Load Calculation, Duct Design, and Air Distribution
This domain tests ACCA Manual J loads, Manual S equipment selection, Manual D duct sizing, and the airflow math that ties them together. Louisiana designs are dominated by latent load, so the exam favors humidity-aware answers.
Load calculation
- Manual J, eighth edition, is room-by-room: orientation, window U-factor and SHGC, insulation, infiltration, duct losses, and internal gains.
- Louisiana summer design is roughly 92 to 95 F dry bulb with indoor 75 F and 50 percent relative humidity; winter design about 30 to 35 F.
- Sensible heat ratio often lands between 0.70 and 0.80. That means 350 to 400 CFM per ton, not a blind 400.
- Manual S selects equipment whose capacities at design conditions cover the load; Manual T selects registers and diffusers for throw and noise.
Airflow math
- Sensible: CFM = Btu/h sensible divided by (1.08 x temperature difference).
- Latent: CFM = Btu/h latent divided by (0.68 x grains difference).
- Total: CFM = Btu/h total divided by (4.5 x enthalpy difference).
Duct design
- Friction rate = available static pressure divided by total effective length, multiplied by 100.
- 0.1 in. w.c. per 100 ft is a starting assumption, not a design. Compute from actual available static and measured effective length.
- Keep supply trunks near 700 to 900 fpm and branches near 500 to 700 fpm to control noise.
Worked example
A room needs 24,000 Btu/h sensible with a 20 F temperature difference: 24,000 divided by (1.08 x 20) equals 1,111 CFM. Available static is 0.50 in. w.c.; subtract 0.10 for registers and grilles; total effective length is 400 ft. Friction rate = (0.40 / 400) x 100 = 0.10 in. w.c. per 100 ft.
Traps that cost marks
- Sizing from square footage or 400 CFM per ton.
- Ignoring attic duct gain and leakage in the load.
- Using outdoor dry bulb where the design calls for the indoor wet bulb.