In this study, thermodynamic performance of R431A and HCFC22 is measured in a heat pump bench tester under air-conditioning and heat pumping conditions. R431A is an near azeotropic mixture composed of 71% propane and 29% HFC152a by mass. It has no ozone depletion potential and low greenhouse warming potential of 43. R431A also offers a similar vapor pressure to that of HCFC22 for possible 'drop-in' replacement. Test results showed that the coefficient of performance of R431A is 3.5-3.8% higher than that of HCFC22 while the capacity of R431A is very similar to that of HCFC22 under both conditions. The compressor discharge temperature of R431A is 21.1-27.3 degrees C lower than that of HCFC22 while the amount of charge for R431A is 50.0-51.9% lower than that of HCFC22 due to its low density. Overall, R431A is a good long term environmentally friendly 'drop-in' alternative to replace HCFC22 in residential air-conditioners and heat pumps due to its excellent thermodynamic and environmental properties. (C) 2009 Elsevier Ltd. All rights reserved.