Selecting the right chiller size is one of the most important decisions in the design of a chilled-water cooling system. A chiller that is too small may struggle to maintain the required temperature during peak conditions, while an unnecessarily large chiller can increase capital and operating costs and may operate inefficiently at low loads.
The correct approach is to determine the project’s actual cooling requirement and then select equipment that can deliver the required capacity under the specific operating conditions.
ASHRAE identifies cooling-load calculations as a primary design basis for chillers and other HVAC components. These calculations account for factors such as outdoor conditions, building materials, windows, occupancy, lighting, equipment, ventilation, and infiltration.
Whether you are selecting a chiller for a commercial building, industrial facility, data centre, process application, or temporary cooling project, the following factors can help you determine the appropriate size.
What Does Chiller Size Mean?
Chiller size refers primarily to its cooling capacity—the amount of heat the equipment can remove over a given period.
Capacity is commonly expressed in:
Tons of refrigeration (TR)
Kilowatts (kW)
British thermal units per hour (BTU/h)
One refrigeration ton is approximately 3.517 kW of cooling capacity.
However, the nominal capacity shown on a chiller’s specification sheet does not necessarily represent the capacity it will provide under every operating condition.
Actual performance depends on factors such as chilled-water temperatures, condenser conditions, flow rates, and ambient conditions.
Therefore, selecting a chiller should involve more than matching a number of tons to the estimated size of a building.
1. Start With a Proper Cooling-Load Calculation
The first step is to determine the project’s cooling load.
Cooling load represents the rate at which heat needs to be removed to maintain the required indoor or process conditions.
For buildings, the load can come from many sources, including:
Solar heat entering through windows
Heat transfer through walls and roofs
Occupants
Lighting
Computers and electrical equipment
Machinery
Outdoor ventilation air
Infiltration
Duct and fan heat
Process equipment
ASHRAE explains that nonresidential cooling loads vary according to external and internal conditions, including building construction, windows, occupancy, equipment, ventilation, and infiltration.
This is why simply calculating the building’s floor area and applying a generic tonnage-per-square-foot figure can produce an inaccurate result.
A proper load calculation should reflect the actual project.
2. Identify the Peak Cooling Load
The chiller needs to cope with the project’s peak requirement—not merely its average daily load.
Peak cooling demand can occur when several heat sources reach high levels at the same time.
For example, a commercial building may experience its highest demand when:
Outdoor temperatures are high
Solar gains are significant
Occupancy is high
Lighting is operating
Office equipment is running
Ventilation requirements are high
ASHRAE describes peak design cooling load as the maximum rate of cooling required to maintain the specified indoor conditions under the selected design conditions.
However, the peak load should be calculated carefully rather than inflated with arbitrary safety margins.
3. Avoid Choosing a Chiller Simply by Floor Area
One of the most common mistakes is choosing a chiller based only on the size of the building.
Two buildings with the same floor area can have completely different cooling loads.
For example, a lightly occupied office and a densely populated facility containing heat-generating equipment may require very different cooling capacities.
Important differences can include:
Building orientation
Window area
Glass type
Insulation
Occupancy
Lighting density
Equipment loads
Fresh-air requirements
Operating schedules
Indoor temperature
Outdoor design conditions
ASHRAE recommends using accurate inputs in cooling-load calculations rather than relying on compounded safety factors that can lead to oversized systems.
4. Consider the Building Envelope
The building envelope can have a significant effect on cooling requirements.
Evaluate:
External walls
Roof
Floors
Windows
Doors
Skylights
Insulation
Shading
Building orientation
Solar heat gain through glazing can be particularly important in buildings with large areas of exposed glass.
A building with good insulation and effective shading may require less cooling than a poorly insulated building of the same size.
5. Calculate Internal Heat Gains
Heat produced inside the building also contributes to the cooling load.
Sources may include:
People
Every occupant contributes sensible and latent heat to the space. A crowded facility can therefore have a substantially different load from a lightly occupied building.
Lighting
Lighting converts electrical energy into heat. Lighting design and operating schedules should therefore be included in the calculation.
Equipment
Computers, servers, production machinery, kitchen equipment, pumps, motors, and other electrical equipment can produce significant heat.
For industrial facilities, equipment and process loads may be among the largest contributors to the cooling requirement.
ASHRAE’s cooling-load guidance specifically identifies occupants, lighting, appliances, and equipment as internal heat-gain sources that should be considered.
6. Consider Ventilation and Fresh-Air Requirements
Cooling is not limited to removing heat from the building’s walls, occupants, and equipment.
Outdoor air introduced into the building can add both sensible and latent cooling loads.
The amount of ventilation air required depends on the application and occupancy.
This is particularly important for:
Offices
Hotels
Shopping facilities
Hospitals
Restaurants
Schools
Industrial facilities
The cooling system must be capable of handling the heat and moisture associated with the required outdoor air.
ASHRAE notes that ventilation, infiltration, moisture migration, and system-related loads all affect cooling-load calculations.
7. Determine the Required Chilled-Water Temperatures
Once the cooling load is known, the next step is to establish the chilled-water operating conditions.
Two important parameters are:
Entering chilled-water temperature
Leaving chilled-water temperature
The difference between these temperatures is commonly referred to as Delta T.
These conditions affect the amount of cooling delivered by the system and the required water flow.
A chiller should therefore be selected according to the actual temperatures required by the connected cooling equipment rather than simply its advertised capacity.
8. Check Chilled-Water Flow
Water flow is another important part of chiller selection.
The required flow depends on the cooling load and the temperature difference across the chilled-water system.
A simplified relationship can be expressed as:
Cooling capacity = mass flow × specific heat × temperature difference
For water systems, engineers commonly use the relationship between flow, Delta T, and cooling capacity to determine the required chilled-water flow.
The chiller manufacturer will also specify acceptable minimum and maximum evaporator flow rates.
This means the selected chiller should be checked against the project’s:
Chilled-water flow
Pump capacity
Pipe size
Pressure drop
Delta T
Entering-water temperature
Leaving-water temperature
9. Consider the Project’s Operating Profile
A building does not necessarily operate at its peak cooling load all day.
For example, an office may have:
Low load overnight
Increasing load in the morning
Peak demand during working hours
Reduced demand in the evening
An industrial facility may have a completely different load profile.
Understanding how cooling demand changes throughout the day helps determine whether one large chiller, multiple smaller chillers, or another configuration is more appropriate.
ASHRAE’s guidance on chiller selection emphasizes accurate determination of required capacity and notes that oversized equipment can lead to inefficient operation and problems such as frequent cycling.
10. Think About Part-Load Performance
A chiller rarely operates at 100% capacity all the time.
For this reason, part-load performance is important when comparing equipment.
A chiller may have excellent full-load efficiency but perform differently at lower loads.
When evaluating options, ask the manufacturer or supplier for performance information at the operating conditions relevant to your project.
Consider:
Full-load efficiency
Part-load efficiency
Minimum operating capacity
Compressor control method
Number of compressors
Variable-speed operation where applicable
Expected annual operating profile
For long-term projects, evaluating lifecycle performance rather than only purchase price can help produce a better overall decision.
11. Don’t Automatically Add a Large Safety Margin
It is common to hear recommendations such as adding 10%, 15%, or 20% to the calculated cooling load.
However, this should not be treated as a universal rule.
ASHRAE’s liquid-chilling guidance specifically states that automatically adding 10–20% to accurate load estimates is unnecessary and can increase equipment and installation costs while reducing efficiency. It also notes that oversized equipment can experience operational problems such as frequent cycling.
There may be legitimate reasons for additional capacity in certain applications, particularly where future expansion, unusual process conditions, or reliability requirements justify it.
But any allowance should be based on the project’s actual requirements rather than an arbitrary percentage.
12. Consider Future Expansion
Future growth can affect chiller selection.
If a facility is expected to expand, adding equipment later may be preferable to purchasing a significantly oversized chiller today.
Depending on the application, a modular arrangement can provide flexibility.
For example, instead of using one very large unit, a project might use several chillers that can operate together at high demand and independently at lower demand.
This can provide:
Better redundancy
Easier maintenance
Improved part-load operation
Capacity flexibility
Easier future expansion
The best configuration depends on the project’s load profile and reliability requirements.
13. Determine Whether You Need Redundancy
Capacity and redundancy are two different considerations.
Suppose a facility requires 1,000 kW of cooling. Installing a single 1,000 kW chiller may satisfy the calculated load, but the facility could lose all cooling if that unit becomes unavailable.
A critical facility might instead use multiple chillers so that some cooling capacity remains available if one machine is offline.
Redundancy can be particularly important for:
Data centres
Hospitals
Manufacturing
Process cooling
Cold-storage applications
Critical commercial facilities
The appropriate redundancy strategy should be determined from the consequences of cooling failure and the project’s operational requirements.
14. Match the Chiller Type to the Application
Chiller size should not be considered separately from chiller type.
Depending on the application, you may need to compare:
Air-cooled chillers
Water-cooled chillers
Screw chillers
Scroll chillers
Centrifugal chillers
Reciprocating chillers
Absorption chillers
The appropriate technology depends on capacity, efficiency requirements, available infrastructure, operating conditions, project duration, and application.
For example, a temporary construction project may prioritise rapid installation and mobility, while a large industrial facility may prioritise efficiency and continuous operation.
15. Consider Ambient Conditions
Chiller performance depends on the conditions under which it operates.
For air-cooled equipment, outdoor temperature directly affects heat rejection.
For water-cooled equipment, condenser-water temperatures and cooling-tower performance are important.
This is particularly relevant when selecting equipment for hot climates.
The chiller should be evaluated at the actual design conditions rather than relying solely on nominal catalogue capacity.
16. Check Electrical Requirements
The selected chiller must be compatible with the site’s available electrical infrastructure.
Confirm:
Voltage
Phase
Frequency
Full-load current
Starting requirements
Maximum current
Available transformer capacity
Generator compatibility where applicable
For rental or temporary projects, electrical capacity can become an especially important constraint because the available power infrastructure may be limited.
If the chiller will operate from a generator, both systems should be evaluated together.
17. Size the Supporting Equipment Too
Choosing the right chiller size is only part of designing a chilled-water system.
The supporting components must also be suitable.
These may include:
Chilled-water pumps
Condenser-water pumps
Cooling towers
Air-handling units
Fan-coil units
Heat exchangers
Expansion tanks
Filters
Valves
Controls
Pipework
An appropriately sized chiller cannot deliver the expected performance if the connected pumps, pipes, coils, or controls are incorrectly selected.
ASHRAE notes that cooling-load calculations affect the sizing of piping, ductwork, air handlers, chillers, coils, compressors, fans, and other system components.
18. Understand the Difference Between Building Load and Chiller Capacity
The building’s cooling load and the chiller’s rated capacity are related, but they are not exactly the same concept.
A building load calculation determines the amount of heat that needs to be removed under specified conditions.
The chiller’s rated capacity describes the cooling output it can provide under particular rating conditions.
The selected equipment must therefore be checked at the actual operating conditions.
A chiller advertised as a particular tonnage may not deliver exactly that capacity when the entering-water temperature, leaving-water temperature, condenser conditions, or flow conditions differ from its rating point.
19. For Rental Chillers, Consider the Temporary Application
If the project requires a rental chiller rather than a permanent installation, the sizing process should still begin with the actual cooling requirement.
Temporary cooling may be required for:
Planned chiller replacement
Emergency cooling
Construction projects
Facility expansion
Events
Industrial processes
Data centres
Temporary production requirements
Rental applications may also have additional constraints involving transport, installation space, temporary pipework, electrical supply, and project duration.
The rental supplier should receive enough technical information to select equipment based on actual operating conditions.
20. Use Professional Load-Calculation Methods
For a significant commercial or industrial project, chiller sizing should be based on a professional engineering calculation rather than a simple online rule of thumb.
ASHRAE’s current guidance identifies the Load Calculation Applications Manual as a comprehensive resource for cooling-load calculations. ASHRAE also states that load calculations should follow appropriate methodologies, with ASHRAE Standard 183 providing requirements for peak cooling and heating load calculations in applicable buildings.
Depending on the project, engineers may use methods such as:
Heat Balance Method
Radiant Time Series Method
Approved load-calculation software
Detailed hourly simulation
The appropriate method depends on the size and complexity of the project.
A Simple Chiller-Sizing Workflow
A practical sizing process can be summarised as follows:
Step 1: Determine the building or process cooling load.
Step 2: Identify the peak design condition.
Step 3: Establish indoor temperature and humidity requirements.
Step 4: Calculate internal and external heat gains.
Step 5: Include ventilation and infiltration loads.
Step 6: Determine chilled-water supply and return temperatures.
Step 7: Calculate the required chilled-water flow.
Step 8: Evaluate the project’s operating profile.
Step 9: Select the appropriate chiller type.
Step 10: Check capacity and efficiency at actual operating conditions.
Step 11: Determine whether redundancy or future expansion is required.
Step 12: Verify pumps, pipework, electrical supply, controls, and other supporting equipment.
Step 13: Compare lifecycle costs and expected operating performance.
Step 14: Have the final selection reviewed by a qualified HVAC professional.
Common Chiller-Sizing Mistakes
Using Only Floor Area
Floor area alone does not represent the complete cooling load.
Automatically Adding 20% Capacity
A blanket safety margin can result in an unnecessarily oversized system. ASHRAE specifically warns against automatically adding 10–20% to accurate load estimates.
Ignoring Part-Load Operation
A chiller that performs well at full load may not be the best option for a building that spends most of its operating hours at partial load.
Ignoring Water Temperatures
The required entering and leaving water temperatures affect chiller performance and should be included in equipment selection.
Forgetting Ventilation Loads
Outdoor air can contribute significant sensible and latent cooling requirements.
Choosing Capacity Without Considering Redundancy
Meeting the calculated load does not automatically mean the system has sufficient backup capacity.
Comparing Equipment Only by Price
The cheapest chiller may not have the lowest lifecycle cost. Efficiency, maintenance, reliability, and operating conditions should also be considered.
Final Thoughts
Choosing the right chiller size starts with understanding the actual cooling requirement of the project. Floor area and simple rules of thumb can provide a rough starting point, but they should not replace a proper cooling-load calculation.
The final selection should consider peak load, internal and external heat gains, ventilation, chilled-water temperatures, flow requirements, ambient conditions, part-load performance, electrical requirements, future expansion, and redundancy.
Most importantly, avoid the assumption that bigger is always better. ASHRAE’s guidance shows that unnecessary oversizing can increase capital costs, operating costs, and efficiency problems, while accurate load calculations provide a more reliable basis for selecting chiller capacity.
For a commercial, industrial, or temporary cooling project, the best approach is to provide a qualified HVAC engineer or chiller supplier with complete project information and have the equipment selected according to the actual design conditions. This helps ensure the chiller provides the required cooling without paying for capacity the project does not need.