Chilled Water System
A central plant that cools water and pipes it to cooling coils around a building.
Quick Answer
A chilled water system is a central cooling system in which a chiller cools water and pumps circulate it through piping to cooling coils in air handlers or fan coil units. The warmed water returns to the chiller to be cooled again. It is common in large commercial, campus, hospital, and data center buildings.
The Full Picture
A chilled water system separates where cooling is produced from where it is used. A chiller in a central plant cools water to a low temperature, often in the range of the low-to-mid 40s degrees Fahrenheit, and pumps send it through a closed piping loop. At each air handler or terminal unit, air passes over a coil carrying the cold water, which cools the air that is then delivered to the space.
The plant typically includes one or more chillers, chilled water pumps, a heat rejection system such as cooling towers or air-cooled condensers, expansion tanks, piping, valves, and controls. Water-cooled chillers reject heat to a condenser water loop and tower, while air-cooled chillers reject it directly to outdoor air. Designers pick between them based on size, efficiency goals, water availability, and space.
Chilled water suits large buildings because distributing cooling by water is more compact than ducting it, and because a central plant can be sized and staged efficiently across many zones. Campuses and districts often share a plant among several buildings. Smaller buildings often use direct-expansion or refrigerant systems instead, because a plant adds equipment and maintenance.
For contractors, chilled water means long-lead equipment, heavy rigging, extensive piping, flushing and water treatment, and a coordination-heavy mechanical room. Chillers can be among the longest-lead items on a job, which makes early procurement and clear scope boundaries important in preconstruction.
Real Examples
Common Misconceptions
People assume: Chilled water is the same as the water from a tap.
Actually: It is a closed loop of treated water that circulates between the chiller and coils. It is not potable and is kept separate from domestic water.
People assume: Chilled water systems are always more efficient than other cooling.
Actually: Efficiency depends on plant size, controls, load profile, and design. For small buildings, simpler systems can be more practical and cost-effective.
Frequently Asked Questions
How does a chilled water system work?
A chiller cools water, pumps circulate it to cooling coils in air handlers or terminal units where it absorbs heat from the air, and the warmed water returns to the chiller to be cooled again.
What is the difference between air-cooled and water-cooled chillers?
Air-cooled chillers reject heat directly to outdoor air. Water-cooled chillers reject it to a condenser water loop, usually served by a cooling tower, which generally adds equipment but can be more efficient at larger sizes.
When is chilled water used instead of direct expansion?
It is typically chosen for larger buildings, campuses, hospitals, and facilities with many zones or high cooling loads, where a central plant and water distribution make sense.
What is a primary-secondary chilled water loop?
It is a piping arrangement where one set of pumps serves the chillers and another serves the building loads, so the chiller flow and building flow can vary independently.