This book is a compilation of selected papers from the 12th International Conference on Cold Climate HVAC and Energy 2025 (CCHVAC2025), held on August 6-8, in Harbin, China. It focuses on novel techniques for renewable and waste energy utilization, sustainable district heating and cooling, heat pump, smart energy system, and zero energy buildings. The contents make valuable contributions to academic researchers, engineers in the industry, and regulators of HVAC and energy authorities. As well, readers will encounter new ideas for realizing a more efficient, low-carbon and sustainable HVAC and energy system.