Duct system calculator
Lay out the duct run from the fan / AHU to every outlet: the tool sums the airflow, sizes the ducts, computes the loss of each section (duct, ASHRAE fittings, Starduct fire dampers, VCDs, VAV boxes, silencers, outlets), finds the index run and the external static pressure (ESP) for fan selection, and gives the resistance table and the bill of quantities of the whole system.
Duct system calculator on the web — 3-minute demo · 2:51
Lay out the duct run from the fan to every outlet, size the ducts, compute the losses, pick Starduct devices with order codes and get the bill of quantities — a three-step walkthrough on the tool's built-in example.
Beta — preliminary results; the designer must check them before use. What you enter stays in your browser.
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1. System settings
As-built mode: enter the installed size of every section (Size column switches to Fixed); the tool reports losses, velocities and the balancing table for that system.
Duct-borne noise (optional)
Enter the fan Lw per octave band from the fan catalogue (blank = not calculated). The tool subtracts straight-duct, elbow, branch-split and end-reflection attenuation per ASHRAE Ch. 49 and Starduct silencer IL, then room Lp (Schultz) → NC. Regenerated noise at elbows, dampers and throttled VCDs is not included.
2. Duct network
Each row is a straight duct section between two junctions. A root section connects to the source (fan / AHU); child sections name their "parent". End sections give the number of outlets and the airflow per outlet — upstream airflows are summed automatically. At a junction the child with the largest airflow is the main run; the others branch off through a tee.
3. Results
Add a duct section to start, or press "Load example".
ESP = loss of the index run from the source outlet, excluding the fan / AHU internal losses (filters, coils…).
IP display converted from SI by hvac-units; Excel / CSV exports stay in SI.
Run diagram
Single-line calculation schematic (drawing convention): root on the left, main run continues straight, branches up/down; orange = index run ★. Junction symbol by the branch tee type (box = 90° tee · Y = wye / dovetail · trapezoid = 45° boot · circle = round branch) with the ASHRAE code; trapezoid at a section start = reducer / expansion; right angle = 90° elbows (×n); diamond = user-added fitting. Devices: crossed box = fire damper · slashed circle = VCD · VAV box · hatched box = silencer · zigzag = flex connector. Each outlet drawn (zigzag = flexible duct to the outlet, scaled by length). Every symbol is a component included in the pressure-drop sum.
Resistance table
ΔP in Pa · ★ = section on the index run
Balancing table (T&C)
Excess at each leaf = index-run ESP − loss to that leaf. Throttling at a VCD / VAV takes the part common to the whole branch below; the remainder goes to the damper at the outlet. ζ required = ΔP / (ρv²/2) of the section. Blade angle is not derived: damper@4 has no ΔP–angle curve yet (AMCA 500-D test pending).
Noise at outlets (NC)
Final NC = max of: duct-borne NC (fan Lw along the run), outlet NC (outlet engine), VAV discharge NC (VAV engine, AHRI 885 App. E). Silencer hint: first branch above the outlet, sized to the section; "transition needed" when the silencer is larger than the duct.
Selected Starduct devices
Each device opens its product page, catalogue, DoP and the line selector (model, size and airflow pre-filled). "Add devices to the project list" writes to the same list every Starduct selector uses, so Submittals can be printed per line.
Bill of quantities (no prices)
Operation / what-if
Enter the fan curve (3–5 Q–P points from the fan catalogue) to find the actual operating point against the system curve P = ESP·(Q/Q_fan)². VAV load 30/60/100 %: box MOP and NC from the VAV engine at the part-load flow, box size unchanged; run ESP ∝ load² (constant-speed fan). Closed-branch scenario: drop the section and its subtree and re-solve.
Operating point:
VAV load 30 / 60 / 100 %
Scenario: close branches (fire damper / VCD shut) — pick sections
Option B
Calculation basis
Duct friction: Haaland (Colebrook–White approximation), roughness by material. Fittings: ASHRAE Duct Fitting Database coefficients (elbow CR3-1 r/W = 1.5; transitions θ = 30°; tees SR5 / ER5). Sizing: rectangular W/H ≈ 1.6 within the max. height, rounded to 50 mm; round to 25 mm. Starduct devices are computed with the same engine as each line's catalogue (Data source column). Core inherited from Starduct VSD. Results are preliminary and exclude fan / AHU internal losses.
