Cooling - Low Suction Pressure

Cooling

Re: Low Suction Pressure

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

NORMAL SUCTION PRESSURE:

• R-22 = 55 - 80 psig
• R410a = 100 - 150 psig

CHECKOUT:

Low Refrigerant Charge

• Low pressure due to low refrigerant charge will be accompanied by a warm suction line and a high superheat (see "Checking Superheat"). If the suction pressure is low and superheat is low (cold suction line) DO NOT ADD REFRIGERANT.
• If the suction pressure is low and the superheat is high try adding refrigerant. If the pressure does not come up and the superheat does not go down, suspect a bad metering device (below).

Bad Metering Device

• A bad metering device will typically result in a low suction pressure and a high superheat.
• If you have a low suction pressure and high superheat and suspect a bad metering device, try adding refrigerant to see if the suction pressure increases and the superheat decreases.
  • If pressures and temperatures improve, the metering device is functioning properly.
  • If the head pressure increases but the suction pressure stays the same or drops, the metering device is bad.

Low Airflow

• Low pressure caused by low airflow will be accompanied by a low superheat.
  • Indoor airflow will affect the suction pressure in the cooling mode.
  • Outdoor airflow will affect the suction pressure in heating mode.

Cold Ambient Temperature

• Cooling
  • In the cooling mode, running in outdoor temperatures below 55 degree without a "low ambient control' will cause symptoms very similar to low refrigerant.
  • The cold outside air will cause the head pressure to drop which, in-turn, causes the suction pressure to drop.
  • Try blocking part of the outdoor fan discharge to increase head pressure.
• Heating
  • As the outdoor temperature drops, the suction pressure will also drop.

Cold Return Air Temperature (cooling)

• The minimum return air temperature for cooling is around 64 deg.
• The colder the return air temperature gets, the lower the suction pressure will go.