An environmental control system for thermally conditioning air within an enclosed space, comprising: a refrigerant circuit comprising a compressor, an indoor heat exchanger for thermally conditioning air within the enclosed space, an expansion device and an outdoor heat exchanger; an exhaust flowpath (44, 426) having a first inlet (420, 428) for receiving an exhaust flow of thermally conditioned air from the enclosed space and an outlet for discharging the exhaust flow to an external environment; and an intake flowpath having a first inlet for receiving an intake flow of fresh air from the external environment and an outlet for discharging the intake flow to the enclosed space, wherein a heat transfer section of the indoor heat exchanger is located in the intake flowpath to thermally condition the intake flow before entering the enclosed space, wherein a heat transfer section of the outdoor heat exchanger is located in the exhaust flowpath (44, 426), and the exhaust flowpath (44, 426) is for directing the exhaust flow of thermally conditioned air through the heat transfer section of the outdoor heat exchanger; wherein the intake flowpath has a second inlet for receiving thermally conditioned air from the enclosed space at a location that is upstream of the heat transfer section of the indoor heat exchanger in the intake flow direction, wherein the exhaust flowpath comprises a second inlet for receiving fresh air from the external environment at a location which is upstream of the heat transfer section of the outdoor heat exchanger in an exhaust flow direction, whereby the exhaust flowpath is suitable for directing a mixture of fresh air and thermally conditioned air to the heat transfer section of the outdoor heat exchanger, and characterized in that the environmental control system comprises a controller which is configured to operate one or more valves or dampers according to a first flow configuration in which the one or more valves or dampers simultaneously: allow an exhaust flow of thermally conditioned air to pass through the exhaust flowpath; allow an intake flow of fresh air to pass through the intake flowpath; and prevent thermally conditioned air from the enclosed space from entering the intake flowpath. a refrigerant circuit comprising a compressor, an indoor heat exchanger for thermally conditioning air within the enclosed space, an expansion device and an outdoor heat exchanger; an exhaust flowpath (44, 426) having a first inlet (420, 428) for receiving an exhaust flow of thermally conditioned air from the enclosed space and an outlet for discharging the exhaust flow to an external environment; and an intake flowpath having a first inlet for receiving an intake flow of fresh air from the external environment and an outlet for discharging the intake flow to the enclosed space, wherein a heat transfer section of the indoor heat exchanger is located in the intake flowpath to thermally condition the intake flow before entering the enclosed space, wherein a heat transfer section of the outdoor heat exchanger is located in the exhaust flowpath (44, 426), and the exhaust flowpath (44, 426) is for directing the exhaust flow of thermally conditioned air through the heat transfer section of the outdoor heat exchanger; wherein the intake flowpath has a second inlet for receiving thermally conditioned air from the enclosed space at a location that is upstream of the heat transfer section of the indoor heat exchanger in the intake flow direction, wherein the exhaust flowpath comprises a second inlet for receiving fresh air from the external environment at a location which is upstream of the heat transfer section of the outdoor heat exchanger in an exhaust flow direction, whereby the exhaust flowpath is suitable for directing a mixture of fresh air and thermally conditioned air to the heat transfer section of the outdoor heat exchanger, and characterized in that the environmental control system comprises a controller which is configured to operate one or more valves or dampers according to a first flow configuration in which the one or more valves or dampers simultaneously: allow an exhaust flow of thermally conditioned air to pass through the exhaust flowpath; allow an intake flow of fresh air to pass through the intake flowpath; and prevent thermally conditioned air from the enclosed space from entering the intake flowpath. allow an exhaust flow of thermally conditioned air to pass through the exhaust flowpath; allow an intake flow of fresh air to pass through the intake flowpath; and prevent thermally conditioned air from the enclosed space from entering the intake flowpath.