Multi-circuit heat pump systems with improved defrost and hot gas reheat modes

A heating, ventilation, and air conditioning (HVAC) system operable to use a refrigerant to heat and cool an indoor space and comprising:

• an indoor heat exchanger being a part of at least two refrigerant circuits with refrigerants flowing therein, each refrigerant circuit comprising a compressor, an outdoor heat exchanger, at least one expansion device, and at least one four-way flow valve in fluid communication with the indoor heat exchanger,
• wherein the at least one four-way flow valve is operable to direct a corresponding refrigerant flow exiting the compressor within each refrigerant circuit while operating in a cooling mode, a heating mode, or a defrost mode,
• wherein only one refrigerant circuit of the at least two refrigerant circuits operates in the defrost mode at a given time, and
• wherein when the one refrigerant circuit is operating in the defrost mode, other refrigerant circuits of the at least two refrigerant circuits is operating in the heating mode so that a supply airflow from the indoor heat exchanger to the indoor space is heated.

A method for using a heating, ventilation, and air conditioning (HVAC) system to heat or cool an indoor space and comprising:

• flowing a first refrigerant through a first refrigerant circuit a first direction while the first refrigerant is operating in a cooling mode and in a second direction, opposite the first direction, while the first refrigerant is operating in a heating mode, and reversing the flow from the second direction to the first direction while the first refrigerant is operating in a defrost mode, wherein the first refrigerant circuit comprises an indoor heat exchanger, a first compressor, a first outdoor heat exchanger, at least one first expansion device, and at least one first flow control valve operable to flow the first refrigerant from the first compressor in the first or second direction;
• flowing a second refrigerant through a second refrigerant circuit the first direction while the second refrigerant is operating in the cooling mode and in the second direction, opposite the first direction, while the second refrigerant is operating in the heating mode, and reversing the flow from the second direction to the first direction while the second refrigerant is operating in the defrost mode, wherein the first refrigerant circuit comprises the indoor heat exchanger, a second compressor, a second outdoor heat exchanger, at least one second expansion device, and at least one second flow control valve operable to flow the second refrigerant from the second compressor in the first or second direction;
• and
• operating only one of the first or the second refrigerant circuit in the defrost mode at any time while operating the other of the first or the second refrigerant circuit in the heating mode so that a supply airflow from the indoor heat exchanger to the indoor space is heated.