Guide · ACCA Manual D
How to size HVAC ductwork, room by room
Six steps. One friction rate for the whole system, one velocity check per run, and no guessing.
1 · Equipment CFM from tonnage
Cooling airflow averages 400 CFM per ton. A 2-ton system moves ~800 CFM; a 3-ton system ~1,200 CFM. (Heating needs roughly 65% of cooling airflow — sizing to cooling covers both.) Punch tons into the calculator's Advanced helper and it fills CFM for you.
2 · Room CFM shares
Room CFM = (room load ÷ whole-house load) × equipment CFM. Example: a 2,000 BTU room in a 24,000 BTU house on a 2-ton (800 CFM) system needs (2,000 ÷ 24,000) × 800 ≈ 67 CFM. Rule of thumb when loads are unknown: ~1 CFM per square foot (≈2 CFM for sun-blasted rooms with big windows).
3 · Available static pressure (ASP)
Read the blower's total external static pressure from the equipment specs (often 0.5 in.wg), then subtract component drops: ~0.03 each for supply register, return grille, and balancing damper, plus the filter's rated drop (~0.10–0.15). Example: 0.5 − 0.03 − 0.03 − 0.15 = 0.29 in.wg ASP. Never design past this budget.
4 · Total effective length (TEL)
Measure the straight duct from the farthest supply outlet, through the equipment, to the farthest return — then add every fitting's equivalent length (elbows, tees, transitions each count as many feet of straight duct). Example: 50 ft of straight + 150 ft of fitting equivalents = 200 ft TEL. Size the whole system to the most restrictive (longest) run.
5 · Friction rate
FR = (ASP × 100) ÷ TEL. With 0.29 ASP and 200 ft TEL: (0.29 × 100) ÷ 200 = 0.145 in.wg/100ft — a generous budget, so standard sizes will do. Most homes land at 0.06–0.10; if you land below 0.06, plan bigger ducts and gentle fittings. Tap “Use this rate” in the calculator to apply it.
6 · Worked examples
Bedroom 150 CFM @ 0.08: exact ≈ 6.9″ → 7″ round at ~560 FPM. Quiet. Rect ≈ 10×6.
Living branch 400 CFM @ 0.08: exact ≈ 10.2″ → 12″ round at 509 FPM. Rect 14×8.
3-ton trunk 1200 CFM @ 0.10: exact ≈ 14.6″ → 16″ round at ~860 FPM. Rect ≈ 22×10.
Flex and fittings — where jobs fail
Size flex about one diameter up from metal at the same friction rate, and only when pulled fully taut and strapped every 4 ft. Compressed or sagging flex can double resistance and starve rooms — the #1 cause of “new system, weak airflow” callbacks. Keep rectangular aspect at or below 4:1; deeper beats wider.