I'm laying out ductwork for a new 3-ton system.

Manual D Duct Sizing Steps:

1. Start with the furnace/air handler - Note the blower's total CFM (typically 400 CFM per ton, so ~1,200 CFM for 3-ton).

1. Lay out the trunk line - This is the main supply duct running from the air handler. Size it based on the total CFM it needs to carry and the available space. For 1,200 CFM, you'll likely use 10" x 16" or 12" x 14" rectangular duct (or 14" round).

1. Work downstream to branches - Each branch takes off from the trunk to feed a room or zone. Calculate the CFM needed for each branch based on Manual J room loads.

1. Size each branch - Use the Manual D worksheet or ductulator. Input the branch's CFM and the maximum friction rate (typically 0.10 inches water column per 100 feet for supply). The tool gives you the required duct size.

1. Check velocity - Ensure velocities stay reasonable (supply: 600-900 FPM, return: 500-700 FPM) to avoid noise.

1. Account for fittings - Add equivalent length of elbows, transitions, and fittings to your total duct length when calculating pressure drop.

Key Rule: The trunk must be sized to carry the sum of all downstream branch CFMs. As you add branches, the remaining trunk CFM decreases, so you may need to reduce trunk size partway down.

Pro tip: Use a ductulator or online calculator - Manual D tables are tedious. Always verify your total system pressure drop matches the blower curve.

If you're doing this for a real install, have a pro review your numbers - improper duct sizing kills efficiency.

Manual D in Plain Terms: The "Friction Rate" Method

You don't guess duct sizes. You calculate them based on available pressure and total effective length (TEL).

1. Find Your Blower's "Budget" (Available Static Pressure - ASP) Look at the equipment specs (blower performance table). Find the static pressure at your design airflow (3-ton = ~1200 CFM).

• Example: Blower delivers 1200 CFM at 0.50" w.c. ESP.
• Subtract component losses: Coil (wet), filter (clean), registers/grilles, balancing dampers.
• Example: Coil (0.15") + Filter (0.10") + Registers (0.06") = 0.31" loss.
• ASP = 0.50" - 0.31" = 0.19" w.c. This is your total "budget" for duct friction.

2. Calculate Total Effective Length (TEL)

• Measure the longest supply run (plenum to boot) + longest return run (grille to plenum) in straight-line feet.
• Add equivalent lengths for every fitting (elbows, transitions, takeoffs, boots) on those two longest paths. Use ACCA Manual D Appendix 3 (or manufacturer data).
• Example: Longest supply = 40 ft + 120 ft fittings = 160 ft. Longest return = 20 ft + 30 ft fittings = 50 ft.
• TEL = 160 + 50 = 210 ft.

3. Calculate Friction Rate (FR) $$FR = \frac{ASP \times 100}{TEL}$$

• Example: $(0.19 \times 100) / 210 = \mathbf{0.09 \text{ in.w.c./100 ft}}$
• This is your design friction rate. Plot this on the ductulator (or use software).

4. Size the Trunks & Branches

• Trunks: Size for total CFM moving through that section at your FR (0.09). Velocity should stay 700–900 FPM (supply) / 600–700 FPM (return).
• Branches: Size each for its specific room CFM (from Manual J) at the same FR (0.09).
• Rule: If a branch size calculates < 4" round, use 4" minimum. If velocity exceeds 900 FPM, go up a size (lower FR locally).

5. Verify & Balance

• Ensure total system CFM matches design.
• Install balancing dampers on every branch.
• Code Check: IRC M1601.1 / IMC 603.2 require ducts sized per Manual D (or approved method). Undersized ducts = high static = compressor failure / frozen coil / noise.

Pro Tip: If your