An environmental control system for aircraft, comprising: a bleed air inlet; a recirculated air inlet configured to receive recirculated air from a chamber to be conditioned; an air cycle machine comprising a compressor and a turbine; at least one heat exchanger; a first flow valve connected between the bleed air inlet and the compressor, in order to divert the flow of the bleed air as to bypass the compressor or as to directly enter the compressor, based on one or more modes of operation; and a second flow valve connected between the recirculated air inlet and the turbine, configured to provide the flow of the recirculated air directly into the turbine based on one or more modes of operation; a flow path between the recirculated air inlet and the compressor; wherein the environmental control system is configured to, during a recirculation chilling mode of the one or more modes of operation, cause the flow of the bleed air to bypass the compressor and provide portions of the flow of the recirculated air directly to the turbine and the compressor, wherein the compressor is configured to adjust the pressure of the recirculated air discharged from the compressor outlet to the same pressure as bleed air supplied by the bleed air source, wherein the recirculated air discharged from the compressor outlet is combined with the bleed air supplied by the bleed air inlet to produce mixed air; wherein the environmental control system is configured to, during a pressure boost mode of the one or more modes of operation, use the second flow valve to cause the recirculated air to provide the recirculated air across the turbine. a bleed air inlet; a recirculated air inlet configured to receive recirculated air from a chamber to be conditioned; an air cycle machine comprising a compressor and a turbine; at least one heat exchanger; a first flow valve connected between the bleed air inlet and the compressor, in order to divert the flow of the bleed air as to bypass the compressor or as to directly enter the compressor, based on one or more modes of operation; and a second flow valve connected between the recirculated air inlet and the turbine, configured to provide the flow of the recirculated air directly into the turbine based on one or more modes of operation; a flow path between the recirculated air inlet and the compressor; wherein the environmental control system is configured to, during a recirculation chilling mode of the one or more modes of operation, cause the flow of the bleed air to bypass the compressor and provide portions of the flow of the recirculated air directly to the turbine and the compressor, wherein the compressor is configured to adjust the pressure of the recirculated air discharged from the compressor outlet to the same pressure as bleed air supplied by the bleed air source, wherein the recirculated air discharged from the compressor outlet is combined with the bleed air supplied by the bleed air inlet to produce mixed air; wherein the environmental control system is configured to, during a pressure boost mode of the one or more modes of operation, use the second flow valve to cause the recirculated air to provide the recirculated air across the turbine.