Book excerpt: Air source heat pumps are mainly used for space heating, and have the advantages of environmental protection, energy saving, and comfort. Written by leading heat pump technology expert Hui Huang, this book summarizes the research and applications of variable volume ratio two-stage vapor compression air source heat pump technology, and its use in cold climate regions. This book can be used for reference by scientific researchers and engineers engaged in research on air source heat pump technology, product development and popularization; and by energy management and policy researchers. It will also be of value to undergraduate and graduate students studying these areas of technology.