Choosing between an air-cooled and a water-cooled chiller involves more than comparing equipment prices. Both systems can deliver chilled water to a building, but their heat-rejection equipment, installation requirements, operating expenses, and maintenance responsibilities differ substantially.
An air-cooled plant generally offers a simpler installation with fewer components. A water-cooled plant can offer lower chiller energy use, but it adds cooling towers, condenser water pumps, piping, and water management responsibilities.
The best choice depends on the complete plant and the building it serves. Cooling demand, operating hours, climate, utility rates, water availability, and maintenance capabilities all influence the decision.
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What Does a Chilled Water Plant Do?
A chilled water plant removes heat from a building and transfers it outdoors. Water carries that heat between the building’s cooling coils and the chiller.
Chilled water flows through coils inside air handling units or fan coil units. As air passes across the coils, heat transfers from the air into the water. The cooled air is then distributed to occupied spaces.
When the coil surface is below the air’s dew point, moisture also condenses on the coil and drains away. This allows the system to provide dehumidification as well as sensible cooling.
The water leaves the coil warmer than it entered and returns to the chiller. Inside the chiller’s evaporator, refrigerant absorbs heat from the water through the heat exchanger surfaces. The water and refrigerant remain separate.
The compressor raises the refrigerant’s pressure and temperature so it can release heat through the condenser. An expansion device then lowers the refrigerant pressure, allowing the cycle to repeat.
The heat path is:
Building air → chilled water → refrigerant → condenser → outdoors
The difference between air-cooled and water-cooled chillers occurs at the condenser. Both produce chilled water; they use different methods to reject heat.
How an Air-Cooled Chilled Water Plant Works
An air-cooled chiller packages the refrigeration equipment and outdoor heat-rejection components into one unit. It is commonly installed on a roof or at ground level.
Fans move outdoor air across finned condenser coils. Refrigerant inside these coils releases heat to the passing air.
The package typically contains the compressors, evaporator, condenser coils, fans, and equipment controls. Chilled water piping connects the evaporator to the building’s cooling coils.
The complete water circuit also needs pumps, an expansion tank, air removal equipment, strainers, valves, and instrumentation. Some chiller packages include pumps and accessories; others require them separately.
The Main Installation Advantage
An air-cooled plant does not need a conventional evaporative cooling tower, condenser water pumps, or the piping connecting a water-cooled chiller to its tower. Eliminating that infrastructure can reduce installation and maintenance costs. Department of Energy
It also avoids tower evaporation and blowdown water consumption. However, its closed chilled water circuit still requires appropriate fluid maintenance.
Airflow Is Essential
An air-cooled chiller needs sufficient clearance for condenser air intake and discharge. Nearby walls, architectural screens, and adjacent equipment can interfere with airflow.
Hot-air recirculation occurs when warm discharge air returns to the condenser intake. The chiller then receives air warmer than the surrounding outdoor conditions, potentially increasing compressor power and reducing available cooling capacity.
Equipment placement should follow the selected manufacturer’s clearance requirements. Architectural screening must be coordinated with airflow rather than treated solely as a visual feature.
Select for Actual Outdoor Conditions
Nominal tonnage does not establish how much cooling a chiller will provide at every outdoor temperature. Selection should use the project’s required cooling load, design ambient temperature, chilled water temperatures, flow, and fluid composition.
Buildings requiring winter cooling also need equipment suitable for low outdoor temperature operation. This is particularly relevant to server rooms, process applications, and buildings with year-round interior cooling loads.
Exposed water circuits require freeze protection where applicable. Depending on the design, this may involve glycol, heaters, heat tracing, or other manufacturer-approved measures. Insulation slows heat loss but does not, by itself, prevent stagnant water from eventually freezing.
How a Water-Cooled Chilled Water Plant Works
A conventional water-cooled plant separates the chiller from the outdoor heat-rejection equipment.
The chiller is commonly located inside a mechanical room. A condenser water circuit connects it to an outdoor cooling tower.
The plant has two separate water circuits:
| Circuit | Function |
|---|---|
| Chilled water | Carries heat from building cooling coils to the chiller evaporator |
| Condenser water | Carries heat from the chiller condenser to the cooling tower |
These circuits do not mix. Refrigerant inside the chiller transfers heat between them.
Condenser water arriving from the tower absorbs heat as it passes through the chiller condenser. A pump circulates the warmer water back to the tower.
Inside a typical open cooling tower, water is distributed over fill, which increases contact between water and moving air. A small portion evaporates, removing heat from the remaining water. The cooled water collects in the basin and returns to the chiller.
The Cooling Tower Does Not Produce Chilled Water
The cooling tower cools condenser water. The chiller’s refrigeration system produces the colder water supplied to the building.
For example, condenser water might enter the chiller at 85°F and leave at 95°F. The tower would then remove heat to bring that water back toward 85°F.
These are illustrative temperatures. Actual operating conditions depend on equipment selection, load, flow, weather, and controls.
Tower Heat Rejection Exceeds the Building Cooling Load
The tower receives the heat removed from the building plus the heat associated with compressor energy input.
Consequently, a tower should be selected for its actual heat-rejection duty, water flow, temperature range, and entering air conditions. Chiller tons and nominal cooling tower tons should not be treated as interchangeable ratings.
Why Climate Changes Chiller Performance
Air-cooled condensers and cooling towers respond to different weather conditions.
Dry-Bulb Temperature and Air-Cooled Chillers
Dry-bulb temperature is the outdoor air temperature measured by an ordinary thermometer.
As outdoor temperature rises, refrigerant generally must condense at a higher temperature to release heat into the air. This increases the compressor’s workload and can reduce available cooling capacity.
A selection that meets the load at mild conditions may therefore be inadequate during the hottest expected weather.
Wet-Bulb Temperature and Cooling Towers
Wet-bulb temperature reflects the combined effects of air temperature and moisture. It indicates the potential for evaporative cooling.
In hot, dry weather, wet-bulb temperature can be considerably lower than dry-bulb temperature. A cooling tower can then supply condenser water well below the outdoor dry-bulb temperature.
In hot, humid weather, the difference is smaller. The higher wet-bulb temperature generally produces warmer condenser water under otherwise comparable conditions.
Cooling Tower Approach
Cooling tower approach is the difference between leaving tower water temperature and entering air wet-bulb temperature:
Approach = Leaving tower water temperature − Entering air wet-bulb temperature
Consider two illustrative examples:
| Condition | Outdoor dry-bulb | Entering air wet-bulb | Leaving tower water | Approach |
|---|---|---|---|---|
| Hot and dry | 100°F | 70°F | 77°F | 7°F |
| Hot and humid | 100°F | 82°F | 89°F | 7°F |
Both examples assume the same approach to isolate the effect of wet-bulb temperature. They are not equipment selection data.
Approach also depends on tower capability, airflow, water flow, heat load, and equipment condition.
Approach Versus Range
Tower range describes the water temperature reduction through the tower:
Range = Entering hot water temperature − Leaving cold water temperature
Approach compares leaving water with wet-bulb temperature. Range compares entering and leaving water.
For example, a tower receiving 95°F water and supplying 85°F water has a 10°F range. If entering air wet-bulb is 78°F, its approach is 7°F.
What Is Refrigerant Lift?
Lift commonly describes the difference between refrigerant evaporating and condensing temperatures, with a corresponding pressure difference that the compressor must overcome.
With the same chilled water requirement, lower condensing temperature generally reduces lift and compressor work.
However, lowering condenser water temperature may require more cooling tower fan energy. The lowest condenser water temperature is therefore not automatically the lowest-energy operating point for the complete plant.
Installation Cost: Compare Complete Systems
The chiller purchase price is only part of installed cost.
For the same building cooling requirement, an air-cooled plant generally requires less heat-rejection infrastructure. A water-cooled plant adds equipment and connections that must be included in the estimate.
| Scope item | Air-cooled plant | Conventional water-cooled plant |
|---|---|---|
| Chiller | Required | Required |
| Chilled water pumps and distribution | Required | Required |
| Valves, insulation, and controls | Required | Required |
| Heat rejection | Packaged condenser coils and fans | Cooling tower |
| Condenser water pumps and piping | Not required | Required |
| Tower makeup water and blowdown | Not required | Required |
| Tower treatment provisions | Not required | Required |
| Equipment location | Usually outdoors | Chiller commonly indoors; tower outdoors |
| Electrical and controls | Chiller and pumping equipment | Chiller, pumping equipment, tower, and associated systems |
Site conditions can change the comparison. Structural support, rigging, electrical capacity, equipment access, and sound requirements can affect either option.
Existing Infrastructure Can Change the Economics
Replacement projects should account for what can be reused.
Serviceable cooling towers, pumps, and condenser water piping may make another water-cooled chiller more economical than converting to air-cooled equipment.
Reuse should follow an assessment of condition, capacity, compatibility, and expected remaining life. Existing infrastructure has value only if it can reliably support the proposed plant.
Equipment quotations also need careful review. Pumps, disconnects, controls interfaces, startup services, and accessories may be included in one quotation and excluded from another.
Operating Cost: Measure Whole-Plant Performance
A lower installed cost does not necessarily produce a lower lifetime cost.
Annual operating expenses include electricity, water, treatment, maintenance, and service. Operating hours determine how much opportunity exists for savings.
Understanding kW per Ton
Kilowatts per ton expresses electrical input relative to cooling output:
kW/ton = Electrical input in kilowatts ÷ Cooling output in tons
At equivalent conditions and the same measurement boundary, a lower value indicates better electrical efficiency.
A chiller-only rating should not be compared directly with a whole-plant figure. Establish which equipment is included before comparing results.
Include Pumps and Fans
An air-cooled plant’s electrical use includes the chiller package and chilled water pumps. Packaged ratings typically include condenser fan power, but the rating basis should be confirmed.
A water-cooled plant adds condenser water pumps and cooling tower fans to the chiller and chilled water pumping energy.
A highly efficient chiller can be paired with inefficient pumping or poorly coordinated controls. Evaluating the complete plant reveals those effects.
Full-Load Efficiency and IPLV
Full-load performance matters when the plant operates near peak demand. Part-load performance matters because cooling demand changes throughout the day and year.
Integrated part-load value, or IPLV, combines equipment performance at standardized load points. It is useful for comparison, but its assumptions do not reproduce every building’s load profile or climate. DOE’s chiller purchasing guidance considers both full-load and part-load efficiency. Department of Energy
Annual energy estimates should reflect actual loads, weather, equipment staging, and control strategies. Utility rates and demand charges then determine the resulting electricity cost.
Maintenance and Water Use
Both plant types need routine maintenance.
Air-cooled heat-rejection maintenance focuses on condenser coils, fans, and motors. Dirt restricts airflow and heat transfer, while outdoor exposure can contribute to corrosion.
Water-cooled plants add cooling tower, condenser water pump, and condenser tube maintenance. Tube deposits reduce heat transfer and can increase compressor energy use.
Water Management Responsibilities
Tower makeup replaces water lost through evaporation, blowdown, and drift. Blowdown removes concentrated water to control mineral buildup. Treatment and monitoring address scale, corrosion, and biological growth.
These requirements create recurring costs for water, chemicals, cleaning, and service. DOE identifies cooling tower management as an opportunity to reduce water and energy consumption. Department of Energy
Cooling towers also need an appropriate water management program to control Legionella growth and spread. CDC identifies effective water management as the primary control strategy. CDC
The owner’s ability to maintain these systems belongs in the selection decision. Potential efficiency savings depend on sustained operating performance.
Pumping Arrangements and Plant Controls
Pumping distributes chilled water while maintaining acceptable flow through the chillers.
As demand falls, two-way coil valves reduce flow. Variable-speed pumps can slow down while maintaining sufficient pressure for coils that still need cooling.
Primary-Secondary Pumping
A conventional primary-secondary arrangement uses primary pumps to maintain approximately constant flow through operating chillers. Separate secondary pumps vary building distribution flow.
A common pipe hydraulically connects the circuits and accommodates differences between their flow rates.
Variable Primary Flow
Variable primary flow varies water flow through both the chillers and the building distribution system. It can reduce pumping equipment, but requires coordinated flow measurement, bypass operation, and chiller staging.
Controls must respect minimum evaporator flow and allowable flow changes. At low building demand, a controlled bypass can maintain required flow through operating chillers. Manufacturer guidance should govern these limits. trane.com
Optimize the Plant, Not Individual Components
Good controls coordinate pump speeds, chiller staging, and tower operation. Temperature reset strategies may offer savings, but must respect equipment limits and the building’s cooling and dehumidification needs.
Changing one setpoint can shift energy use elsewhere in the plant. Performance should be evaluated at the system level.
Low Delta-T: Why More Flow May Be Required
Chilled water delta-T is return temperature minus supply temperature.
For water, a useful approximate relationship in customary U.S. units is:
Cooling capacity in tons ≈ GPM × Delta-T ÷ 24
This approximation assumes water properties typical of HVAC applications. Glycol mixtures require adjusted calculations.
For a 100-ton load:
| Supply | Return | Delta-T | Approximate required flow |
|---|---|---|---|
| 44°F | 56°F | 12°F | 200 GPM |
| 44°F | 50°F | 6°F | 400 GPM |
At the same load, halving delta-T requires approximately twice the flow. This does not mean pump power automatically doubles; pumping energy depends on flow, head, efficiency, and system behavior.
Low delta-T can contribute to higher pumping demand, distribution limitations, and additional chillers operating to provide flow.
Possible causes include excessive coil flow, bypassing valves, dirty coils, inadequate airflow, and control problems. Low delta-T does not automatically indicate lost chiller capacity.
Verify measurements before diagnosing the system. Also consider operating load: a lower delta-T at light load is not necessarily evidence of a fault.
Freeze Protection and Glycol
Glycol can protect exposed circuits against freezing, but it changes fluid properties.
Compared with water, glycol mixtures generally have lower specific heat and higher viscosity. Depending on concentration and temperature, this can affect capacity, required flow, pressure drop, and pumping energy.
Chillers, coils, and pumps should be selected for the intended fluid mixture. Avoid choosing glycol concentration solely by rule of thumb; coordinate it with the required protection temperature and equipment guidance.
Reliability and Backup Capacity
Neither heat-rejection method automatically guarantees greater reliability.
A plant with multiple chillers may still depend on a shared pump, electrical supply, controller, or tower header. Those shared components can affect available cooling during a failure.
Define how much cooling must remain available when equipment is out of service. Evaluate redundancy at the plant level, including pumps, power, controls, isolation, and maintenance access.
Lifecycle Cost and Payback
A lifecycle comparison weighs installed cost against operating expenses, repairs, replacements, and expected service life.
For example, if one plant costs $100,000 more to install and saves $10,000 annually in total operating expenses:
Simple payback = $100,000 ÷ $10,000/year = 10 years
These figures are illustrative. Savings should account for electricity, water, treatment, and maintenance.
Simple payback omits financing, discount rates, changing utility costs, major replacements, and residual value. A lifecycle analysis provides a fuller comparison.
Which Chilled Water Plant Fits Your Project?
An air-cooled plant may be attractive where water availability is limited, operating hours are moderate, or simpler maintenance is a priority.
A water-cooled plant may justify its additional infrastructure where loads and operating hours create sufficient energy savings and the owner can support ongoing maintenance.
Before selecting either option, establish:
- Peak cooling load and annual load profile.
- Local outdoor design conditions.
- Required chilled water temperatures and fluid mixture.
- Complete installed scope and available infrastructure.
- Electricity, water, and applicable sewer costs.
- Maintenance resources and service arrangements.
- Reliability and future expansion requirements.
There is no universal building-size threshold that settles the decision. Compare complete plants under equivalent conditions.
Choosing the Plant Beyond the Chiller
Air-cooled plants offer fewer heat-rejection components and a relatively straightforward installation. Water-cooled plants can deliver lower chiller energy use, with additional infrastructure, water consumption, and service responsibilities.
For both arrangements, equipment selection, controls, distribution performance, and maintenance determine the results.
Which factor matters most on your projects: installation cost, energy savings, water use, or maintenance? Share your experience in the comments.
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