Airflow: the HVI rule of thumb
The Home Ventilating Institute publishes the simplest starting point there is. For a hood against a wall it recommends 100 CFM for every linear foot of cooking surface, with 40 CFM per foot as the minimum. Over an island, where air moves in from every side and pushes the plume around, both figures rise by half.
Airflow is measured in cubic feet per minute. HVI warns that inflated ratings are common on hoods that are not HVI-certified, so the figure worth comparing is the certified one, printed on the label or in the listing.
| Hood width | Recommended | Minimum |
|---|---|---|
| 30 inches | 250 CFM | 100 CFM |
| 36 inches | 300 CFM | 120 CFM |
| 48 inches | 400 CFM | 160 CFM |
Professional-style burners change the math
High-output sealed burners, a griddle or an infrared element put far more heat and smoke into the air than a standard residential surface of the same width. HVI's guidance for those is to follow the manufacturer of the cooking equipment, whose installation manual usually states the ventilation it expects.
Why more is not automatically better
A bigger blower needs a bigger duct, makes more noise at the speeds people actually use, and pulls more conditioned air out of the house. Past a certain size, building codes also ask for make-up air, which is its own project. Pick the airflow the cooking needs, then size the duct to deliver it.
Source: Home Ventilating Institute — How Much Ventilation Do I Need? — 100 CFM recommended and 40 CFM minimum per linear foot against a wall, 150 and 50 over an island; 250/300/400 CFM for 30/36/48-inch wall hoods; follow the manufacturer for professional-style surfaces; inflated ratings are common on uncertified hoods; downdraft needs higher volume and cannot equal a canopy
Width: covering the burners you use
Smoke and steam widen as they rise. A canopy narrower than the cooking surface leaves the outer burners' plumes partly outside its reach, and those are often the burners used for searing. Matching the hood width to the cooking surface is the usual floor; going a size wider is the common upgrade for heavy cooks and for islands.
Depth matters too. A shallow under-cabinet hood over a deep surface leaves the front burners outside the canopy, which is why so much smoke escapes at the front edge of an ordinary kitchen.
Island hoods lose the wall that helps a wall hood. With no back surface to guide the plume upward, smoke drifts toward whichever side has the strongest draft, from an open door, a return grille or people walking past. That is part of why HVI raises the airflow over islands, and why many kitchens pair an island canopy with a width a size larger than the burners beneath it.
- Measure the full width of the cooking surface, including side trim.
- Measure its depth from the wall to the front of the front burners.
- Compare both with the canopy dimensions in the hood's specification sheet.
- Over an island, allow for air moving in from all four sides.
Mounting height: close enough to catch, far enough to work under
Every manual prints a height window, and the windows differ. Broan's BXT1 under-cabinet series asks for the bottom of the hood to sit between 20 and 25 inches above the cooking surface for best capture; its BCSD series says no less than 18 and no more than 24. Vent-A-Hood recommends no more than 30 inches for both its wall and island hoods.
Lower catches more, because the plume has less room to spread. Higher is easier on tall cooks and on the forehead, but the airflow needed for the same capture rises with every inch.
Two manuals, one height
The hood's range of heights and the cooking surface's minimum clearance above it both apply. Where they overlap, that overlap is the allowed window; where they do not, the pairing does not work and one of the two has to change.
Source: Broan — BXT1 Series Range Hood Installation Instructions — For best capture the bottom of the hood should be 20 to 25 inches above the cooking surface; Broan — BCSD Series Range Hood Installation, Use and Care Manual — The bottom of the hood must be no less than 18 inches and no more than 24 inches above the cooking surface; Vent-A-Hood — Wall Mount Range Hood Installation Instructions — Bottom edge no more than 30 inches above the cooking surface; 300 CFM single blower on 6-inch round duct, 600 CFM dual blower on 8-inch round; never reduce the duct size; use only smooth galvanized metal duct, no flexible or corrugated, with as few turns as possible; Vent-A-Hood — Island Range Hood Installation Instructions — Island hood bottom edge no more than 30 inches above the cooking surface
Matching airflow to the duct behind it
A rating is only reached through the duct it was tested with. Vent-A-Hood's wall instructions pair its 300 CFM single blower with 6-inch round duct and its 600 CFM dual blower with 8-inch round, and state that the duct size must never be reduced. A strong blower on a small or long duct delivers far less than its label.
- The duct diameter the manual lists for that blower, carried the whole way out.
- Smooth metal duct rather than flexible or corrugated.
- As few elbows as the route allows.
- A wall or roof cap sized to the duct, not a smaller one.
Source: Vent-A-Hood — Wall Mount Range Hood Installation Instructions — Bottom edge no more than 30 inches above the cooking surface; 300 CFM single blower on 6-inch round duct, 600 CFM dual blower on 8-inch round; never reduce the duct size; use only smooth galvanized metal duct, no flexible or corrugated, with as few turns as possible
