A safe, effective range hood installation starts with the hood manufacturer's rated performance and installation instructions. Model residential codes require makeup air when a kitchen exhaust system can exhaust in excess of 400 cubic feet per minute; when makeup air is required it should be supplied at a rate approximately equal to the exhaust rate[1][2]. High‑CFM exhaust without appropriate makeup air can create negative house pressure and risk backdrafting of combustion appliances, which is the safety rationale for the code trigger and a reason to confirm appliance venting after installation[1][11].
Sizing CFM: manufacturer guidance first, heuristics second
Always prioritize the hood manufacturer's recommended CFM and any tested capture performance; the manufacturer's installation manual is the controlling instruction and dictates permitted duct types, connector sizing, and maximum equivalent duct length[6]. If you need a quick estimate before checking model specs, industry rules‑of‑thumb used by many retailers and installers include roughly 100 CFM per 10,000 BTU of cooktop output or about 100 CFM per linear foot of cooktop — these are capture‑power heuristics and should be secondary to manufacturer performance and tested capture efficiency[10][3][4].
Standard ventilation guidance also treats kitchen local exhaust as a spot exhaust; a common baseline in standards and guidance is about 100 CFM intermittent (or a continuous equivalent) for kitchen local exhaust, while NKBA planning methods calculate local and whole‑house needs and use the greater value[3][4].
Duct type, connector sizing, and equivalent length
Range hoods must discharge to the outdoors (not into attics, crawl spaces, garages, or wall cavities). The serving duct should be smooth‑interior, single‑wall metal, airtight, and equipped with a backdraft damper per IRC/IMC requirements and manufacturer guidance[1][6]. Flexible foil or plastic ducts reduce airflow, trap grease, and are discouraged or prohibited by code and many manufacturers; smooth‑walled rigid metal ducts deliver more effective flow and are preferred[1][6][11].
Manufacturer literature commonly illustrates connector sizes for under‑cabinet residential hoods like rectangular 3‑1/4" × 10" or 6–7" round; always check the hood's manual for the exact connector and allowable equivalent duct length[6]. Minimize equivalent length and elbows because each adds equivalent feet that reduce delivered CFM; if the run exceeds the manufacturer's maximum allowable equivalent length (or contains too many turns), change the design by using larger ducts, reducing elbows/length, or adding a listed in‑line booster/centrifugal fan per manufacturer rules[6][1].
Termination: outdoors only and use a listed cap with a backdraft damper
Do not terminate the hood duct into enclosed spaces such as attics, crawlspaces, garages, or wall cavities. Terminate to a listed exterior cap (wall or roof) with a backdraft damper per code and manufacturer instructions[1][6]. For guidance on wall vs. roof terminations and routing short, straight runs, see related venting guidance for exhaust fans[6] — the same principles (short runs, smooth ducts, and listed exterior termination) apply; for more on wall vs. roof termination details see our article on how to vent a bathroom fan correctly. wall vs. roof termination details
Makeup air options when required (>400 CFM)
When a hood can exhaust more than ~400 CFM, codes require makeup air; makeup air should approximate the exhausted rate[1][2]. Makeup air may be provided passively (an automatic gravity damper sized to open when the hood runs) or actively (a fan‑assisted makeup‑air unit). Manufacturer and product solutions exist — for example, fan‑assisted units that can be tied to hood operation to control airflow and temper incoming air[1][7][8]. When makeup air is installed, follow the makeup‑air product and hood manufacturer instructions for controls and interlocks[6].
Safety checklist before and after installation
Pick a hood sized to the cooktop width and a model with manufacturer capture guidance; follow the manual for allowed duct types and connector sizing[6].
Determine required CFM using the manufacturer's specs. Use ASHRAE/NKBA guidance or BTU/width rules only as secondary heuristics; remember makeup‑air triggers apply at ~400 CFM[6][3][1].
Select smooth, rigid metal duct sized to the model connector and keep runs short and straight to minimize equivalent length; avoid flexible foil/plastic ducts unless the manufacturer explicitly permits them[1][6][11].
Provide a backdraft damper and listed exterior termination; never terminate into enclosed spaces[1][6].
Provide makeup air when the hood is capable of >400 CFM — passive or powered units tied to the hood are acceptable when they meet code and manufacturer requirements[1][7][8].
Follow the hood's maximum equivalent duct length; if you exceed it, revise the design (larger duct, fewer elbows, or listed in‑line booster) per the manual[6][1].
After installation, test delivered airflow and verify that combustion appliances do not backdraft when the hood and makeup air are operating[6][11].
Costs & realistic expectations
Entry under‑cabinet hoods can cost a few hundred dollars; higher‑end island or professional hoods commonly range into $1,000–$3,000+ depending on CFM and finish. Installation and minor ducting are commonly estimated in consumer guides at roughly $400–$1,500, with averages often near $600–$800 for straightforward installs. Complex jobs with roof penetrations, long ducts, or makeup‑air systems can push total project cost into the thousands[5][10][9][13][14].
Common installer rules‑of‑thumb (use cautiously)
Retailers and installers often use quick rules (about 100 CFM per 10,000 BTU or about 100 CFM per linear foot) to estimate needs during planning. Use them only as a starting point and confirm with the hood manual or test capture performance for your specific cooktop and hood geometry[10][3][4][6].
Final note: follow the manual and local code
The manufacturer's installation manual and local code requirements are the controlling documents. Manufacturers specify permitted duct types, connector sizing, and maximum equivalent lengths that determine rated performance; follow them to maintain the hood's tested capture and to meet code[6][1].