A cooling load calculation estimates how much heat must be removed from a space to hold the design indoor condition on a design day. It is the foundation of everything that follows: airflow, coil capacity, chiller size, duct and pipe sizes, and electrical service. Get it wrong and every downstream decision inherits the error.
Commercial cooling loads are harder than heating loads because they change hour by hour. The sun moves, people arrive and leave, and the building mass stores heat and releases it later. This guide explains the components of a cooling load, the methods used to calculate it, and how to review the results.
Heat gain versus cooling load
Two terms are often confused:
- Heat gain is the rate at which heat enters or is generated in a space at a given moment.
- Cooling load is the rate at which heat must be removed from the room air to hold its temperature.
They differ because radiant heat (from the sun, lights and people) is first absorbed by floors, walls and furniture, then released to the air later. Only convective gains become cooling load immediately. A heavy concrete building can delay and flatten the peak by several hours compared with a lightweight one. This is why simply adding up instantaneous gains overstates the peak.
Components of a commercial cooling load
| Component | Type | Notes |
|---|---|---|
| Solar gain through glazing | Sensible | Often the largest perimeter load; depends on orientation, SHGC and shading |
| Conduction through glazing | Sensible | Driven by outdoor to indoor temperature difference |
| Conduction through walls and roof | Sensible | Delayed by mass; sun-heated surfaces raise effective temperature |
| People | Sensible and latent | Varies with activity level |
| Lighting | Sensible | Part radiant, part convective; return plenums can capture some |
| Equipment and plug loads | Sensible, sometimes latent | Computers, kitchen equipment, process loads |
| Infiltration | Sensible and latent | Small in pressurized buildings, but not zero |
| Ventilation (outdoor air) | Sensible and latent | A coil load, not a room load, unless delivered untreated |
| Fan and duct heat gain | Sensible | Supply fan heat and duct gains in hot spaces |
Room loads set the supply airflow. Coil loads include room loads plus ventilation and system gains, and set coil and plant capacity. Mixing the two is a common source of error. For detail on the internal components, see internal heat gains from people, lighting and equipment, and for windows, solar heat gain through glazing.
Calculation methods
ASHRAE Handbook, Fundamentals describes the accepted methods.
Heat balance method
The heat balance method solves energy balances on every surface and the room air for each hour. It is the most rigorous approach and is the engine behind tools based on EnergyPlus and similar simulation programs.
Radiant time series method
The radiant time series (RTS) method is a simplified procedure derived from heat balance. It splits each gain into radiant and convective parts, then uses time series factors to spread the radiant portion over the following hours. It is well suited to design calculations and is transparent enough to check by hand.
Older methods
Earlier methods such as CLTD/CLF and TETD/TA are still embedded in some legacy spreadsheets. They are no longer recommended by ASHRAE for new work, and results should be compared with a current method if they are used.
Commercial software (for example Carrier HAP, Trane TRACE 3D Plus or IES VE) implements these methods. The tool matters less than the quality of the inputs and the engineer's review.
Inputs that drive the answer
- Outdoor design conditions: dry-bulb and coincident wet-bulb for the location, typically from NBC Appendix C or ASHRAE climatic data. See selecting outdoor design conditions for Canadian cities.
- Indoor design condition: commonly 23 to 24 °C with a relative humidity target, chosen in line with ASHRAE Standard 55.
- Envelope: actual U-values including thermal bridging, glazing SHGC, shading devices and orientation.
- Internal gains and schedules: realistic occupancy density, lighting power density and plug loads, with hourly schedules.
- Ventilation rates: from ASHRAE 62.1 or the applicable code, with any energy recovery accounted for.
- Building mass and construction: affects time lag and peak timing.
- Zoning: each exposure peaks at a different time, so zone loads and block loads differ.
Zone peak versus block load
Each zone is sized for its own peak. An east office peaks in the morning; a west office peaks late afternoon. The air handler or chiller serving both sees the block load, the largest simultaneous sum, which is always smaller than the sum of zone peaks.
| Item sized | Load to use |
|---|---|
| Terminal units, diffusers, branch ducts | Zone peak |
| Air handler supply airflow (VAV) | Block load airflow |
| Cooling coil | Block coil load at the peak hour |
| Chiller plant | Block load for all systems served, plus distribution gains |
Sizing a chiller to the sum of zone peaks is one of the most common causes of oversized plants. See diversity and safety factors in load calculations for how to handle this properly.
Reviewing a load report
Before equipment is selected, a reviewer should check:
- The design month and hour of the peak make sense for the orientation and glazing.
- Occupant density matches the actual use, not a code maximum applied everywhere.
- Lighting and plug loads reflect current LED lighting and modern office equipment.
- Ventilation is entered once, not both as a room load and a coil load.
- Safety factors are applied once, at a stated location, and are documented.
- Normalized results (W/m² of floor area, m² per kW) fall within a reasonable range for the building type. Use these only as a check, never as a substitute for the calculation.
Canadian considerations
In most of Canada, cooling design wet-bulb temperatures are moderate, and outdoor air loads can be dominated by sensible heat. Southern Ontario and parts of Quebec have more humid summers where latent loads matter more. Code requirements for ventilation and energy (NBC Part 6, NECB 2017 or 2020 depending on the province) affect inputs, and some jurisdictions require load calculations to be submitted with permit drawings. Confirm requirements with the authority having jurisdiction.
Frequently asked questions
Can I size a rooftop unit from floor area alone?
Not reliably. Floor area ignores glazing, orientation, occupancy and ventilation. Square-metre rules can be useful to sanity check a calculation, but they routinely produce oversized equipment.
Why does the peak occur in September for some spaces?
South-facing glazing receives more direct solar gain when the sun is lower in the sky. For heavily glazed south zones in Canada, the cooling peak can occur in late summer or early fall rather than July.
Should ventilation be included in the room load?
Only if outdoor air is delivered directly to the room without being conditioned. In most systems, it is a coil load and is handled separately.
Key takeaways
- Cooling load differs from heat gain because building mass delays radiant gains.
- Use a current ASHRAE method such as heat balance or radiant time series.
- Size zones on zone peaks and central equipment on the block load.
- Input quality, especially occupancy, lighting, plug loads and glazing data, drives accuracy.
- Apply safety factors once and document them; a careful review prevents costly oversizing.
- When a load needs an independent check, our HVAC design team can review the calculation.
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