People spend four figures on a hood and then connect it to whatever duct is already in the wall. The blower is rated at 1200 CFM in a test rig with a straight, short, smooth duct. Your kitchen is not a test rig. Here is what actually determines the airflow you end up with.
Duct diameter is the hard limit
Airflow through a duct scales with its cross-sectional area, and area scales with the square of the radius. A ten-inch duct has almost three times the area of a six-inch duct. If your 1200 CFM blower is squeezed into a six-inch pipe, the pipe is now the appliance, and it does not matter what the box said.
Every hood manufacturer publishes a minimum duct diameter for each blower size. Meeting it is not a suggestion. Reducing it is the most common reason a new hood feels weaker than the one it replaced.
Flex duct is not the same as rigid duct
Corrugated flexible duct has enormously more internal friction than smooth rigid pipe. A run of flex can cost you a third of your airflow compared to the same length in rigid, and it also gets crushed behind cabinetry where nobody can see it.
We use rigid duct wherever it is physically possible, with smooth transitions and long-radius elbows instead of sharp ones. Where flex is unavoidable, we keep it as short and as straight as it can possibly be.
Every elbow costs you
A 90-degree elbow adds the equivalent resistance of several feet of straight duct. Two elbows and a long horizontal run through a joist bay can quietly halve a hood's real output.
The best duct route is the shortest one that gets outside. Straight up through the roof usually beats a long horizontal run to a side wall, even though it looks like more work on paper.
Mounting height and capture area
Airflow is only half of ventilation. The hood also has to physically sit over the cooking surface with enough capture area to catch rising steam and grease before it escapes into the room.
A few rules that hold up in practice:
- The hood should be at least as wide as the cooking surface, and wider is better on a pro range.
- Mount inside the manufacturer's published height range, typically 30 to 36 inches above a gas cooktop.
- Deeper hoods capture more, especially over front burners where most searing happens.
- Island hoods need more CFM than wall hoods, because there are no walls helping contain the plume.
Get those right and a 600 CFM hood will outperform a badly installed 1200 CFM one every single time. Our range hood installation service starts with the duct route, not the box.
Make-up air, the thing nobody mentions
Above roughly 400 CFM, many Bay Area jurisdictions require a make-up air system, because a powerful blower in a tightly sealed modern home can depressurise the building and pull combustion gases back down a water heater flue. That is a genuine safety issue, not a paperwork issue.
If you are specifying a high-output hood, this needs to be part of the plan from the start. We flag it during the site inspection so it does not become a surprise at final inspection.
