How to Calculate HVAC Load

Start with design conditions

A load is the rate of heating required or cooling removed to maintain a selected indoor condition at a selected outdoor design condition. Record the location, indoor temperature and humidity target, and the design outdoor condition before entering any building data.

Build the component list

Component	Information needed	Common effect
Envelope	Area, U-value, temperature difference	Heating loss and cooling gain
Windows and solar	Area, orientation, glazing, shading	Often changes cooling peak timing
Outdoor air	Ventilation/infiltration airflow and conditions	Sensible and latent load
Internal gains	People, lighting, equipment, schedules	Cooling gain; may offset heating loss

Use the right level of method

For a preliminary review, aggregate documented component loads and keep every assumption visible. For residential equipment sizing, ACCA Manual J is a recognized load procedure; Manual S then uses the load with manufacturer data to select equipment. Commercial projects generally need a detailed ASHRAE load method appropriate to the building and schedules.

Example: transparent preliminary total

Suppose a worksheet has 12,000 BTU/h of envelope load, 3,000 BTU/h of solar, 1,500 BTU/h from people, 1,024 BTU/h from 300 W of lighting, and 2,160 BTU/h of outdoor-air sensible load. The visible sensible total is 19,684 BTU/h, or 1.64 tons. This is not a final cooling selection because latent load, time effects, and equipment performance still require review.

Common mistakes

Using floor area as the full model instead of documenting envelope and air-side inputs.
Mixing climates, temperatures, units, or schedules from different scenarios.
Turning a load in BTU/h directly into equipment selection without checking performance data and distribution.

Next step

After establishing the load, convert capacity only for comparison, then check airflow and duct design separately.