Engineering Question
How Does a Chilled Water System Work?
Chilled water absorbs heat at building coils, returns to the chiller, rejects that heat at a cooling tower or air-cooled condenser, and is pumped back out as cold supply water.
Reviewed by
Michael K. Frempong, PE, LEED AP BD+C
President & Principal Engineer, BEED Engineering
Michael is a professional engineer and LEED AP BD+C accredited professional with more than 20 years of consulting engineering experience. He leads BEED Engineering's mechanical, electrical, and plumbing design work for commercial, healthcare, laboratory, education, industrial, and other building projects throughout Central Florida and the Treasure Coast.
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A chilled water system works in a loop. Pumps send cold water to cooling coils, where the water absorbs heat from building air. The warmer water returns to the chiller, which removes that heat and sends cold water back out. The chiller then rejects the collected heat through a cooling tower or air-cooled condensers. Controls stage equipment so the plant matches the building load.
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The loop in plain language
Start with what a chilled water system is. Operation is a repeating cycle:
- The chiller produces cold supply water.
- Pumps send that water to air-handling unit coils.
- Warm building air passes over the coils. The water absorbs heat and the air is cooled and dehumidified.
- Warmer water returns to the chiller.
- The chiller transfers that heat to condenser water or to air-cooled condensers.
- A cooling tower, if used, rejects heat to the outdoor air.
Two water circuits
Most water-cooled plants have two hydronic circuits:
- Chilled water: building side. Cold supply, warmer return.
- Condenser water: heat-rejection side. Warm water from the chiller to the tower, cooler water back.
Air-cooled chillers skip the tower and reject heat directly to outdoor air. They can simplify water treatment but have different efficiency and sound characteristics.
What the controls do
Building automation typically:
- Starts pumps before chillers
- Stages chillers with load
- Resets water temperature when conditions allow
- Protects against low flow and freeze conditions
- Coordinates air-handler valves and plant capacity
Without a defined sequence of operations, a plant can run inefficiently even if the chiller itself is new.
Where systems fail in the field
- Dirty coils and incorrect water flow
- Control valves that do not close
- Pumps operating against closed paths
- Tower issues in coastal or high-humidity climates
- Mismatched coil entering-water temperatures
When to involve an engineer
Involve an engineer when adding load to an existing loop, replacing a chiller, converting constant-flow to variable-flow pumping, or investigating comfort and energy complaints. Those problems are design problems, not only maintenance problems.
Common Questions
What temperature is chilled water?
Many commercial plants supply water around 42–45°F and see return water several degrees warmer. Exact temperatures are a design choice tied to coil performance and efficiency.
What is condenser water?
On water-cooled chillers, condenser water carries heat from the chiller to a cooling tower, where it is rejected to outdoor air.
Filed under Mechanical Engineering
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Related Questions
What Is a Chilled Water System?
A chilled water system uses a chiller to cool water that is pumped to air-handling units and other coils, then returned to the plant to be cooled again.
What Is Building Automation?
Building automation is the control system that starts, stops, sequences, and monitors HVAC and related equipment so a building operates as designed.
What Is HVAC Engineering?
HVAC engineering is the design of heating, ventilating, and air-conditioning systems that control temperature, humidity, ventilation, and indoor air quality.
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Chiller plants, pumping, air-handling equipment, and controls for larger commercial and institutional facilities.
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BEED Engineering is based in Palm Bay and serves building projects throughout Brevard County, the Treasure Coast, Port St. Lucie, Orlando, and surrounding areas.
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