In November 2022 I was in a 1940 row house in Astoria doing kitchen ventilation work. The gas range hood — installed six years earlier — vented through 3.25-inch-by-10-inch rectangular duct, two 90-degree elbows, and 22 feet total to the outside. Hood was rated at 400 CFM. I measured 140 CFM at the outlet. On all four gas burners, kitchen CO2 reached 1,400 ppm and particulates hit levels that would raise flags in an occupational setting. The hood was moving air, just not through ductwork that matched its rated capacity.
What actually limits a range hood’s performance
A hood’s rated CFM is measured at zero static pressure — essentially no ductwork. Add a foot of duct, a grease filter, and an elbow, and you have static pressure losses. The actual airflow depends on the fan curve, which is how CFM changes as the duct system resists. Most residential range hood fans are not high-static fans. As duct resistance increases, flow falls steeply.
Twenty-two feet of duct with two 90-degree elbows, one damper, and grease filters is equivalent to roughly 55 to 60 feet of straight duct in static pressure terms. Against that resistance, a 400-CFM fan delivers 130 to 160 CFM — roughly what I measured. To get 300 to 400 CFM through that run you need a fan rated at 700 to 800 CFM and 7-inch or larger round duct throughout.
Duct sizing and configuration rules
Round duct moves more air per inch of diameter than rectangular duct of the same cross-sectional area because the circular shape minimizes surface contact. For a hood over a typical Queens row house gas range:
- 6-inch round minimum for hoods rated up to 400 CFM
- 7- or 8-inch round for hoods rated 400 to 600 CFM
- Keep the run under 15 feet of equivalent length including all fittings. Each 90-degree elbow adds roughly 5 to 10 feet of equivalent length depending on radius.
- Straight runs beat turns: every elbow the duct makes cuts available CFM. If the hood has to go sideways before it can go up or out, use a low-resistance swept elbow rather than a standard sheet-metal 90.
- Smooth rigid duct throughout: flexible duct for range hood exhaust traps grease and causes flow loss. Rigid galvanized or aluminum throughout to the exterior cap.
What happens when combustion gases stay in the kitchen
A gas range produces CO2, water vapor, particulates from cooking oil, and nitrogen dioxide as combustion byproducts, along with CO at lower levels if burners are not tuned. The EPA’s indoor air quality guidance identifies gas cooking as a significant contributor to kitchen NO2, which in poorly ventilated kitchens can reach levels above its outdoor air quality standard. NO2 is not something you smell at low concentrations, and the remedy is straightforward: exhaust it before it migrates to the rest of the house.
In Astoria the kitchen opened directly to the living room through an archway, so without effective exhaust, cooking at peak output was ventilating the entire first floor. A properly sized and ducted 400 CFM hood captures 70 to 90 percent of cooking plume before it escapes the range canopy. A recirculating charcoal-filter hood removes some odors and particulates but does not remove CO2, water vapor, or combustion gases. In a gas kitchen, exterior exhaust is not optional.
Make-up air in a tight house
In newer or heavily air-sealed construction, a 400 CFM exhaust fan can depressurize the house enough to cause backdrafting in gas appliances. In a 1940 Astoria row house this is rarely the issue; in a gut-renovated house with spray foam and new windows, verify with a blower door test. Make-up air options: passive kitchen inlet ($80 to $200) or an automated motorized damper tied to the hood switch ($600 to $1,500 installed).
The Astoria remodel outcome
In the Astoria house, the existing duct was stripped and replaced with 7-inch round rigid aluminum, routed up through the cabinet interior and straight through the soffit to the exterior with a single sweep elbow at the transition — total equivalent length dropped from 60 feet to about 20 feet. The existing 400-CFM hood moved 320 CFM after the reduct. The homeowner later chose to upgrade to a 600-CFM unit to get full capture over the back burners on the 36-inch gas range. The kitchen duct and hood replacement was handled by a Queens kitchen remodeling contractor who knew the routing constraints in the soffit above the original 1940 kitchen layout.
Frequently asked questions
Why is my range hood not capturing cooking smoke even at high speed? Usually the duct run is too long, has too many elbows, or uses undersized or flexible duct that creates too much resistance. The fan delivers rated CFM only at the bench. Measure your duct: if it is longer than 12 to 15 feet equivalent or uses 3.25-inch-by-10-inch rectangular duct, the duct is the bottleneck, not the fan.
What size duct does a 400 CFM range hood need? A minimum 6-inch round duct, with the total equivalent duct length kept below 15 feet. A 7-inch duct gives more margin for a run with any elbows. Rectangular duct of equivalent area underperforms round duct at the same static pressure because of higher surface friction.
Does a gas range really require exterior exhaust, or does recirculating work? For gas cooking, exterior exhaust is strongly preferred. A recirculating charcoal filter removes some grease and odor but does not remove CO2, water vapor, NO2, or combustion CO from the kitchen air. The EPA and CDC both identify inadequately ventilated gas cooking as a meaningful source of indoor NO2 and particulates.