The objective of this study is to investigate the performance characteristics of an injection CO2 (carbon dioxide) HPWH (heat pump water heater) at various operating conditions. In the standard and standby heat loss test conditions, the performance of a two-stage CO2 HPWH adopting a SCVI (sub-cooler vapor injection HPWH) cycle was measured and analyzed by varying the compressor frequency, the water flow rate, and the injection ratio at various outdoor temperatures. In the standard tests at the outdoor temperature of -15 degrees C, the COP (coefficient of performance) of the optimized SCVI HPWH was 7.6% higher than that of the non-injection HPWH. During the standby loss tests at the water flow rate of 200 kg h(-1), the COP of the optimized SCVI HPWH was 7.1% higher than that of the non-injection HPWH. (C) 2014 Elsevier Ltd. All rights reserved.