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