What size duct for 800 CFM on a flex branch?
| Round ducttheoretical 17.1″ rounded up to a size that exists | 18″ |
|---|---|
| Actual velocitytarget for a flex branch is 500 fpm | 453fpm |
| Target velocityinsulated flex to a register — the corrugated wall costs you velocity | 500fpm |
| Rectangular equivalentsame area at 8″ deep — the dimension a joist bay usually limits | 32″ × 8″ |
| Cross-section | 1.767ft² |
| Noise riskbelow the threshold where duct noise becomes noticeable | Normal |
Notes
- **18″ is the smallest stock size that carries 800 CFM at 500 fpm.** The arithmetic gives 17.1″; ducts come in whole inches, so the real answer rounds up and the real velocity drops to 453 fpm. Rounding *down* to save a size is the most common way a system ends up noisy.
- **Velocity is a trade, not a target to minimise.** Slower air means bigger duct, more sheet metal and more ceiling given up. Faster air means smaller duct and a system you can hear from the sofa. At 453 fpm this run sits in the quiet band.
- **Flex is not rigid with a different label.** The corrugated inner wall raises friction loss substantially over smooth metal of the same diameter, and flex that is not pulled tight — the normal condition in a real attic — is worse again. The lower target velocity above already allows for it.
- **The duct is half the answer; total static pressure is the other half.** Every elbow, boot and takeoff adds equivalent length, and a system can have every duct correctly sized and still fail because the total run is too long for the blower. Size the ducts here, then measure static pressure at the air handler.
- Rectangular equivalents are matched on area at 8″ deep. Flatter ducts of the same area carry less air than round: the more the shape departs from a circle, the more wall there is per unit of flow, and friction follows wall.
800 CFM on a flex branch
18″, running at 453 fpm. About 32″ × 8″ if the run has to be rectangular.
Where the number comes from
Area equals airflow divided by velocity, and diameter follows from area. No manufacturer table involved — every step is arithmetic you can redo.
The target velocity for a flex branch is 500 fpm. Ducts come in whole inches, so the real answer rounds up and the real velocity lands at 453.
Velocity is a trade, not a target
Slower air means bigger duct, more sheet metal, more ceiling given up. Faster air means smaller duct and a system you can hear from the sofa.
Above about 900 fpm people notice. Rounding down a size to save money is the most common way a system ends up noisy.
Flex is not rigid with a different label
The corrugated inner wall raises friction loss substantially over smooth metal of the same diameter — and flex that is not pulled tight, which is the normal condition in a real attic, is worse again.
Size flex as flex.
The duct is half the answer
Every elbow, boot and takeoff adds equivalent length. A system can have every duct sized correctly and still fail because the total run is too long for the blower.
Size the ducts here, then measure total external static pressure at the air handler. That measurement is the one that tells the truth.
Nearby sizes
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- What size duct for 800 CFM on a rigid main trunk?