Chiller Air Handling Unit (AHU) Cooling tower water pump chillers are usually located in basement or roof, depending on the type of chillers used. Roof chillers are usually “air-cooled”, while underground chillers are usually “water-cooled”, but they all have the same function, that is, to generate cold water for air conditioning by removing excess heat from buildings. The only difference is how the chillers dissipate excess heat.