The enhanced vapor injection air source heat pump (EVI-ASHP) has better thermal performance at a low temperature

which has received much attention to supply hot water in cold region in recent years due to the growing space heating load and concern for environmental degradation. Environmental temperature is often below -5℃ in winter in cold region of China

and changes greatly throughout the year. Thus coefficient of performance (COP) under nominal working conditions (dry-bulb temperature of 7℃) is difficult to accurately reflect the actual energy-saving effect of the system. So

a set-up of EVI-ASHP system was built in Lanzhou

and thermal performance at different environmental temperatures and humidities was determined. COP of the EVI-ASHP system could reach above 6.5 when electromagnetic valve for vapor injection was off

and linear change of COP was observed. At a low temperature

when electromagnetic valve was on

COP was about 2.0. The fitting equations of the experimental data were obtained and verified

with average relative error below 3% compared with the experimental data from the set-up. An effective prediction method was established for thermal performance of the EVI-ASHP system at changeable environmental temperature and humidity

supplying a theoretical guidance for the application of the system in cold climate.