The project: a 2,400 sq ft, 2-story, 4-bedroom single-family residence in a mixed-humid climate (Climate Zone 4), 2x6 walls at R-19, R-49 attic, low-E double-pane windows. Every downstream step — equipment selection, duct design, ventilation, controls, commissioning — is sized against the load this step calculates.
ACCA Manual J is the industry-standard method for calculating a residence's actual heating and cooling load — room by room, based on the house's real envelope (wall/roof/window R-values), orientation, infiltration rate, and internal gains, not a rule-of-thumb like "500 sq ft per ton." Manual J load calculation is the reason two houses of identical square footage can need meaningfully different equipment sizes: envelope quality, window area and orientation, and air infiltration rate all drive the actual load independently of floor area.
For this house, the envelope assumptions above (R-19 walls, R-49 attic, low-E double-pane windows) put it solidly in "code-minimum-plus" territory for Climate Zone 4 — not a high-performance envelope, but not the loose, high-infiltration construction that inflates load calculations on older housing stock. A Manual J run against this envelope, at design-day conditions for a mixed-humid climate, typically lands this size and construction quality of house in the 2.5–3 ton range for cooling — this project uses 2.5 tons (30,000 BTU/h cooling) as the calculated design load, with a corresponding heating load sized for the heat pump and backup-heat sizing in Step 2.
Manual J output isn't just a single whole-house tonnage number — it's a room-by-room load breakdown, which is what Step 3's duct design actually sizes against (a bedroom with two exterior walls and west-facing windows needs meaningfully more supply airflow than an interior bathroom of the same square footage). Oversizing the equipment "to be safe" is a common and genuinely costly mistake: an oversized system short-cycles, never runs long enough to properly dehumidify in a mixed-humid climate, and wears out faster from more frequent starts — Manual J exists specifically to prevent that by sizing to the real calculated load, not a rounded-up guess.