A vapor compression system (20; 120; 220; 320) comprising: a first compressor (22A; 122A; 222A); a second compressor (22B; 122B; 222B); a first heat exchanger ; a second heat exchanger ; one or more expansion devices (52; 52A, 52B); and means (32A, 32B; 32A, 32B, 126A, 126B; 32A, 32B, 232A, 232B) for switching the system between operation in a first mode or a second mode: wherein the first mode is a cooling mode in which: the first compressor compresses refrigerant; the compressed refrigerant is cooled in the first heat exchanger; the cooled refrigerant is expanded in at least one of the one or more expansion devices; the expanded refrigerant absorbs heat in the second heat exchanger and returns to the first compressor; and the second compressor is offline; and wherein the second mode is a heating mode in which: the second compressor compresses refrigerant; the compressed refrigerant is cooled in the second heat exchanger; the cooled refrigerant is expanded in at least one of the one or more expansion devices; the expanded refrigerant absorbs heat in the first heat exchanger and returns to the second compressor; and the first compressor is offline. a first compressor (22A; 122A; 222A); a second compressor (22B; 122B; 222B); a first heat exchanger ; a second heat exchanger ; one or more expansion devices (52; 52A, 52B); and means (32A, 32B; 32A, 32B, 126A, 126B; 32A, 32B, 232A, 232B) for switching the system between operation in a first mode or a second mode: wherein the first mode is a cooling mode in which: the first compressor compresses refrigerant; the compressed refrigerant is cooled in the first heat exchanger; the cooled refrigerant is expanded in at least one of the one or more expansion devices; the expanded refrigerant absorbs heat in the second heat exchanger and returns to the first compressor; and the second compressor is offline; and wherein the second mode is a heating mode in which: the second compressor compresses refrigerant; the compressed refrigerant is cooled in the second heat exchanger; the cooled refrigerant is expanded in at least one of the one or more expansion devices; the expanded refrigerant absorbs heat in the first heat exchanger and returns to the second compressor; and the first compressor is offline. wherein the first mode is a cooling mode in which: the first compressor compresses refrigerant; the compressed refrigerant is cooled in the first heat exchanger; the cooled refrigerant is expanded in at least one of the one or more expansion devices; the expanded refrigerant absorbs heat in the second heat exchanger and returns to the first compressor; and the second compressor is offline; and the first compressor compresses refrigerant; the compressed refrigerant is cooled in the first heat exchanger; the cooled refrigerant is expanded in at least one of the one or more expansion devices; the expanded refrigerant absorbs heat in the second heat exchanger and returns to the first compressor; and the second compressor is offline; and wherein the second mode is a heating mode in which: the second compressor compresses refrigerant; the compressed refrigerant is cooled in the second heat exchanger; the cooled refrigerant is expanded in at least one of the one or more expansion devices; the expanded refrigerant absorbs heat in the first heat exchanger and returns to the second compressor; and the first compressor is offline. the second compressor compresses refrigerant; the compressed refrigerant is cooled in the second heat exchanger; the cooled refrigerant is expanded in at least one of the one or more expansion devices; the expanded refrigerant absorbs heat in the first heat exchanger and returns to the second compressor; and the first compressor is offline. <EOS>