By Tamika Brooks · Updated August 18, 2026 · 7 min read
Short answer
Chiller sizing starts with the peak cooling load in tons, found with a load calculation or measured flow and temperature difference: tons = GPM x delta-T / 24. Apply diversity for loads that do not peak together, add only a modest safety margin, and check part-load behavior, because chillers rarely run at full capacity. Oversizing causes short cycling, poor efficiency and humidity or control problems.
Chiller sizing means choosing a capacity, in tons of refrigeration, that covers the real peak cooling load without leaving the machine badly oversized for the other 95 percent of the year. The core relationship is simple: tons equal gallons per minute times the water temperature difference, divided by 24. The difficult part is knowing the load itself, deciding how much diversity and margin to include, and checking how the plant behaves when it is not at peak.
How do you calculate chiller tons?
One ton of refrigeration equals 12,000 BTU per hour. For a water loop:
Tons = GPM x delta-T (°F) / 24
This comes from the heat equation for water, where roughly 500 BTU per hour is moved per GPM per °F (BTU/hr = 500 x GPM x delta-T), divided by 12,000.
Worked example: a process loop flows 240 GPM and the water rises 10°F across the load, from 44°F supply to 54°F return.
- 240 x 10 / 24 = 100 tons.
For a glycol mix, the heat capacity is lower than for water, so the multiplier drops, and the actual factor depends on the concentration and temperature. Use the glycol supplier’s data. See glycol chillers explained for context.
This formula is useful for checking an existing system with measured flow and temperatures, or for working backward from a process requirement. For a new building, a load calculation comes first.
How is the building or process load determined?
For comfort cooling
Cooling loads come from a heat-gain calculation that accounts for:
- Envelope: walls, roof and windows, including solar gain
- Occupants and their activity level
- Lighting and equipment (plug loads)
- Outdoor air for ventilation, often a large share of the total
- Infiltration and moisture (latent load)
Engineers typically use ASHRAE methodology from the ASHRAE Handbook of Fundamentals, usually with calculation software. Simple square-foot rules of thumb vary widely with climate and building type, and they are a poor basis for final selection.
For process cooling
Process loads are usually easier to define and harder to forecast:
- Heat released by the equipment, from motor power, friction, chemical or thermal processes, often given in the manufacturer’s data.
- Required fluid temperature and flow at each user.
- Duty pattern: continuous, batch or intermittent.
- Future expansion: planned lines or machines.
Measure where possible. Logged flow and temperature data from an existing plant tell you more than a nameplate sum does.
What is diversity, and why does it matter?
Diversity recognizes that not every load peaks at the same moment. If a campus has ten buildings and each has a peak load of 100 tons, the plant may never see 1,000 tons at once, because the buildings peak at different hours and some are partly unoccupied.
- Block load is the actual simultaneous peak of the whole system.
- Sum of peaks is the arithmetic total of every individual peak.
- Diversity factor is the block load divided by the sum of peaks, always 1.0 or lower.
Sizing to the sum of peaks is a common route to an oversized plant. But diversity must come from real data or a careful analysis, not from a guess. For process plants where everything runs together, diversity may be close to 1.0.
How much safety margin should you add?
There is no universal figure. Practice varies, and the right answer depends on how reliable the load data is. General guidance:
- Do not stack margins. If the load calculation includes a margin, the equipment selection includes another, and a designer adds a third, the total can be large and not visible.
- Margin for uncertainty, not for comfort. A modest allowance, often in the single-digit to low double-digit percent range, is typical when the load is well understood. Larger uncertainty can justify more, but a modular plant handles it better than a bigger machine.
- Plan for growth separately. Reserve space, piping and electrical capacity for a future chiller instead of installing today’s oversized machine.
- Check the rating conditions. Capacity published at standard conditions (for example 44°F leaving water and 95°F ambient for air-cooled machines) changes at hot ambient or colder fluid temperatures. Confirm the capacity at your actual design conditions. See air-cooled vs water-cooled chillers for why ambient matters.
What is part-load operation and why is it important?
A chiller runs at its design load only a small fraction of the hours in a year. The AHRI 550/590 standard weights part-load efficiency (IPLV) with 1 percent of hours at 100 percent load, 42 percent at 75 percent, 45 percent at 50 percent and 12 percent at 25 percent, reflecting typical building patterns. Real plants differ, but the principle holds: the efficiency at partial load is what drives energy cost. How these ratings are built is explained in chiller efficiency: kW per ton, COP and IPLV.
Ways to handle part load:
- Multiple chillers. Two or three machines let the plant stage capacity and keep each one in an efficient range.
- Variable speed drives. Especially effective on centrifugal and screw compressors.
- Compressor staging. Used on scroll and some screw designs.
- Variable primary flow and good control sequencing to avoid low delta-T problems.
- Thermal storage, which shaves the peak by shifting load to off-peak hours. See thermal energy storage explained.
What goes wrong when a chiller is oversized?
| Problem | Why it happens |
|---|---|
| Short cycling | A large machine satisfies the load too fast and shuts off, then restarts repeatedly. This wears compressors and starters. |
| Poor part-load efficiency | Many machines lose efficiency at very low load, and some cannot unload below a minimum (for example centrifugal surge limits). |
| Unstable temperature control | Narrow turndown range makes leaving water temperature hunt. |
| Higher first cost | Larger machine, larger electrical service, larger piping and pad. |
| Humidity problems | In comfort systems, short run times remove less moisture. |
| Hot gas bypass use | Some machines waste energy bypassing refrigerant to stay running at low load. |
What goes wrong when a chiller is undersized?
An undersized plant cannot hold setpoint on the hottest days. Leaving water temperature drifts up, process quality or space comfort suffers, and the machine runs flat out with no reserve. For critical loads such as data halls, labs or food safety, losing capacity is a real risk. This is why redundancy (N+1) is often considered separately from capacity sizing. In an N+1 plant, the required load can still be met when the largest machine is out of service.
A step-by-step sizing checklist
- Define the design supply temperature and temperature difference for every user.
- Calculate or measure the peak load of each user.
- Apply diversity based on real schedules and data.
- Add a documented margin and a future-growth allowance, separately.
- Choose a machine type and size range. See types of chillers.
- Split the load into two or more machines when part-load operation or redundancy matters.
- Verify capacity and efficiency at actual design conditions, including ambient or condenser water temperature, glycol percentage and fouling.
- Model the annual load profile to compare part-load performance.
- Confirm pump, pipe, electrical and refrigerant requirements, including the refrigerant outlook in refrigerants: what is changing in 2026.
Frequently asked questions
How many tons of cooling do I need per square foot?
There is no single answer. Rules of thumb for offices, for example, have historically ranged from about 250 to 400 square feet per ton, but climate, glazing, ventilation, occupancy and equipment can move the real number a long way. A load calculation is the only reliable method, and benchmarks should serve only as a sanity check.
Is it better to install one large chiller or several small ones?
For loads that vary a lot or must stay online, two or more machines are usually better. They improve part-load efficiency and give redundancy. A single large machine can be cheaper per ton and simpler for steady, high-load process work, if downtime risk is acceptable.
Does glycol change the size of the chiller?
Yes. Glycol lowers heat capacity and raises viscosity, and a lower fluid temperature reduces compressor capacity. The flow rate, pump head and the machine capacity all need to be rechecked at the actual glycol concentration and temperature.
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