Usually, we say air source heat pump for the heat pumps whose heat source is air, which means they absorb heat from the air and transfer the heat to water. However, there are different types of air source heat pumps if we classify them based on the application and the different technology used on the heat pumps. Based on the application, air source heat pumps includes air source heat pump for swimming pool water heating (usually we call it swimming pool heat pump) and air source heat pump for house heating or domestic hot water. Based on the different compressor technology, there are on-off (constant frequency) air source heat pump, DC inverter air source heat pump and EVI air source heat pump which works at minus 25 degree.