The by-product of chilled water production for air conditioning is hot water. For years, because energy was cheap, we discarded the hot water from the from the air conditioning process by rejecting it to cooling towers usually located on the roof. This hot water is essentially free and can be utilized to do domestic hot water preheating, reheating in air conditioning, and preheating of outside air. By sizing a modular chiller (known as a “Dedicated Heat Recovery Chiller”) to match the simultaneous heating and cooling loads in a commercial building we can significantly reduce utility bills and reduce carbon footprint. Modular chillers are uniquely suited to this simultaneous load and the production of hot water of temperatures up to 180F.