YHM - Low Pressure Fault (Heating)

York Heat Pump: YHM

Re: Low Pressure Fault - Heating

The YHM heat pump has a 5-stage Mitsubishi compressor a DC Inverter board.

A small control board below the Inverter Board has the diagnostic and status indications.

A Low Pressure Fault is indicated by the following LED pattern on the outdoor unit control board:

Soft Lockout:

• Red = 2 Flashes
• Green 1 = Off
• Green 2 = 4

Hard Lockout:

• Red = 3 Flashes
• Green 1 = Off
• Green 2 = 4

Low Pressure Switch

• If the low-pressure switch opens for more than five seconds, the control will cause a soft lockout condition and display the appropriate fault codes.
• If four LPS openings occur within a twelve-hour period, it causes
  a hard lockout which can be reset by cycling system power, or by shorting
  the TEST pins for more than 2 seconds.
• The control will ignore the low pressure switch input and not cause a soft lockout condition if it opens during the following conditions.
  • Defrost operation
  • First two minutes of compressor operation
  • Two minutes following the completion of a defrost cycle
  • TEST input shorted with Y1 input energized

CHECKOUT:

1. Check Unit Operation

• Make sure the thermostat is set correctly and calling for Heat.
• Allow up to 5 minutes for compressor to start.
  • Outdoor Unit Operating → Step 2. Check Refrigerant Pressures
• Check the Diagnostic Indication Lights.
  • Unit in Lockout ⇒ Reset the control by removing 24VAC power or by jumping the Test pins for more than 2 seconds.

2. Check Refrigerant Pressures

• Hook-up refrigerant gauges to the pressure ports on the unit.
  • Blue --> "True Suction" Low Pressure
  • Red --> High Pressure (either refrigerant line)

• Ensure that the unit is running at full capacity (H4).
  • It is possible to force the unit into High Speed by moving the dip switch on the small control board to "Hi".
• Monitor pressures as the system operates.

• Normal Pressures + Low Pressure Lockout = Bad Pressure Switch

Low Pressure Issues

• 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.)
  • 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 + Low Discharge + Clear Sight Glass + Expansion Valve Frosting = Bad O/D Expansion Valve