Load Calculations, Duct Design, and Installation Practices
Sizing is a calculation, not a rule of thumb
Manual J produces the room-by-room load from design conditions, envelope construction, windows, infiltration, and internal gains. Manual S selects equipment to match that load - including sensible and latent capacity at the actual entering conditions, not just nominal tons. Manual D sizes the ducts; Manual T handles register selection and throw. A question offering "500 square feet per ton" as the method is testing whether you know that shortcuts are not acceptable practice.
Duct design
The equal-friction method is standard: pick a friction rate, then read duct size from a duct calculator for each run. Friction rate = (available static pressure × 100) ÷ total effective length, where TEL includes the longest path plus fitting equivalent lengths. Residential total external static is commonly around 0.5 in. w.c., so a design that starts near that ceiling leaves nothing for a dirty filter. Velocity limits keep trunk runs roughly 600–900 fpm and branches lower to control noise.
Installation practice
Seal with mastic, not tape alone; support flex duct at the manufacturer's spacing, keep bends gentle, and keep runs short and straight. Ducts in a Texas attic need real insulation (commonly R-6 to R-8) and a sealed, insulated connection at the air handler. Returns matter as much as supplies: undersized returns raise static and cut airflow.
Worked example
Available static 0.30 in. w.c., total effective length 250 ft. Friction rate = (0.30 × 100) ÷ 250 = 0.12 in. w.c. per 100 ft. Use that number on the duct calculator for every run.