A typical office worker generates about 120 watts of heat—equivalent to a bright incandescent bulb—just by sitting at their desk. This metabolic heat output, often overlooked in preliminary calculations, can account for 20-30% of a room's total cooling load in densely occupied spaces.
The Formula
Let's break down the cooling load formula variable by variable. The core equation is: coolingLoad = envelopeLoad + occupantLoad + equipmentLoad + lightingLoad. Each term represents a distinct heat source that HVAC systems must counteract.
First, tempDiff = |outdoorTemp − indoorTemp| calculates the temperature gradient driving heat transfer through the building envelope. This absolute value ensures we're always calculating heat gain, regardless of whether it's cooling or heating season. The envelope load term area × tempDiff × 6.0 × (ceilingHeight / 2.7) deserves special attention. The constant 6.0 represents the overall heat transfer coefficient in W/m²·K for typical construction, while the ceiling height adjustment normalizes to a standard 2.7-meter room height. This scaling factor accounts for increased wall surface area in taller spaces, which directly impacts conductive heat transfer.
The occupant load occupants × 120 models sensible heat from people at sedentary activity levels. This 120W per person represents a reasonable average between the 75W minimum for resting and 150W for light office work. Equipment and lighting loads are user-specified because these vary dramatically by space type—a server room versus a conference room, for example. The formula's elegance lies in its separation of external (envelope) and internal (occupant, equipment, lighting) heat gains, allowing engineers to analyze and optimize each component independently.
Worked Example 1
Consider a 50 m² office with 3-meter ceilings, designed for 8 occupants. Outdoor design temperature is 35°C with an indoor target of 24°C. Equipment includes computers and printers totaling 800W, while LED lighting adds 400W.
First, calculate temperature difference: tempDiff = |35 − 24| = 11°C
Envelope load: 50 × 11 × 6.0 × (3.0 / 2.7) = 50 × 11 × 6.0 × 1.111 = 3,666W
Occupant load: 8 × 120 = 960W
Total cooling load: 3,666 + 960 + 800 + 400 = 5,826W
Convert to practical units: 5.8 kW or approximately 1.66 tons (5,826 ÷ 3,517). This tells us we need an air conditioner capable of removing 5.8 kilowatts of heat continuously to maintain comfort.
Worked Example 2
Now examine a 1,200 ft² retail space with 12-foot ceilings (3.66 meters), 15 customers plus staff, and significant lighting and display equipment. Outdoor temperature is 95°F (35°C) with indoor target of 75°F (24°C). Convert area: 1,200 ft² ≈ 111.5 m².
Temperature difference remains 11°C (20°F difference).
Envelope load: 111.5 × 11 × 6.0 × (3.66 / 2.7) = 111.5 × 11 × 6.0 × 1.356 = 9,985W
Occupant load: 15 × 120 = 1,800W
Assume equipment and lighting total 2,500W for displays and ambient lighting.
Total cooling load: 9,985 + 1,800 + 2,500 = 14,285W
This converts to 14.3 kW or approximately 4.06 tons. The higher ceiling and larger area significantly increase the envelope load compared to the office example.
What Engineers Often Miss
First, many engineers use average rather than design temperatures. The 35°C outdoor temperature in our examples represents a design condition—the temperature exceeded only 1-2% of hours annually. Using average summer temperatures (typically 5-8°C lower) would undersize equipment by 30-40%. Second, solar heat gain through windows often equals or exceeds the entire envelope load calculated here. Even with the formula's 6.0 W/m²·K coefficient, unshaded west-facing windows can add 300-500 W/m² during peak afternoon hours. Third, equipment heat loads evolve. That 800W office equipment estimate might double with monitor upgrades, additional peripherals, or server additions, requiring capacity headroom that many specifications omit.
Try the Calculator
While understanding the mathematics is essential, practical engineering requires efficient tools. The Cooling Load Calculator implements this exact formula with unit conversions and instant results, letting you focus on design decisions rather than arithmetic. Use it to quickly compare scenarios or validate manual calculations during preliminary HVAC sizing.
Originally published at calcengineer.com/blog
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