Subcooling and charge

Continuing the discussion of the quality installation (QI) process, the next step is ensuring that the cooling equipment is properly charged.

Original Equipment Manufactures (OEM) provide us with detailed guidance on charging procedures and very specific superheat and subcooling target values to compare to the measured values for use in evaluating and adjusting refrigerant charge as accurately as possible.

  • However, those target superheat and subcooling values provided by the OEM are based on the premise that the furnace or air handler is operating at design air flow.

So, before you reach for the manifold gauges and that drum of refrigerant, system air flow must be measured to ensure that the equipment is operating at the design Cfm. Here’s why.

Verifying design airflow across the indoor coil is a perquisite to ensuring the proper operation of any air conditioning system.

  • Low airflow across the indoor coil can create symptoms that may appear to be refrigerant charge related. Those include evaporator icing or freezing and low system capacity, which can be mistaken for low charge.
  • High airflow can reduce the ability of the indoor coil to dehumidify the indoor air leading to poor humidity control.  That can also be mistaken for low charge.

Temperature drop alone across the indoor coil cannot be relied upon to determine proper system charge or performance.

  • The condition of the air entering the indoor coil will not normally impact the temperature drop across the coil.
  • But is does impact the temperature of the air leaving the coil.

To achieve the design output capacity, the cooling system must operate at the airflow required by the OEM prior to confirming superheat and subcooling settings.

  • Once design air flow has been confirmed, subcooling or superheat measurements can be made and refrigerant charge can be adjusted accurately to the OEM target value.