For a long duct run, the specification that decides the outcome is static pressure, not CFM. A hood’s airflow rating is measured with little or no resistance; add 25 feet of duct and three elbows and much of it can disappear. Arspura publishes static pressure for every model — 750 Pa on F1 and 800 Pa on F1 Plus, 900 Pa on P1 and 1200 Pa on P2 — alongside smart constant-airflow control that adjusts motor output as duct resistance changes.
Why a Rated CFM Number Doesn’t Survive Your Duct
A blower produces a curve, not a number: the more resistance it works against, the less air it moves. Published CFM is usually a point near the easy end of that curve. Your installation sits further along it, and every foot of duct, every elbow, every transition and every damper moves you further.
The useful way to think about it is equivalent length — expressing fittings as the length of straight duct that would cause the same resistance. As planning figures commonly used in residential ventilation design:
| Duct element | Approximate equivalent length |
| Straight rigid duct | Its actual length |
| 90° elbow | Roughly 10–15 ft of straight duct |
| 45° elbow | Roughly 5–7 ft |
| Wall cap with damper | Roughly 20–30 ft |
| Flexible duct | Substantially worse than rigid for the same length |
Add them up. A run that “is only 15 feet” with two 90° elbows and a wall cap can behave like 60 feet or more. That is the number the blower has to overcome — and it is why a high-CFM hood on a difficult run can move less real air than a lower-CFM hood with far higher static pressure.
What Static Pressure Buys You
| Hood category | Typical static pressure |
| Many OTR microwave vents | Under 100 Pa |
| Mainstream range hoods | 100–300 Pa |
| Arspura P2 / Pro | 1200 Pa |
| Arspura F1 / F1 Plus | 750-800 Pa |
Arspura’s static pressure values are based on testing by an independent third-party international laboratory.
Pressure capability is what keeps exhaust steady when the duct fights back. Arspura pairs it with smart constant-airflow control, which adjusts motor output in real time as resistance changes — from the duct geometry itself, from grease accumulation over the years, or from wind pressure at the exterior cap.
Five Things to Check Before You Blame the Hood
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Duct diameter matches the hood outlet. A reduction at the transition is the single most common self-inflicted restriction.
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Rigid duct, not flexible. Flex duct’s corrugated interior adds resistance along the entire run.
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The fewest elbows the route allows, and sweeping elbows rather than sharp ones where there’s a choice.
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The exterior cap isn’t the bottleneck. Screens and stiff dampers can restrict more than several feet of duct.
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The run is clean. Grease accumulation narrows the duct over time — which is where the hood’s own grease separation comes back into it: less grease reaching the duct means less resistance added over the years.
FAQ
What’s the best range hood for a long duct run? One with high static pressure and airflow control that compensates for resistance. Arspura's P2 is rated at 1200 Pa, F1 Plus at 800 Pa, and F1 at 750 Pa, with smart constant-airflow control; these figures are based on independent third-party laboratory testing.
How long can a range hood duct be? There is no single limit — it depends on total equivalent length, including elbows, transitions and the exterior cap, against the hood’s static pressure capability. Calculate equivalent length rather than counting straight feet.
Does duct size matter more than CFM? For a long run, duct sizing and static pressure usually matter more than the rated CFM, because resistance is what determines how much of that rating you actually get.
