Transfer → Step 6. Check Indoor Airflow

6. Check Indoor Airflow

• Check the air filter.
• Check the indoor coil.
• If Indoor Coil is OK ⇒ Re-check pressures with the filter removed.
  • Pressure Becomes Normal = Clogged or Restrictive Air Filter
  • No Change = Restricted Ductwork or Low Blower Speed 
    • X-13 Motor - Check the position of the "White" wire on the motor and increase if possible.

7. Check Duct Static Pressure

• Check the return static pressure (between the air handler and filter if possible).
• Check the supply static pressure.
  • High Static = Above 0.5" w.c.
    • High Return Static + Low Supply Static = Restricted Return Ductwork
    • Low Return Static + High Supply Static = Restricted Supply Ductwork