How Does a Chilled Water System Work?

The loop in plain language

Start with what a chilled water system is. Operation is a repeating cycle:

1. The chiller produces cold supply water.
2. Pumps send that water to air-handling unit coils.
3. Warm building air passes over the coils. The water absorbs heat and the air is cooled and dehumidified.
4. Warmer water returns to the chiller.
5. The chiller transfers that heat to condenser water or to air-cooled condensers.
6. 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.

That sequence work is part of how BEED has approached campus plants such as Meadowlane Intermediate School and Lake Nona Middle School, where one chilled-water plant serves mixed occupancies.

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.