A chiller is often the largest single piece of mechanical equipment in a building, and one of the longest-lived. The decision made during a replacement or new construction project shapes energy costs, maintenance budgets, and reliability for well over a decade, often two.
Yet many commercial chillers are selected by default. The old unit’s nameplate tonnage becomes the new unit’s size, or the most common option in the market becomes the choice without much evaluation. The result is often a chiller system that is oversized for the building’s actual cooling load and poorly matched to how the building operates.
This guide walks facility managers through how chillers work, the main types of chillers used in commercial buildings, which building characteristics should drive the decision, and how sizing, maintenance, and lifespan factor into getting the choice right.
How Chillers Work
Most HVAC systems in smaller buildings cool air directly, using packaged rooftop units or split systems. Chillers take a different approach. Instead of cooling air at the unit, a chiller cools a fluid, typically water, and circulates it throughout the building to wherever cooling is needed. That central approach makes chillers the standard choice for large buildings, campuses, and facilities that need precise temperature control, and many also serve industrial processes that rely on chilled process water or other cooling liquids.
How Chilled Water Systems Distribute Cooling
In chilled water systems, the chiller produces cold water, usually in the low to mid 40s °F, and pumps move it through a piping loop to coils in air handlers, fan coil units, and other terminal equipment. As building air passes over those coils, the water absorbs heat and returns to the chiller as warm water, typically 10 to 15 degrees warmer than it left. The chiller removes that heat, and the loop repeats.
This distinction matters for selection. The pumps, piping, and coils that make up the distribution system often outlive the chiller itself, so a replacement chiller has to work with infrastructure that is already in place.
Heat Rejection and the Cooling Process
Inside the chiller, a vapor compression refrigeration cycle does the work. Its main components are the evaporator, compressor, condenser, and expansion valve.
- Evaporator: The cooling process begins here. Liquid refrigerant passes through a heat exchanger, absorbing heat from the returning warm water and boiling into refrigerant vapor
- Compressor: Raises the vapor to high pressure and temperature so it can release heat
- Condenser: The refrigerant moves to the condenser, where it rejects heat to outside air across condenser coils or to water circulated from a cooling tower, condensing back into a liquid
- Expansion valve: This expansion device creates a pressure drop that cools the refrigerant before it returns to the evaporator, where the process begins again
The heat pulled from the building has to go somewhere. How a chiller rejects it, to air or to water, is the first major selection decision.
Air-Cooled Chillers vs. Water-Cooled Chillers
Air-cooled chillers reject heat directly to the ambient air using fans and condenser coils, much like a large rooftop condenser. Water-cooled chillers reject heat to condenser water, which is pumped to a cooling tower where evaporation carries the heat away. Each approach carries different costs, space demands, and maintenance requirements.
| Factor | Air-Cooled | Water-Cooled |
| Efficiency | Lower, especially in hot weather | Higher, particularly for large loads with long runtimes |
| Upfront cost | Lower | Higher, including cooling tower, condenser pumps, and piping |
| Supporting equipment | None beyond the chiller | Cooling tower, condenser water pumps, water treatment |
| Water use | None | Ongoing, from evaporation and blowdown |
| Location | Outdoors on a pad or roof | Indoor mechanical room plus a cooling tower |
| Maintenance | Less maintenance; mainly coil and fan care | More involved, including tower service and proper water treatment |
| Noise levels | Higher, from condenser fans | Chiller indoors; tower fans outdoors |
| Extreme heat | Capacity and efficiency drop as outside air temperature rises | Less affected by high ambient temperatures |
In practice, air-cooled systems tend to fit small to mid-sized buildings, facilities with limited mechanical room space, locations where water is expensive or scarce, and owners who want to keep maintenance simple. Water-cooled chillers typically make more sense for large buildings and campuses with high cooling loads and long operating hours, where the efficiency advantage adds up over thousands of hours each year and justifies the added infrastructure.
Neither option is universally better. The right choice depends on the building’s size, runtime, climate, available space, and water costs, which is why the decision should follow a load and site evaluation rather than precede it.
Understanding Compressor Types
The compressor is the heart of any vapor compression chiller, and it largely determines a chiller’s cooling capacity, part-load behavior, and maintenance profile. Capacity ranges overlap, but in commercial buildings each chiller type tends to fit a distinct size of load.
| Chiller Type | Typical Capacity | Part-Load Behavior | Maintenance Profile |
| Scroll | Up to about 200 tons, using multiple compressors | Steps capacity by cycling individual compressors on and off | Simplest; sealed compressors are replaced rather than rebuilt |
| Screw | Roughly 200 to 500 tons, with larger models available | Modulates smoothly down to about 20% of capacity | Moderate; oil management and periodic compressor inspection |
| Centrifugal | Mostly above 500 tons, up to several thousand | Most efficient near full load; variable speed drives improve part-load performance | Most specialized; requires technicians trained on the specific equipment |
Scroll Chillers
Scroll chillers use several small compressors within one unit and bring them online as the cooling load rises. That staging provides built-in redundancy, since the loss of one compressor reduces capacity rather than eliminating it. They are common in small to mid-sized buildings, are often air-cooled, and are well suited to modular installations that can grow with the building.
Screw Chillers
Screw chillers fill the middle of the market and are available as both air-cooled and water-cooled models. They modulate capacity smoothly and handle part-load conditions well, which makes them a frequent choice for mid-sized commercial buildings and replacement projects. Because many screw chillers rely on a single compressor, a compressor failure can take the entire unit offline.
Centrifugal Chillers
Centrifugal chillers are the standard for large buildings, hospitals, universities, and campuses with high, sustained cooling loads. They offer the highest full-load efficiency of the three and are almost always water-cooled. Older constant-speed models can lose efficiency at light loads, but variable-speed drives and newer designs have narrowed that gap, making centrifugal chillers more energy efficient across a wider operating range.
Absorption Chillers
Absorption chillers replace the mechanical compressor with a heat-driven process, using steam, hot water, natural gas, or waste heat from other equipment. They use far less electricity but are larger, costlier, and less efficient on their own. They make sense mainly where a free or low-cost heat source already exists, such as facilities with cogeneration or excess steam capacity.
Matching a Chiller System to Your Building
The right chiller system is the one that fits how a specific building uses cooling, not the one that fits a general category. Before comparing equipment, it helps to define the building’s cooling requirements across several factors:
| Building Characteristic | What to Consider | Tends to Favor |
| Peak load size | Total tonnage required on the hottest days | Scroll for smaller loads, screw for mid-range, centrifugal for large loads |
| Load profile | How much the load swings by hour and season | Multiple chillers, screw chillers, or variable speed drives for highly variable loads |
| Runtime hours | Seasonal operation vs. year-round cooling | Water-cooled for long runtimes where efficiency pays back |
| Climate | Frequency and severity of high outdoor temperatures | Water-cooled in hot climates with high cooling hours |
| Water availability and cost | Local water rates, restrictions, and treatment costs | Air-cooled where water is scarce or expensive |
| Space | Mechanical room size, roof structural capacity, site footprint | Air-cooled where indoor space is limited; water-cooled where roof or site space is limited |
| Noise sensitivity | Proximity to occupied spaces, neighbors, or local ordinances | Indoor water-cooled chillers, or air-cooled units with low-noise options |
| Redundancy needs | Consequences of losing cooling | Multiple chillers or modular units |
| Future expansion | Planned additions or changes in use | Modular or multi-chiller designs that can add capacity over time |
Facilities that also serve industrial applications, such as manufacturing processes or laboratory equipment, should account for those loads separately, since process cooling often runs on different schedules and temperature requirements than comfort cooling.
Critical Loads Like Data Centers and Hospitals
In some buildings, the cost of losing cooling far outweighs the cost of the equipment. Data centers, hospitals, and research facilities depend on continuous cooling, and equipment such as MRI machines often requires dedicated chilled water to operate. For these facilities, unplanned downtime can halt operations or put patients and sensitive equipment at risk.
That changes the selection criteria. Redundancy, often an N+1 arrangement with one more chiller than peak load requires, becomes a baseline requirement, and cooling solutions should be evaluated as much for reliability and serviceability as for efficiency.
Sizing to Actual Load, Not Nameplate Tonnage
When a chiller reaches the end of its life, the simplest path is to replace it with a unit of the same tonnage. However, if that chiller was sized 20 years ago, that may not be possible today. Over a chiller’s life, many changes can reduce the actual cooling load:
- LED lighting retrofits that cut internal heat gain
- Window, roof, and insulation upgrades
- Lower occupancy density or hybrid work schedules
- Repurposed spaces, such as data rooms consolidated or relocated
- Original designs that included generous safety margins
Many original chillers were oversized from the start, and those changes widen the gap. Replacing on nameplate tonnage carries that oversizing forward for another two decades. Before selecting a replacement, the cooling load should be re-evaluated using a current load calculation and, where available, operating data from the building automation system showing how much capacity the existing chiller actually uses.
Part-Load Performance and Real-World Energy Efficiency
Chillers rarely run at full capacity. The industry-standard Integrated Part Load Value (IPLV) rating assumes a chiller spends only about 1% of its operating hours at full load, with most hours between 50% and 75%. That makes part-load efficiency a better predictor of operating cost than the full-load rating.
An oversized chiller spends even more time at light loads, where it may short-cycle, wear its compressor faster, and lose efficiency. Comparing chillers on IPLV and specifying variable speed drives for buildings with variable loads supports efficient operation across the range of conditions the building actually sees.
Multiple-Chiller Plants and Energy Savings
Splitting total capacity across two or more chillers is often more effective than relying on a single large unit. Chillers can be staged on as the load rises, allowing each to operate closer to its most efficient range, and lead-lag rotation evens out wear. Some plants pair a smaller chiller for nights, weekends, and shoulder seasons with larger units reserved for peak demand. These configurations can significantly reduce energy use while adding redundancy, so the building keeps partial cooling if one chiller goes down.
Maintenance Demands and Lifespan by Chiller Type
Maintenance requirements vary significantly by chiller type, and they directly affect how long a chiller lasts. Water-cooled chillers, housed indoors and operating at lower condensing pressures, generally outlast air-cooled units exposed to weather year-round, but they depend on a cooling tower and water treatment program that adds its own upkeep.
| Chiller Type | Key Maintenance Tasks | Typical Lifespan |
| Air-cooled (scroll or screw) | Condenser coil cleaning, fan and motor inspection, refrigerant leak checks, electrical and controls testing | 15 to 20 years |
| Water-cooled screw | Condenser tube cleaning, oil analysis, water treatment, refrigerant leak checks | 20 to 30 years |
| Water-cooled centrifugal | Condenser tube cleaning, oil and vibration analysis, periodic tube testing, water treatment | 25 years or more, often 30 |
| Cooling tower (water-cooled systems) | Fill and basin cleaning, fan and drive service, water treatment, and Legionella management | 15 to 20 years typical, longer with proper maintenance |
These ranges assume reasonable care. The biggest factor within them is maintenance quality. Scaled or fouled condenser tubes, dirty coils, and neglected water treatment all force a chiller to work harder, raising energy use and accelerating wear on the compressor. Consistent, proper maintenance keeps a chiller at optimal performance and gives it a much better chance of reaching the upper end of its range, or beyond.
Lifespan also belongs in the selection decision itself. A lower-cost air-cooled chiller may need replacement five to ten years sooner than a water-cooled alternative, so comparing options on total cost over the building’s planning horizon, rather than purchase price alone, gives a more accurate picture of value.
Replacing a Chiller Within Existing HVAC Systems
New construction lets the chiller plant be designed around the building from scratch. Replacement projects don’t have that luxury. The new chiller has to work with pumps, piping, controls, electrical service, and a mechanical room that were built around the old one. Before finalizing a selection, facility managers should evaluate:
- Cooling tower condition and capacity: A tower near the end of its life may need to be replaced alongside the chiller, and a new chiller with different capacity or efficiency may require different condenser water flow
- Pumps, piping, and coils: Flow rates and temperature differentials must match what the existing distribution system can deliver
- Electrical service: Larger chillers, new variable speed drives, or a switch in chiller type may require panel or service upgrades
- Rigging and access: Getting the old chiller out and the new one in may require crane lifts, structural review for rooftop units, or knockdown chillers that can be disassembled to fit through existing doorways
- Temporary cooling: Buildings that can’t go without cooling during changeover need rental chillers, which should be planned and budgeted early
- Changing chiller types: Switching from water-cooled to air-cooled can eliminate cooling tower and water treatment costs, while the reverse can improve efficiency for buildings with long runtimes; either change affects site layout and infrastructure
- Current requirements: Most new comfort cooling chillers now must use lower-GWP refrigerants under EPA rules, and replacement equipment must meet the energy code efficiency minimums in effect in your jurisdiction
Addressing these items during selection, rather than after equipment is ordered, prevents change orders and schedule delays.
Common Chiller Selection Mistakes
Most costly chiller decisions trace back to a few recurring mistakes:
- Replacing like-for-like on nameplate tonnage: Carrying forward the old chiller’s size locks in any oversizing for another 15 to 30 years.
- Comparing price tag cost alone: Purchase price is a fraction of what a chiller costs over its life; energy, maintenance, and expected lifespan belong in the comparison.
- Evaluating full-load efficiency only: Since most operating hours happen at part load, IPLV and part-load performance are better predictors of real energy use.
- Overlooking supporting infrastructure: A new chiller paired with a failing cooling tower, undersized electrical service, or incompatible piping creates problems that surface after installation.
- Ignoring redundancy until a failure: A single-chiller plant has no backup, and emergency replacements leave little room to evaluate options.
Each of these is avoidable with a load evaluation and site assessment before selection begins.
Choose and Install the Right Chiller With Ambient Enterprises
Selecting commercial chillers is a decision that shapes a building’s operating costs and reliability for decades. Ambient Enterprises helps facility managers get it right from the start, evaluating actual building load, assessing existing infrastructure, and recommending the chiller type and plant configuration that fit how the building operates today and how it may change.
From selection through rigging, installation, and startup, our commercial cooling services keep the project coordinated under one team, so the chiller you choose performs the way it was designed to.
Contact our team to plan your next chiller replacement or new construction project.