A range hood's fan rating assumes it's connected to the right duct. Undersize the duct, add too many turns or run it too far, and the fan moves a fraction of the air it's rated for while making more noise doing it. Installation guides handle this with two numbers: a required duct size and a maximum length counted in equivalent feet, where every elbow and cap adds a set amount.
What two guides require
KitchenAid KXW-series canopy hoods
The dimension guide for the KXW8730Y, KXW8736Y and KXW8748Y commercial-style wall-mount hoods says:
- •A 10-inch round vent system is required; the hood's exhaust opening is 10 inches round.
- •Maximum equivalent length is 60 feet. Each 45-degree elbow adds 2.5 equivalent feet; each 90 adds 5.
- •Use no more than three 90-degree elbows, keep at least 24 inches of straight duct between elbows when there's more than one, and don't install two elbows together.
- •Use metal vent only; rigid metal is recommended. Plastic or foil vent is not recommended, and flexible vent creates back pressure that greatly reduces performance.
- •The duct must end outdoors, not in an attic or other enclosed area, and must have a damper. Don't use 4-inch laundry-type wall caps.
- •Keep the duct a uniform size, seal joints with clamps and caulk around the exterior opening.
The guide's worked example: 8 feet of straight duct, one 90-degree elbow (5 feet) and a wall cap (counted as 0 feet in that example) comes to 13 feet, well within the limit.
GE over-the-range microwaves
GE's microwave oven installation instructions, Pub. No. 49-8699-3, cover a much smaller fan:
- •The microwave's exhaust is built to connect to standard 3 1/4- by 10-inch rectangular duct. If round duct is needed, a rectangular-to-round transition must be used, and the duct must be at least 6 inches in diameter.
- •Total length should not exceed 140 equivalent feet.
- •Its chart lists approximate equivalents: 40 feet for a wall cap, 24 feet for a roof cap, 5 feet for the rectangular-to-round transition, 5 feet for a 45-degree elbow, and 10 or 25 feet for a 90-degree elbow depending on the elbow type shown.
GE's caps count for far more than KitchenAid's example does. Use the values in your own guide rather than mixing charts.
How equivalent length works
Air slows as it rubs along duct walls, and it loses much more energy when it turns a corner or squeezes through a cap and damper. Each fitting is assigned the length of straight duct that would cause the same loss. Add the straight runs and the fitting values together, and you get one number to compare with the limit.
Some practical consequences:
- •Reducers choke everything. Shrinking below the hood's outlet size throttles the entire run; GE's 6-inch floor for round duct exists for that reason.
- •Elbows are expensive. On the GE chart, two of the 25-foot 90-degree elbows plus a wall cap use up 90 of the 140 available feet before any straight duct is counted.
- •Slow air drops grease. In an undersized or overlong run, grease and moisture settle in the duct instead of leaving.
Tallying your own duct
- Find the required size in your hood's guide and check the hood's outlet collar.
- Follow the duct's path from hood to exterior cap, whether that's through a cabinet, a wall, the joists or the roof.
- Measure each straight piece and tally the elbows by type, 45 or 90.
- Convert everything to equivalent feet with your own guide's numbers. As an illustration, with the KitchenAid KXW chart, 30 feet of straight duct plus two 90-degree elbows (10 feet) and a wall cap counted at 0 feet totals 40 feet, under the 60-foot limit.
- Check the termination. The run has to finish outside, at a wall or roof cap whose damper actually opens, never in an attic, soffit or ceiling cavity.
- Look for reducers or flexible sections anywhere along the run.
DC rowhouse and condo realities
- •Kitchens in the middle of the house. Many renovated rowhouses put the kitchen mid-plan, with no exterior wall nearby. Ducting up through joists to the rear wall or roof adds length and elbows quickly, so a model with a generous length allowance helps.
- •Historic districts. In districts such as Capitol Hill, Georgetown, Dupont Circle and Mount Pleasant, new exterior vents go through historic preservation review. The DC Historic Preservation Office lists ventilating and mechanical installations among minor work it can review at staff level when visibility is limited, which favors rear walls and roofs over street facades.
- •Condos and co-ops. Many buildings don't allow individual units to vent outside, which is why recirculating hoods and microwaves are so common. Check building rules before buying a ducted hood.
- •Permits. DOB's instant permits cover replacing a kitchen exhaust fan in kind in a one- or two-family home. New duct routes through framing or exterior walls go beyond that; ask the Department of Buildings what applies.
Signs the duct is the problem
- •The hood is loud on high but barely clears steam.
- •Smoke rolls past the hood even when it's mounted at the right height.
- •The outside damper flap doesn't move when the fan runs.
- •Grease drips from the hood or appears at duct joints.
Who handles it
- •Openings in exterior walls or roofs, plus flashing around a roof cap, are contractor or roofer work.
- •Upsizing a duct through framing may need a permit; ask DOB.
- •A new hood circuit is licensed electrical work.
Ductwork can't make up for a hood hung at the wrong height; our notes on hood placement over the cooktop and microwave mounting clearances cover that side. If the duct checks out and the fan still underperforms, the blower motor or controls may be failing, which our range repair technicians can test.
Documents referenced
- •KitchenAid KXW8730Y/KXW8736Y/KXW8748Y dimension guide: duct diameter, 60-foot equivalent limit, elbow values and spacing, damper and termination rules.
- •GE microwave oven installation instructions, Pub. No. 49-8699-3: rectangular and round duct sizes, 140-foot equivalent limit, fitting values.