How Duct Length and Elbows Cut Your Range Hood's Real CFM
Your range hood says 900 CFM. The wall says 612.
A kitchen designer in Hamburg wrote to our team last spring with a problem that comes up every week. Her client had purchased a 900 CFM T-shape stainless steel range hood rated for the kitchen layout, and the install was clean — a single 90-degree elbow at the top, a 4-metre run through the ceiling, and a wall cap. The hood ran, but the smoke collection was clearly undersized for the wok burner on the island. She had measured at the wall cap: 612 CFM. The hood was brand new; the ductwork looked correct on paper.
The 288 CFM difference was not a hood defect. It was the duct doing what duct does. The single 90-degree elbow, the 4-metre straight run, and the wall cap together absorbed enough of the fan's available static pressure that the fan had to settle into a slower operating point on its performance curve. Same hood. Same motor. Different airflow.
The point of this article is to make that kind of gap visible before the wall cap goes in, not after. The fix is rarely the hood — it is almost always the duct.
The equivalent-length math that quietly drives CFM loss
The HVAC industry solved the "what does my ductwork cost me" question decades ago with the concept of equivalent length. Every fitting in a duct run is converted to an equivalent length of straight duct; the total equivalent length is summed; and that total is read against the hood's fan curve to find the operating airflow.
Three numbers matter for the calculation:
- Actual straight duct length — measured along the run from the hood collar to the termination cap.
- Equivalent length of each fitting — converted via standard tables.
- Hood fan curve — a graph of CFM versus static pressure for the specific hood model. The HVI (Home Ventilating Institute) publishes standard test procedures for range hood airflow at hvi.org, and equivalent-length values are tabulated in the ASHRAE Handbook of Fundamentals.
The standard source for the equivalent-length values is the JILU ductwork sizing reference; equivalent-length tables are also published by ASHRAE in the Handbook of Fundamentals and by HVAC trade bodies in most jurisdictions.
Once you have the equivalent length, you compute the friction loss for the equivalent diameter (typically using the Darcy-Weisbach equation or a simplified residential table), and you read the resulting static pressure against the hood's fan curve. The intersection point is the operating airflow. The number will always be lower than the rated CFM because rated CFM is measured at 0 inches water gauge static pressure, an idealised reference that you will never see in the field.
What an elbow actually costs you, by geometry
The elbow is the most CFM-expensive single component in a typical residential duct run. The geometry of the elbow changes the cost significantly.
| Elbow type | Equivalent length (ft of 8" smooth rigid metal duct) | Notes |
|---|---|---|
| Long-radius 90° (sweeping turn, centreline radius ≥ 1.5× duct diameter) | ~5 to 7 ft | Default choice for any new install where the framing allows |
| Standard 90° (three-piece stamped elbow, centreline radius ~1.0× duct diameter) | ~8 to 12 ft | The most common residential 90; the dominant CFM penalty in most installs |
| Sharp 90° (mitre or pressed tight) | ~15 to 25 ft | Rare in modern installs but appears in retrofits where the framing forces a tight turn |
| 45° elbow | ~3 to 5 ft | Useful for offsetting around a beam or joist without paying for a 90 |
| Two 45° elbows in series (to offset and re-align) | ~6 to 10 ft | Often cheaper than a single 90 if the geometry allows |
What this table means in a real install: a single standard 90° elbow buys you the equivalent of about 8 to 12 feet of straight duct. Two 90s plus a wall cap plus the actual straight length is why a "short" kitchen duct run routinely adds up to 30 equivalent feet even when the geometric path is short.
Smooth rigid metal vs flexible duct — where the CFM goes
The duct material is the second-largest lever after fitting count.
| Duct material | Friction rate vs smooth rigid metal (rough multiplier) | Where it appears in residential installs |
|---|---|---|
| Smooth rigid galvanized steel | 1.0× (baseline) | Trunk runs through ceiling cavities and soffits |
| Rigid aluminium | ~1.0× to 1.1× | Light commercial and some residential retrofits |
| Semi-rigid aluminium (forms by hand) | ~1.2× to 1.5× | Transitions where rigid duct won't fit |
| Flexible non-insulated (stretched straight) | ~1.5× to 2.0× | Final connection from rigid trunk to hood collar in tight spaces |
| Flexible non-insulated (compressed or coiled) | ~3× to 6× or worse | The single most common reason a new install under-delivers |
| Flexible insulated (acoustic duct) | ~2× to 3× stretched, much worse compressed | Quiet range hood installs where noise is the dominant constraint |
| PVC or metal pipe (incorrect material for range hood exhaust) | Not applicable — fail on heat or condensate | Should not be used for range hood ducting in any standard install |
The flexible duct penalty is the one installers most often ignore. A flexible duct that was cut long and left coiled at the hood collar can lose 30 to 50 percent of total airflow on its own. Stretch the duct; support it with a radius; treat it as the worst section of the run, not the easiest.
Wall caps, roof caps, and the back-pressure nobody budgets for
The termination cap is the third major CFM penalty, and the easiest to underestimate. Most wall caps carry an equivalent length of 15 to 30 feet when you read the manufacturer's test data against a typical hood flow — a number that catches first-time installers by surprise.
The reason is that the cap has to do two jobs at once: exhaust the air and keep weather out. A flap that closes against back-draft adds resistance; a screen mesh adds more; a poorly designed louvre adds more still. The wall-mounted hood termination caps we test against are typically in the 15 to 25 equivalent-feet range for the gravity-flap design, and the better designs drop to around 10. Roof caps are similar to 20 to 40 equivalent feet depending on the design and the prevailing wind load.
Three installation rules that follow from this:
- Specify a low-resistance termination cap. The manufacturer's equivalent-length data should appear on the cut sheet. If it doesn't, switch caps.
- Don't add a screen mesh at the cap "to keep insects out." A mesh is an additional 10 to 20 equivalent feet that nothing in the spec budgets for. For indoor air quality and combustion-product venting context, the U.S. EPA indoor air quality reference covers residential kitchen ventilation as part of whole-home IAQ management.
- Use wall cap rather than roof cap where the kitchen layout allows. Wall caps are typically lower equivalent length than equivalent-quality roof caps in residential installs.
Five installation patterns that consistently lose 30% CFM
The ductwork patterns below show up again and again on residential and light-commercial installs. Each one typically costs 25 to 35 percent of the hood's rated airflow. They are easy to recognise on the plan; they are hard to recognise in the field until the install is done.
Pattern 1 — Compressed flexible duct at the hood collar
The flexible connector between the rigid trunk and the hood is the most common culprit. A 250 mm section of compressed flex reduces flow by enough to drop the whole system 10 to 15 percent. The fix is always mechanical: a longer flex, a smoother transition, or a rigid connector.
Pattern 2 — Two close 90° elbows in series
When a duct has to offset around an obstruction and recover, two close 90s cost the equivalent of 16 to 24 feet of duct. Two long-radius 90s separated by at least three duct diameters can be cheaper.
Pattern 3 — 6-inch duct for a long run
Six-inch duct loses friction at the fifth power of the diameter. A 6-inch run longer than about 12 feet is almost always undersized; switching to 8 inches reduces friction by roughly a factor of four at the same airflow.
Pattern 4 — Wall cap with mesh screen
A wall cap plus a mesh insect screen can add 25 to 40 equivalent feet. Use the cap's built-in louvre instead, or specify a no-mesh model.
Pattern 5 — Termination in a high-wind exposure
A wall cap on the windward side of a coastal or hilltop install sees wind pressure that effectively raises its equivalent length to 40+ feet. Relocate the cap or specify a sealed back-draft damper.
When the BOFU math flips the spec
The interesting design question is not "what hood should I buy," but "what system airflow do I actually need, given the duct I have or the duct I can run?" Three scenarios are the ones buyers hit most often.
Scenario A — the duct is friendly
If you are specifying residential equipment and need the underlying safety reference, the U.S. Consumer Product Safety Commission publishes guidance on cooking appliance installation; the CPSC framework complements the airflow-target sizing discussed below.
If the duct path is short (under about 12 equivalent feet), the hood's rated CFM is within 10 percent of what reaches the cap. Spec the hood to the cooking load, run the duct, move on. This is the easy case.
Scenario B — the duct is constrained but rerouteable
When the duct has to run long, has multiple elbows, or has to terminate in a less-than-ideal location, the fix is usually duct-side, not hood-side. Upsize the duct from 6 to 8 inches, replace the standard 90s with long-radius 90s, swap the wall cap for a low-resistance model. Most "underperforming 900 CFM" kitchens are actually solvable at the duct, for less money than the upgrade to a 1200 CFM hood.
Scenario C — the duct is fixed and unfriendly
Some installs have a fixed path the duct has to follow — usually a retrofit in an older building with a structural obstruction. Where the duct terminates through a fire-rated assembly, the National Fire Protection Association standards (NFPA 96 for commercial cooking) define the through-penetration protection that must accompany the airflow sizing. In that case, the math has flipped and the upgrade is to a higher static-pressure-rated hood, not a higher rated-CFM hood. A 600 CFM hood rated for 1.0 inches water gauge will outperform a 900 CFM hood rated for 0.4 inches water gauge when the system static pressure is 0.7 inches. Read the fan curve, not the headline.
Quiet range hoods and ductwork — two pressures, one duct
"Quiet range hood" is a category that has grown sharply in 2025–2026 across European and North American residential markets. Buyers increasingly spec the hood by sones (a measure of perceived loudness) rather than CFM, and they ask for sones below 1.0 on the boost setting. Quiet range hoods and ductwork have a non-obvious relationship: a quiet range hood does not save you from a noisy duct system.
The fan noise comes from the motor. The airflow noise comes from the ductwork — turbulence at fittings, vibration along straight runs, the cap flapping. A hood with a quiet motor running against an undersized duct often ends up louder than the spec promised, because the airflow noise dominates once the duct static pressure rises. The same motor at the same airflow against a properly sized duct is much quieter.
For buyers who spec quiet, two rules apply. First, run the largest equivalent-length duct that the framing allows — 8-inch preferred, 10-inch if the framing accepts it. Second, use long-radius fittings, low-resistance termination, and rigid metal where possible. These choices deliver both lower static pressure (more airflow) and lower airflow noise (quieter operation).
Where JILU specs against this
Our T-shape stainless steel range hood portfolio carries published fan curves for each model so that the duct designer can read the operating airflow at the actual system static pressure, not just at 0 inches. We publish the duct collar diameter, the recommended equivalent-length maximum, and the recommended duct diameter for typical kitchen layouts. For projects where the duct path is constrained, our technical team can run the equivalent-length calculation against a specific plan and recommend the right hood for the duct you have, not the duct you wish you had.
For buyers working on a new build, the right sequence is: pick the duct path first, then pick the hood. The hood you spec against an early architectural drawing, when the duct path is negotiable, is usually a different hood than the one you spec against a finished plan. Get the duct right; the hood spec usually follows.
Frequently asked questions
How much CFM do you lose per foot of duct?
It depends on duct diameter, duct type, and air velocity. A smooth rigid metal duct at typical residential airflow loses roughly 0.08 to 0.12 inches water gauge static pressure per 10 linear feet. Flexible duct loses 1.5 to 2 times more than rigid metal for the same length because the corrugated interior creates turbulence.
How much CFM does each elbow cost?
Each 90-degree elbow in a smooth rigid metal duct behaves like roughly 8 to 10 linear feet of straight duct. A 45-degree elbow behaves like roughly 3 to 5 feet. Sharp-radius elbows cost more than long-radius elbows. Two 90s plus a termination cap can easily account for 30 to 40 percent of total system static pressure.
Why does my range hood sound louder than the spec sheet says?
Because the motor is working against higher static pressure than it was rated for, the fan runs off its performance curve, and noise increases. The same hood running at 0.4 inches water gauge sounds different from the same hood running at 0.9 inches water gauge. Quiet range hoods require both a quiet motor and a quiet duct system.
Does a larger duct diameter really help?
Yes, and the math is non-linear. Going from a 6-inch duct to an 8-inch duct reduces friction losses by roughly a factor of four at the same airflow. A long run that struggles at 6 inches is often no problem at 8 inches.
Should I run flexible duct or rigid metal duct?
Use rigid metal duct for any run longer than a few feet if you can route it. Flexible duct is acceptable for short connections, but should be stretched as straight as possible. Compressed or coiled flexible duct is the single most common cause of high CFM loss in residential installs.