This paper describes experiments comparing a commercially available R410A heat pump and prototype R744 (carbon dioxide) system in heating mode. The R744 system utilizes aluminum microchannel heat exchangers and a prototype semi-hermetic compressor. System performance is compared for two configurations: first, when the R744 compressor speed is set to match the baseline system's heating capacity at 8.2 degreesC outdoors and 21.1 degreesC indoors; and second, when the air conditioning capacity is matched at 35 degreesC outdoors and 26.5 degreesC indoors. The R744 system operates with a slightly lower heating COP, but its higher capacity at lower outdoor temperatures reduces the need for less efficient supplementary heating capacity. (C) 2003 Elsevier Science Ltd. All rights reserved.