15% compressed flex duct pressure drop at 300 CFM
| Pressure drop over this run | 0.112in w.c. |
|---|---|
| Same run in rigid metalwhat the design assumed | 0.022in w.c. |
| Branch size300 CFM at about 700 fpm | 9″ |
| Multiplier against rigid | 5× |
| Share of the 0.5 in w.c. blower budgetand this is ONE branch of many | 22% |
| Ductthe ordinary sagging install nobody flags at handover | flex, 15% compressed |
Notes
- Flex duct is not the problem; slack flex duct is. Pulled properly taut it costs about twice what rigid metal costs in pressure. Left sagging the way this one is, the inner liner corrugates and the loss multiplies — 5× here. Nothing about that is visible once the ceiling is closed, which is exactly why it keeps happening.
- A single branch eating 22% of the blower budget is the whole story. A residential air handler is designed for about 0.5 in w.c. of total external static, shared between the filter, the coil, the grilles and every duct. One branch taking this much means the system was over budget before the second branch was counted.
- The fix costs almost nothing at install time and a fortune afterwards. Stretch every flex run to at least 95% of its length, support it every four feet with wide saddle hangers so it cannot sag between them, and cut it to length instead of coiling the excess. Retrofitting that once the drywall is up means opening the ceiling.
- Use rigid for the trunk and short flex for the last connection. That is the compromise most good installers settle on: metal where the air travels far and fast, a couple of feet of flex at the boot to absorb vibration and to make the final alignment easy. All-flex is fast to install and slow to run.
- Elbows in flex are worse than elbows in metal, and there is no chart for them. A tight radius bend in flexible duct closes the inner liner into a partial obstruction. Keep the bend radius at least one duct diameter, and never turn a corner by simply letting the duct fall around it.
- Figures here are for a 25 ft branch at 300 CFM. The base equation is the standard ASHRAE friction relation for galvanised round duct; the flex multipliers come from laboratory measurements and are a range rather than a constant, so treat them as the size of the problem rather than a precise answer.
0.112 in w.c. for one branch
A 9″ branch carrying 300 CFM over 25 ft of 15% compressed flex duct costs 0.112 inches of water column — against 0.022 for the same run in rigid metal.
That is 22% of the entire 0.5 in w.c. budget a residential blower is designed for, spent on one branch of many.
Flex is not the problem. Slack flex is.
Pulled genuinely taut, flexible duct costs about twice what rigid metal costs, because the liner is corrugated rather than smooth. Left slack, the corrugations deepen and the loss multiplies several times over.
The worst part is that it is invisible. A run left in gentle waves across the attic looks like a tidy installation, passes every visual inspection, and quietly halves the air reaching that room. Once the ceiling is closed, nobody sees it again.
What good practice looks like
Stretch every run to at least 95% of its length. Support it every four feet with wide saddle hangers so it cannot sag between them. Cut the excess off rather than coiling it in the ceiling. Keep any bend radius to at least one duct diameter, and never turn a corner by letting the duct simply fall around it.
The compromise most good installers settle on
Rigid metal for the trunk, where air travels far and fast, and a short length of flex at the boot to absorb vibration and make the final alignment easy. All-flex is quick to install and expensive to run, every hour, for the life of the system.
Nearby sizes
Read more
- Installing Flex Duct So It Carries the Air It Is Rated ForFlex duct is rated in a laboratory pulled tight and straight. Almost none of it is installed that way.
- Sealing Ducts — Mastic, Not Duct TapeThe product named after the job is the one product that does not do it.
- Duct Insulation — R-Value, Where It Is Required and Why It SweatsInsulating a duct is only half the job. Keeping humid air away from the cold surface inside is the other half.
- Balancing a Duct System — Dampers at the Branch, Not Vents at the RoomEvery duct system is out of balance when it is built, and most of them are never balanced.