Manual S and Manual D: the other two letters

ACCA's residential design sequence has three steps, and permit offices are asking for more of them every year.

Manual S — equipment selection

Takes the Manual J load and proves the specific unit you're installing matches it, using the manufacturer's expanded performance data at your design conditions — not the nominal tonnage on the box. Sizing limits apply (for cooling, typically up to ~115–125% of the sensible load depending on equipment type; heat pumps get more latitude in heating-dominated climates). This is how reviewers catch the 5-ton condenser on the 2.5-ton house.

Manual D — duct design

Turns room-by-room airflows from the load calc into duct sizes: friction rates, trunk and branch dimensions, register selection. Required mostly on new construction and full-system replacements. Undersized returns — the most common duct sin — choke otherwise correctly sized equipment.

What your permit likely needs

Equipment changeout: Manual J, often + S. New construction or new duct system: J + S + D. When in doubt, the rejection letter will tell you — or ask the plan desk before you submit, since resubmittals cost you a week either way.