Cooking releases heat, steam, grease and, with gas, combustion products such as nitrogen dioxide and carbon monoxide. A range hood captures them at the source. How much airflow it needs depends on the cooktop width and where the hood hangs, on the power of gas burners and on the size of the kitchen. The calculator works out each rule and keeps the largest.
The rules
| Rule | Airflow |
|---|---|
| Wall hood | 100 CFM per foot of cooktop width |
| Island hood | 150 CFM per foot of cooktop width |
| Gas cooktop | 100 CFM per 10,000 BTU/h of total burner output |
| Room volume | volume (cu ft) × 15 air changes per hour ÷ 60 |
Island hoods need more airflow because air flows in from every side and drafts carry fumes away before they reach the hood.
Example
A 30 in electric range under a wall hood needs 250 CFM by the width rule. In a 12 × 10 ft kitchen with an 8 ft ceiling, the room volume rule gives 240 CFM, so 250 CFM is enough. A 36 in gas range with 60,000 BTU/h of burners would need 600 CFM.
Hood size and height
The hood should be at least as wide as the cooktop, and for an island 3 in wider on each side. Mount it 24 to 30 in above an electric cooktop and 30 to 36 in above gas, or as the manufacturer says. A deeper hood captures better than a more powerful one.
Makeup air
The International Residential Code requires makeup air for kitchen exhaust over 400 CFM: an opening or a powered inlet that brings in outside air as fast as the hood removes it. Without it, a powerful hood depressurizes the house, works poorly and can pull combustion gases down the chimney of a furnace, water heater or fireplace.
Ducted or recirculating?
Ducted hoods vent outside and remove heat, moisture and fumes. Recirculating hoods filter grease and odors through charcoal but return the heat and humidity to the kitchen. Use the duct size the hood requires (often 6 in for up to 400 CFM, 8 in or more above), keep the run short and avoid flexible duct.
Noise
A hood that is too loud is turned off. Look for a sone or decibel rating at the speed you will use most, and consider a larger hood running at low speed rather than a small one at full blast.