What Manual J Actually Is
ACCA Manual J (published by the Air Conditioning Contractors of America) is the industry-standard method for calculating the heating and cooling load of a residential building — the rate at which heat enters or leaves the space that the HVAC equipment has to overcome to maintain a target indoor temperature. It is not a single formula but a structured procedure: room-by-room, it accounts for every meaningful heat gain and loss path — walls, roof, windows, doors, floors, infiltration, occupants, and internal equipment — and sums them into a total design load, expressed in BTU/hr.