Abstract: A bi-directional flow auxiliary heat exchanger is disposed in proximity to the outdoor heat exchanger of a heat pump. The auxiliary fin and tube coil is active in both the heating and cooling modes to subcool refrigerant fluid flowing from the condenser. During operation of the heat pump system in the cooling mode, the entire auxiliary coil is used for subcooling; however, a portion of the subcooler is bypassed during heating mode operation to reduce the effective size of the subcooler. The bypassed portion of the auxiliary coil is used for storing excess refrigerant charge. This arrangement provides the system with the optimum amount of subcooling in each mode of operation, and adjusts the refrigerant charge for optimum performance in both the heating and cooling modes.