Low pressure bleed architecture

A bleed air conditioning system (34;134) within an aircraft including a low-pressure bleed air line (28) and a high-pressure bleed air line (30). The system includes an air-to-air heat exchanger (38) and an un-cooled bleed air line (40) to connect the air-to-air heat exchanger to the low-pressure bleed air line (28) and the high-pressure bleed air line (30). The un-cooled bleed air line (40) carries a flow of un-cooled bleed air from at least one of the low-pressure bleed air line (28) and the high-pressure bleed air line (30) to the air-to-air heat exchanger (38). The system also includes a cooled bleed air line (36) connected to the air-to-air heat exchanger (38) for carrying cooled bleed air from the air-to-air heat exchanger and a low-pressure bypass line (42) connecting the low-pressure bleed air line (28) to the cooled bleed air line (36), bypassing un-cooled bleed air from the low-pressure line (28) around the air-to-air heat exchanger (38) to the cooled bleed air line (36).