Rheem Heat Pump: RPNL / RPQL

Re: Cooling - Low Pressure Lockout - Code L21

These heat pump models have a control board which monitors equipment operation.

If the low pressure switch trips three times during a run-cycle, a "Hard-Lockout" Code L21 is generated and must be manually reset.

CHECKOUT:

1. Check Refrigerant Pressures

• Hook-up refrigerant gauges to the pressure ports on the unit.
• Allow the unit to run for 5-10 minutes.
• Monitor pressures as the system operates.
  • Normal R410a Suction Pressure = 100 - 150 psig

2. Check Superheat and Sub-Cooling

• Superheat
• Check the Suction Line Temperature near the outdoor unit.
• Record the "Evaporation Temperature" from the Suction pressure gauge.
  • Superheat = Suction Line Temperature - Evaporation Temperature
    • Normal Superheat = 8 - 18 deg.
• Sub-Cooling
• Check the Liquid Line Temperature near the outdoor unit.
• Record "Saturation Temperature" from the discharge pressure gauge.
  • Sub-Cooling = Saturation Temp - Liquid Line Temp
    • Normal Sub-Cooling = 3 - 12 deg. (Sight-Glass Clear)
• Low Suction Pressure + High Superheat + High Sub-Cooling = Bad Expansion Valve
• Low Suction Pressure + Low Superheat + High Sub-Cooling = Low Indoor Heat Transfer
• Low Suction Pressure + High Superheat + Low Sub-Cooling = Low Refrigerant Charge or Low Outdoor Air Temperature

3. Check Indoor Temperature Drop

• Check the Return Air Temperature at the air handler or furnace.
  • Return Air Temperature below 64 deg. will cause low pressure issues.
• Check the Supply Air Temperature
  • Temperature Drop = Return Air Temperature - Supply Air Temperature
    • Normal Temperature Drop = 15 - 20 deg.
      • Low Temperature Drop = Bad Expansion Valve or Low Refrigerant Charge
      • High Temperature Drop = Low Heat Transfer