A chiller is a large piece of cooling equipment that produces cold water and circulates it through a building to remove heat. Instead of blowing cooled air through long ductwork the way a residential air conditioner does, a chiller cools water at a central location, then pumps that water to individual rooms and zones where smaller units use it to cool the air locally. Chillers are the standard cooling method for large commercial buildings, hospitals, universities, and industrial facilities.
How a Chiller Cools a Building
A chiller works by exploiting the same basic principle as your refrigerator, just on a much larger scale. A chemical refrigerant cycles through four stages in a continuous loop, absorbing heat from the building’s water supply and dumping that heat outside.
In the first stage, liquid refrigerant enters a component called the evaporator, where it absorbs heat from the building’s water. That heat causes the refrigerant to boil into a vapor, and the water leaves the evaporator cold, typically around 6 to 7°C (about 44°F). A compressor then squeezes the refrigerant vapor, raising its pressure and temperature dramatically. The hot, pressurized vapor moves to a condenser, where it releases all that collected heat to the outdoors (either to the air or to a separate water loop connected to a cooling tower). As the refrigerant sheds heat, it condenses back into a liquid. Finally, an expansion valve drops the refrigerant’s pressure back down, cooling it and preparing it to absorb heat again. The cycle repeats continuously.
The chilled water produced by this process flows through insulated pipes to air handling units or fan coil units positioned throughout the building. Those units blow room air across coils filled with cold water, cooling the space. The now-warmed water returns to the chiller to be cooled again. This loop of chilled water is why the system is sometimes called “hydronic cooling.” One central chiller plant can serve dozens of zones, or even multiple buildings on a campus, all controlled from a single location.
Air-Cooled vs. Water-Cooled Chillers
The two main categories of building chillers differ in how they reject heat from the condenser side of the cycle.
Air-cooled chillers sit outdoors and use large fans to blow ambient air over the condenser coils, carrying heat away directly into the atmosphere. They’re simpler to install because they don’t need a cooling tower or a separate condenser water loop. The tradeoff is that they’re noisier, consume more energy to power those fans, and top out at a smaller capacity, generally up to about 500 tons of cooling. They’re a common choice for mid-sized office buildings, retail centers, and facilities that lack the roof or ground space for a cooling tower.
Water-cooled chillers pass the condenser heat into a loop of water (often mixed with glycol for freeze protection) that flows to a cooling tower on the roof or nearby. The cooling tower exposes that warm water to outdoor air, allowing evaporation to pull the heat away. This approach is more energy-efficient, especially for facilities that run around the clock, and water-cooled chillers scale up to 4,000 tons of cooling capacity. The downside is added complexity and maintenance: the cooling tower needs regular condenser tube cleaning, water treatment, and freeze protection in cold climates.
Compressor Types and Building Size
The compressor is the component that does the heavy lifting, and the type of compressor inside a chiller generally matches the size of the building it serves.
- Scroll compressors use two interlocking spiral plates to compress refrigerant. They handle about 3 to 510 tons of cooling, making them suitable for smaller commercial buildings like restaurants, clinics, or small office buildings.
- Screw compressors (also called rotary compressors) trap and squeeze refrigerant between rotating helical screws. They cover a wide range, from 3 to 2,000 tons, and are available in both air-cooled and water-cooled configurations. Many mid-rise office buildings and hospitals use screw chillers.
- Centrifugal compressors spin refrigerant at high speed using a turbine-like impeller, converting that kinetic energy into pressure. They’re always water-cooled and handle the largest loads, from 60 to 6,000 tons. You’ll find these in skyscrapers, large hospital complexes, and university central plants.
Electric vs. Absorption Chillers
Most building chillers are vapor compression systems powered by electricity. An electric motor drives the compressor, and electricity is the single energy input. These are by far the most common type in commercial buildings.
Absorption chillers take a fundamentally different approach. Instead of a mechanical compressor, they use a heat source (natural gas, steam, or waste heat from an industrial process) to drive the refrigerant cycle through a chemical absorption process. Buildings that have access to cheap steam or waste heat sometimes choose absorption chillers to reduce their electricity demand. They’re less energy-efficient overall, but they can make financial sense when heat is readily available and electricity is expensive.
How Long Chillers Last
Water-cooled chillers typically last 20 to 30 years. Air-cooled chillers have a shorter lifespan of roughly 15 to 20 years, partly because their outdoor condenser coils and fans are exposed to weather year-round. In both cases, hitting the upper end of that range depends on consistent maintenance.
Most HVAC professionals recommend at least two inspections per year. Routine tasks include cleaning or replacing air filters, inspecting heat exchanger surfaces for wear or corrosion, and checking refrigerant levels. For water-cooled systems, water treatment is critical. As water evaporates in the cooling tower, dissolved minerals like calcium and magnesium concentrate in the remaining water. If left unchecked, those minerals form scale on condenser tubes, insulating them and forcing the chiller to work harder. Corrosion and biological growth (algae, bacteria) are the other common threats, so treatment programs typically include scale inhibitors, corrosion inhibitors, and biocides.
Why Buildings Use Chillers Instead of Large AC Units
Water is a far better medium for moving heat than air. A pipe carrying chilled water can deliver the same cooling as a duct many times its size, which matters enormously in a 30-story building where duct space would eat into usable floor area. Centralized chiller plants also allow precise zone-by-zone temperature control. Each floor or section of a building can have its own fan coil unit receiving chilled water, letting an office on the sunny side of the building get more cooling than a shaded conference room, all from the same plant.
Operational efficiency is the other major advantage. A single large chiller running near its design capacity is more efficient per unit of cooling than dozens of individual rooftop AC units. Many large buildings install two or three chillers so they can stage capacity up or down depending on the season, running one chiller on a mild spring day and all three during a July heat wave. This staging keeps each chiller operating closer to its most efficient point rather than cycling on and off wastefully.