Low Suction Pressure (Heating)

CHECKOUT:

1. Check Equipment Status

• Ensure that the outdoor coil is not frozen = Defrost Problem → Step 3. Check Defrost
• Ensure that the outdoor fan is operating in the heating mode.
  • Compressor Running + No Outdoor Fan = Defrost Mode, Bad Fan Motor/Capacitor or Bad Control Board
    • Ensure unit is not in defrost mode (suction line should be hot in heating mode, cold in defrost mode)
    • Check for 230VAC between the "Common" (Black) and "Run" (Purple or Red) wires going to the motor.
      • Power + No Fan = Bad Cond Motor / Capacitor ⇒ Check Capacitor
      • No Power = Bad Control Board

2. Check Refrigerant Pressures

• If the outdoor coil is frozen → Step 3. Check Defrost (Do Not Attempt to Add Freon with a Frozen O/D Coil!)
• Hook-up refrigerant gauges to the pressure ports on the unit.
  • Blue --> "True Suction" Port (Not The Suction Line)
  • Red --> High Pressure (either refrigerant line)
• Monitor pressures as the system operates.

• 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 Pressure + Clear Sight Glass + Expansion Valve Frosting = Bad Expansion Valve (See "Expansion Valve Issues" in Related Articles)
  • Low Suction Pressure + Low Discharge Pressure + 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 Expansion Valve (See "Expansion Valve Issues" in Related Articles)

3. Check Defrost

• Cycle the heat pump through a defrost by shorting the TEST pins on the defrost board.
• Observe refrigerant pressures (may need to switch Red hose to liquid line for cooling High Pressure)
• Unit should terminate defrost somewhere around the following pressures:
  • 125 - 140 Suction 
  • 300 to 400 Discharge.

4. Check Air Temperature Rise

• With the auxiliary heaters disconnected (remove "W1" and "W2" form the air handler terminal block):
  • Check the Return Air Temperature near the unit or in the filter compartment.
  • Check the Supply Air Temperature.
  • Normal Temperature Rise = 15-20 deg.
    • Abnormally Low Return Air Temperature ⇒ Check Return Ductwork for Cold Air Infiltration
    • Low Temperature Rise = Inefficient Operation
    • High Temperature Rise = Low Airflow
  • RE-HOOK "W1" AND "W2" WIRES ON THE AIR HANDLER!