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R22 Cooling - Low Suction Pressure

Article ID: 335
Last updated: 29 Sep, 2017

Heat Pump: Cooling

Re: Low Suction Pressure

A heat pump or air conditioner runs with a lower-than-normal suction pressure.

Customer typically reports low or no cooling.


Qty Possible Causes:
Low Refrigerant Charge
1 Bad Expansion Valve
Low/No Airflow
Cold Outdoor Air
Cold Return Air

CHECKOUT:

1. Check Refrigerant Pressures

  • Hook-up refrigerant gauges to the pressure ports on the unit.
    • Blue --> "True Suction" Low Pressure
    • Red --> High Pressure (Small liquid line)
  • Allow the unit to run for 5-10 minutes.
  • Monitor pressures as the system operates.
    • Normal R-22 Suction Pressure =  60 - 80psig

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
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Article ID: 335
Last updated: 29 Sep, 2017
Revision: 1
Access: Public
Views: 7247
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