It is tempting to size HVAC equipment by rule of thumb — a certain number of square feet per ton, for example. But buildings vary widely in orientation, glazing, insulation, occupancy and internal loads. A load calculation accounts for those differences and gives the design a defensible starting point.
What a load calculation considers
| Factor | Why it matters |
| Envelope | Walls, roofs and windows gain and lose heat at different rates. |
| Orientation & glazing | Solar gain through glass can dominate the cooling load. |
| Occupancy | People add sensible and latent heat. |
| Lighting & equipment | Internal gains add to the cooling load. |
| Ventilation | Outdoor air must be heated, cooled and dehumidified. |
The cost of oversizing
Oversized cooling equipment tends to short-cycle: it satisfies the thermostat quickly but may not run long enough to remove moisture effectively, and frequent cycling can reduce efficiency and comfort.
The cost of undersizing
Undersized equipment may not maintain comfortable conditions on design days, leading to complaints and costly corrections after occupancy.
Loads also drive the rest of the design
- Airflow for each space, which sets duct and diffuser sizes
- Ventilation and exhaust quantities
- Energy-code documentation for the mechanical systems
Starting with an accurate load calculation makes every downstream decision — equipment, ductwork and controls — easier to justify during design and review.
