A 2 ton system moves about 800 CFM. Split between a 900 sq ft layout of five rooms and sized at 0.08 in. w.g. per 100 ft, that needs a 20 × 8 in trunk at the air handler stepping down to 8 × 8 in, and round branches of 10, 8, 8, 7 and 5 in. This is the equal friction method from ACCA Manual D in its simplest form; a full Manual D design adds each fitting's pressure loss and the blower's real static pressure.
How the ductwork is calculated
- System airflow. Tons × 400 CFM, the typical airflow per ton Trane gives for central air conditioners and heat pumps. Or enter each room's CFM yourself, for example from a Manual J room-by-room report.
- Room airflow. Each room gets the system airflow in proportion to its floor area. Floor area stands in for the room's load, which is the rough part: a sunny room with big windows deserves more than its share.
- Branch size. Every branch is sized so its friction equals the design rate, using the round galvanized duct equation published by Engineering ToolBox, then rounded up to the next common round size.
- Trunk size. The trunk is divided into equal sections, one per branch, in the order you list the rooms. Each section carries the air still in it and is sized to the same friction rate, then converted to a rectangle of your chosen height with the Huebscher equivalent diameter.
Branch diameter (in) = (0.109136 × CFM^1.9 ÷ friction rate)^(1 ÷ 5.02)
Rectangular trunk: width where 1.30 × (w × h)^0.625 ÷ (w + h)^0.25 = the round diameter
Worked example
System: 2 tons, 800 CFM, friction rate 0.08, a 40 ft trunk 8 in high, 26 gauge.
| Room | Floor area | Airflow | Branch | Velocity |
|---|---|---|---|---|
| Living room | 320 sq ft | 284 CFM | 10 in | 522 fpm |
| Kitchen | 180 sq ft | 160 CFM | 8 in | 458 fpm |
| Bedroom 1 | 200 sq ft | 178 CFM | 8 in | 509 fpm |
| Bedroom 2 | 150 sq ft | 133 CFM | 7 in | 499 fpm |
| Bathroom | 50 sq ft | 44 CFM | 5 in | 326 fpm |
The trunk leaves the plenum at 800 CFM, 13.36 in round, which at 8 in high is 20 × 8 in (720 fpm). After the living room it drops to 14 × 8 in, then 12 × 8 in, then 8 × 8 in for the last two sections: three reducers. The calculator never makes the trunk narrower than it is tall.
Ductwork sizing and velocity
Friction sizing keeps pressure even; velocity keeps it quiet. ACCA Manual D's recommended maximums, as quoted by Energy Vanguard, are 900 fpm for supply ducts and 700 fpm for returns, and the velocity check flags any trunk section or branch above 900. For ducts in an unconditioned attic, Energy Vanguard cites 600 to 750 fpm as a better target. If a branch fails, raise its size one step in the duct size calculator or lower the friction rate.
The friction rate itself should come from the blower: Energy Vanguard defines it as the available static pressure × 100 ÷ the total effective length of the longest run, fittings included. Its example works out to 0.073, and it cautions that the popular 0.10 can leave ducts undersized.
Duct material list
The list under the result counts what to buy:
- Round pipe by size, in sections of the length you enter (5 ft by default; check your supplier), with the waste allowance: 45 ft of 8 in pipe plus 10% is 10 sections.
- Elbows, two per branch by default, take-off collars and register boots, one per branch, and a register per room.
- Rectangular trunk by size in feet, plus the reducers and an end cap.
- Insulation wrap for the outside surface of all the duct, 348 sq ft in the example, 383 sq ft with 10% for overlaps. IECC 2021 R403.3.1 asks R-8 on ducts in attics and R-6 elsewhere outside the heated space; ducts inside the envelope need none.
- Mastic or foil tape for every joint. ENERGY STAR estimates that 20 to 30% of the air in a typical duct system leaks out, and says to seal with mastic or metal tape, not duct tape.
Ductwork weight
Sheet metal weight is the duct surface times the sheet thickness times 41.82 lb per sq ft per inch, the basis of the manufacturers' standard gauge. At 26 gauge (0.0217 in, about 0.91 lb per sq ft) the example's 348 sq ft weighs about 316 lb, before seams, laps and hangers. Pick the gauge your code and supplier call for per size; lighter 28 or 30 gauge is common for small round pipe.
Transitions and offsets
Every trunk step-down is a reducer (a transition); the calculator counts one wherever the trunk width changes. Offsets and extra elbows around joists add pressure drop that a full Manual D converts to equivalent feet of duct. If a run has more than two turns, raise the elbows-per-branch field so the list is right, and consider one size larger for that branch.
Before this, and after
- A room's cooling or heat load: the BTU calculator.
- System size: the AC size calculator or the furnace size calculator.
- Airflow per room from air changes, or a single duct's CFM: the CFM calculator.
- Insulating the rest of the attic while the ducts are exposed: the insulation calculator.