A hood's fan rating assumes the right ductwork. Installation guides specify the diameter or shape of duct a hood or over-the-range microwave must connect to, and 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.
In San Diego homes the duct often runs through an attic in single-story tract homes, through a wall to a side yard in townhomes, or doesn't exist at all in many condos. Each of those has its own pitfalls.
What the guides say
- •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 (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. In GE's chart, a 90-degree elbow counts as 10 feet and a wall cap as 40.
- •Whirlpool (general guidance): It is not recommended to vent with more than two elbows; refer to the product manual for specifics.
Duct size follows the fan. A large canopy hood needs a large round duct; an over-the-range microwave uses a smaller one.
Why duct size and length matter
- •Resistance. Air slows against duct walls, turns and caps. A smaller duct or longer run means the fan moves less air.
- •Equivalent feet. Each fitting is converted into the length of straight duct that resists 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.
- •Reducers. Shrinking the duct below the outlet size chokes the system, which is why GE sets a 6-inch minimum for round duct.
San Diego situations
- •Attic terminations. Some older homes have a hood duct that ends in the attic rather than at a roof cap. That deposits grease and moisture in the house, and it should be extended to the outside.
- •Coastal caps. Wall and roof caps near the ocean in Pacific Beach, Ocean Beach, La Jolla and Coronado see salt air that stiffens damper hinges. A damper stuck partly closed adds resistance the chart doesn't show.
- •Condos without exterior ducts. Many units in Downtown and Mission Valley run hoods or microwaves in recirculating mode. Any new exterior penetration usually needs HOA approval.
- •Big hoods in remodels. A large hood paired with a pro-style range can move a lot of air out of a tight modern house. California's codes include kitchen ventilation requirements, and some jurisdictions raise makeup-air questions for large hoods. Ask your building department before buying.
How to check your run
- Find the duct size your hood's guide requires and the size of its outlet collar.
- Trace the duct from the hood to the outside cap, through the cabinet, wall, attic or roof.
- Measure straight sections and count every elbow, noting 45- and 90-degree turns.
- Add up equivalent length with your guide's values. With the KitchenAid KXW guide's values, 30 feet of straight duct plus two 90-degree elbows (5 feet each) comes to 40 equivalent feet, within its 60-foot limit. Guides count caps differently (GE's chart adds 40 feet for a wall cap), so use the values for your own hood.
- Confirm 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 goes wrong
- •The hood is loud but weak, and smoke escapes into the kitchen.
- •Grease builds up in long runs and low spots.
- •A stuck damper or crushed section cuts airflow further.
- •Ducting into an attic deposits grease and moisture in the house.
What we check on a service visit
We start at the hood: filters, fan blades or blower wheel, motor speed and the damper at the outlet. Then we feel for airflow at the exterior cap with the fan on high. If the fan is healthy and airflow outside is weak, the duct is the problem and we'll tell you what we found so a contractor can address it. Our range repair and microwave repair pages cover hoods and over-the-range microwaves.
Height matters as much as ductwork; see range hood mounting height and, for microwaves, over-the-range microwave mounting height.
Work for other trades
- •Cutting through an exterior wall or roof, and flashing a roof cap, is work for a contractor or roofer.
- •Upsizing a duct through framing may need a contractor and a permit.
- •A new hood circuit is a job for a licensed electrician.
Frequently asked questions
Can I connect a new hood to my old, smaller duct with a reducer? The guides discourage it. A reducer chokes the fan, so a hood that needs a larger duct won't perform as rated on a smaller one.
Is flexible duct OK for a range hood? Check your hood's guide. Smooth rigid metal duct has the least resistance and is easier to keep clean of grease.
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)
If your hood fan runs but barely clears the air, you can book a visit online and we'll tell you whether the fan or the duct is holding it back.