Abstract: A heat exchanger liquid refrigerant defrost system disclosed herein specifically designed to defrost the coil subsystems used on an outdoor heat exchanger used on a building heat pump unit or combination heat pump/air condition unit. The outdoor heat exchanger contains at least two coil subsystems each including having a first secondary bypass check valve, a secondary liquid line, a bypass solenoid, a suction solenoid, and a metering device. During use, the flow of warm liquid refrigerant through the coil subsystems is selectively controlled to defrost the coil subsystem one chamber at a time. The other coil subsystems continue to exchange heat and warm the building. When all of the coils systems are sequentially defrosted, all of the coil subsystems may operate in a heating mode or begin the defrost cycle again. Two important benefits of the system over a conventional heat exchanger are the amount of energy required to defrost the coil subsystem is reduced and a supplemental heat source is not needed.