Chilled water is used in large HVAC systems because it is an efficient and cost-effective way to cool large areas. Chilled water systems use a refrigerant to cool water, which is then circulated through a system of pipes to cool the air in the building. This method of cooling is more efficient than traditional air conditioning systems because it does not require the use of compressors or fans. Additionally, chilled water systems are more cost-effective because they require less energy to operate and can be used to cool multiple areas at once.