A hood's fan rating assumes the right ductwork, so installation guides specify range hood duct size, meaning the diameter or shape of duct a hood or over-the-range microwave must connect to, along with a maximum length counted in "equivalent feet." A duct that's too small or too long cuts airflow far below what the fan is rated to move.
What the spec actually says
- •KitchenAid KXW-series commercial-style canopy hoods: The dimension guide requires a 10-inch round vent system, with a maximum length of 60 feet, where a 45-degree elbow counts as 2.5 feet and a 90-degree elbow as 5 feet. It recommends no more than three 90-degree elbows and keeping the run as short as possible.
- •GE over-the-range microwaves (installation instructions 49-8699-3): The exhaust mates with a standard 3 1/4- by 10-inch rectangular duct; round duct needs a transition and must be at least 6 inches in diameter. The total should not exceed 140 equivalent feet, with elbows, caps and adapters counted toward it. In GE's chart, a 90-degree elbow counts as 10 feet and a wall cap as 40.
- •Whirlpool (general guidance): Whirlpool's hood sizing article says it is not recommended to vent with more than two elbows, and to refer to the product manual for specifics.
Duct size follows the fan. A large canopy hood needs a large round duct, while an over-the-range microwave uses a smaller duct.
Why manufacturers require it
- •Resistance: Air slows against duct walls, turns and caps. A smaller duct or a longer run adds resistance, and the fan moves less air.
- •Equivalent feet: Each fitting is converted into the length of straight duct that would resist airflow as much. Adding them up gives one number to compare with the limit.
- •Noise: A fan pushing against too much resistance runs louder for the same result.
- •Grease and moisture: Slow air lets grease and steam settle in the duct instead of leaving the house.
- •Reducers: Shrinking the duct below the hood's outlet size chokes the whole system, which is why GE sets a 6-inch minimum for round duct.
How to check your home against it
- Find the duct size your hood's guide requires and the size of its outlet collar.
- Trace the existing duct from the hood to the outside cap, through the cabinet, wall, attic or roof.
- Measure the straight sections and count every elbow, noting 45-degree and 90-degree turns.
- Add up equivalent length using your guide's values. For the KitchenAid KXW guide, 30 feet of straight duct with two 90-degree elbows and a wall cap comes to 40 feet, because the guide's own example counts a wall cap as 0 feet. That's within its 60-foot limit.
- Check that the duct ends outdoors, at a wall or roof cap with a working damper, not in an attic or soffit.
- Look for a reducer anywhere along the run.
What happens when it's ignored
- •The hood is loud but weak, and smoke escapes into the kitchen.
- •Grease builds up inside an oversized run or at low spots.
- •A damper that doesn't open, or a crushed section, cuts airflow further.
- •Ducting into an attic deposits grease and moisture in the house.
Height matters as much as ductwork. See range hood mounting height, and for microwaves, over-the-range microwave mounting height.
When it needs a licensed pro
- •Cutting through an exterior wall or roof, and flashing a roof cap, is work for a contractor or roofer.
- •Upsizing a duct through framing or between floors may need a contractor and a permit. Ask your building department.
- •A new hood circuit is a job for a licensed electrician.
- •Large hoods may trigger makeup-air rules in some codes. Ask your building department before installing one.
Sources
- •KitchenAid KXW canopy hood dimension guide (10-inch round duct; 60-foot limit; elbow equivalents; three-elbow limit)
- •GE: Over-the-range microwave installation instructions 49-8699-3 (3 1/4 by 10 inches or 6-inch round minimum; 140 equivalent feet)
- •Whirlpool: Range hood dimensions (no more than two elbows recommended)
Quick check: Count the elbows between the hood and the outside. If there are more than two or three, the run is probably working against the fan.
If your hood fan runs but barely moves air even with a good duct, call TrueFix Long Island at (631) 985-3690 or book through our range repair page.