A chilled water system is a type of air conditioning that cools buildings by circulating cold water through a network of pipes instead of blowing refrigerant directly to each room. The water, typically supplied between 43°F and 47°F, absorbs heat from indoor air at various points throughout a building, then returns to a central chiller to be cooled again. These systems are the standard cooling method for large commercial buildings, hospitals, universities, and data centers, where window units or rooftop systems simply can’t keep up with demand.
How the System Works
At the heart of every chilled water system is a refrigeration cycle, the same basic process that keeps your kitchen refrigerator cold. A liquid refrigerant inside the chiller evaporates as it absorbs heat from the building’s water loop. That refrigerant vapor is then compressed, which raises its temperature and pressure. The hot, pressurized vapor moves to a condenser where it releases that heat (either to outdoor air or to a separate water loop connected to a cooling tower). Finally, the refrigerant passes through an expansion valve that drops its pressure, cooling it back down so the cycle can repeat.
The chilled water itself never mixes with the refrigerant. It flows in a separate closed loop. Pumps push the cold water from the chiller out to air handling units and fan coil units scattered throughout the building. Inside those units, warm indoor air blows across coils filled with chilled water. The air cools down and enters the occupied space, while the now-warmer water (the “return” water) flows back to the chiller to be cooled again. Supply water typically leaves the chiller between 43°F and 47°F, and returns several degrees warmer after picking up heat from the building.
Main Components
A chilled water system has five core components that work together in a continuous loop:
- Chiller: The central piece of equipment that removes heat from the water using the refrigeration cycle. This is where the cooling actually happens.
- Cooling tower (in water-cooled systems): A structure, usually on the roof, that rejects the collected heat to the outdoor air by evaporating a small amount of water.
- Pumps: Circulate chilled water from the chiller to the building and back. Larger systems often have separate primary and secondary pump sets. Many modern systems use variable frequency drives to adjust pump speed based on demand, saving significant energy.
- Air handling units and fan coils: Located throughout the building, these blow indoor air across chilled water coils to cool occupied spaces.
- Piping and valves: The distribution network connecting everything. Two-way control valves at each coil regulate how much chilled water flows based on the cooling needs of each zone.
Air-Cooled vs. Water-Cooled Chillers
The two main categories of chilled water systems differ in how they dump heat outdoors. Air-cooled chillers use fans to blow outdoor air across condenser coils, similar to how a car radiator works. Water-cooled chillers instead send heat to a cooling tower, where evaporating water carries the heat away. This distinction drives most of the practical trade-offs between the two types.
Water-cooled systems are roughly twice as energy efficient as air-cooled ones. They condense refrigerant at lower temperatures because evaporating water can reach cooler conditions than dry air alone. They also take up less physical space on the equipment side. The downside is high water consumption and the added cost and maintenance of cooling towers. Air-cooled systems avoid water use entirely, which makes them a better fit in areas with water scarcity or expensive water utilities. They’re also simpler to install since there’s no cooling tower, no condenser water piping, and fewer components overall.
For smaller buildings or projects where installation cost matters more than long-term energy bills, air-cooled chillers often win. For large campuses or buildings that run heavy cooling loads year-round, water-cooled systems typically pay for themselves through lower operating costs.
District Cooling: Scaling Up
In dense urban areas, multiple buildings can share a single massive chilled water plant instead of each building operating its own. These district cooling systems pipe chilled water underground from a central facility to office towers, malls, and residential buildings across an entire neighborhood.
The energy savings are substantial. A district cooling system typically uses about 35% less energy than individual air-cooled systems in each building, and about 20% less than if each building had its own water-cooled chiller plant. Part of this comes from economies of scale (larger chillers run more efficiently), and part comes from load diversity: not every building peaks at the same time, so the central plant can be sized smaller than the combined total of individual systems.
Buildings connected to district cooling also reclaim significant space. Removing rooftop chiller plants and indoor mechanical rooms frees up an estimated 75% of the plant room area that would otherwise be dedicated to cooling equipment. That space becomes usable for tenants, rooftop gardens, or other purposes. Noise, vibration, and waste heat from equipment are concentrated at the central plant, where they can be better controlled.
Maintenance That Matters
Chilled water systems need regular attention to run efficiently and safely. The basics include cleaning evaporator and condenser coils, checking refrigerant levels, lubricating moving parts, tightening electrical connections, and verifying that the system starts up and shuts down correctly. Dirty coils alone can quietly drive up energy costs by forcing the chiller to work harder to transfer heat.
Water-cooled systems carry an additional responsibility: preventing bacterial growth in cooling towers. Cooling towers create warm, moist conditions that are ideal for Legionella, the bacterium that causes Legionnaires’ disease. OSHA recommends cleaning and disinfecting cooling towers at least twice a year, typically before startup in spring and after shutdown in fall. Between cleanings, periodic biocide treatments keep bacterial levels in check. Maintaining free chlorine levels above 0.5 parts per million in cooling tower water helps prevent growth, as long as the water’s pH stays below 8.0. Systems with heavy biological buildup may need more frequent cleaning.
Refrigerant Changes on the Horizon
The refrigerants inside chillers are changing due to environmental regulations. Most chillers have historically used hydrofluorocarbons (HFCs), synthetic chemicals that don’t damage the ozone layer but are potent greenhouse gases. Some HFCs trap hundreds to thousands of times more heat than the same amount of carbon dioxide. One common HFC, HFC-23, has a climate impact 14,800 times stronger than CO₂.
The U.S. EPA began restricting higher-impact HFCs in new refrigeration and air conditioning equipment starting January 1, 2025. New residential and light commercial systems installed after January 2026 must use refrigerants with a global warming potential below 700. The industry is shifting toward alternatives including newer synthetic refrigerants with much lower climate impact, as well as natural options like ammonia, propane, and CO₂ itself. If you’re specifying or replacing a chiller in the next few years, the refrigerant it uses will be a bigger part of the conversation than it was a decade ago.