How to Identify the Critical Duct Path
The critical path — the index run — is the single route through the system that the fan has to overcome. Get it wrong and the fan is selected on the wrong number. This guide shows how to enumerate paths, convert fittings to effective length, compare them, and turn the winner into a fan duty.
Last updated September 2026
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What makes a path critical
Air leaving the fan splits and rejoins at every tee, and each parallel route absorbs whatever pressure it needs. The fan only has to produce enough static pressure for the worst of them; the rest is thrown away at balancing dampers. That worst route is the critical path.
The critical path is about total loss, not distance. Length, fitting count, fitting quality, terminal type and any in-line device — fire damper, attenuator, coil, filter box — all count.
Step by step
- Mark every terminal on the layout and trace its route back to the fan discharge.
- For each route, sum the straight duct length section by section.
- Convert every fitting on the route to an equivalent length at that section's size and velocity, or total the fittings directly as C × Pv if you are working in pressure.
- Add the two to get the total effective length (TEL) for the route.
- Apply the design friction rate to the TEL, then add the terminal, damper and any in-line device loss to get the route total.
- The highest route total is the critical path. Size the fan on it.
Worked comparison of four paths
| Route | Straight duct | Fittings (equiv. length) | Total effective length | Duct loss (in. w.g.) | Route total |
|---|---|---|---|---|---|
| Path 1 — east perimeter VAV | 210 ft | 95 ft | 305 ft | 0.305 | 0.655 |
| Path 2 — west perimeter VAV | 150 ft | 190 ft | 340 ft | 0.340 | 0.690 |
| Path 3 — core VAV | 90 ft | 70 ft | 160 ft | 0.160 | 0.510 |
| Path 4 — conference, via fire damper | 165 ft | 235 ft | 400 ft | 0.400 | 0.850 |
Design friction rate 0.10 in. w.g. per 100 ft. Terminal and device losses added separately.
Path 4 — conference, via fire damper is the critical path, even though Path 1 has the most straight duct. The fire damper, the extra offsets and the higher terminal loss on that route outweigh 45 extra feet of clean duct elsewhere.
Total your own path fitting by fitting in the fitting loss calculator, and turn the result into a fan duty with the total external static pressure guide.
Field signs you picked the wrong path
- One zone never reaches design airflow no matter how far its damper opens — that zone is the real critical path.
- Every balancing damper in the building ends up nearly wide open; the fan was sized on an easier path and is short of pressure.
- Conversely, every damper is nearly shut and the system is noisy: the fan was sized on a path harder than any that exists.
- Airflow is fine at commissioning and fails a year later — the filter's dirty pressure drop was not in the path total.
Method and assumptions
- Method
- Total effective length comparison, then pressure totalling on the governing route.
- Friction rate
- 0.10 in. w.g. per 100 ft used in the worked example for illustration only.
- Fitting equivalents
- Fitting equivalent lengths must be derived at each section's own size and velocity; the example values are pre-computed.
- Terminal losses
- Entered as fixed values from the terminal schedule, not calculated here.
Limitations of this method
What the calculation on this page does not account for. Read these before using a number on a drawing or a submittal.
- Total effective length is a comparison shortcut. Once the likely critical path is identified, total it in actual pressure, section by section, before selecting a fan.
- The critical path can move. At part load in a VAV system, with a dirty filter, or after a tenant fit-out adds a branch, a different route can become governing.
- Equivalent lengths are velocity-dependent. Borrowing a fitting equivalent from one section and applying it to another size will mis-rank close paths.
- Return, relief and outside-air paths have their own critical routes and must be assessed separately from the supply side.
- This is a preliminary design exercise. Fan selection, system effect factors at the fan inlet and outlet, and code compliance must be confirmed by a qualified professional.
Frequently asked questions
What is the critical path in a duct system?
The critical path, also called the index run, is the route from the fan to a terminal that has the highest total pressure loss. It sets the external static pressure the fan must produce; every other path is balanced down to match it.
Is the critical path always the longest run?
No. It is the path with the greatest total loss, which is length plus fittings plus terminal. A shorter branch stacked with elbows, a fire damper and a throttled VAV box often beats a long, straight run.
How do I find the critical path quickly?
List every fan-to-terminal route, compute the total effective length of each — straight duct plus the equivalent length of every fitting — and take the largest. Then total the actual pressure on that one path including coil, filter and terminal losses.