Manual J load calculation

Manual J, published by the Air Conditioning Contractors of America (ACCA) and recognized as an ANSI standard, estimates heat loss and gain through the building envelope, windows, air leakage, occupants and internal loads at local design temperatures. Manual S guides choosing equipment to match those loads, and Manual D covers duct design. Many building codes reference these methods.

Sizing by square-footage rules of thumb tends to oversize equipment, causing short cycling, poor dehumidification, comfort complaints and higher cost.

For product teams, load calculations matter when building HVAC controls, digital twins or energy analytics, because the building's load profile determines how equipment should be sized and controlled.