What is acceptable ductwork size for range hood?

[stevenmikramer]

kitchen remodeling.

Broan Range hood with 100 to 450 CFM exhaust rate (4 settings).

Contractor wants to use 4" flexible ducting for a 7 foot run to the outside wall.

Is 4" duct too small?

My math says that the air speed in the duct would be 60 MPH at max CPM.

Anyone have any ideas?

[paul52446m]

kitchen remodeling.

Broan Range hood with 100 to 450 CFM exhaust rate (4 settings).

Contractor wants to use 4" flexible ducting for a 7 foot run to the outside wall.

Is 4" duct too small?

My math says that the air speed in the duct would be 60 MPH at max CPM.

Anyone have any ideas?

FAQ check this site.
I don't know what static pressure your blower can build up, but 4" is too small.
I would guess between 6 & 8" Paul

[HayZee518]

a 2 inch X 8 inch rectangular trunk line would be sufficient. a 4 inch aluminum dryer line is too small. there would be too much back pressure and the fan would run slower and draw more amps.

[pushkins]

Flexible ducting is not permitted in almost all (if not all) codes for over range venting.

[paul52446m]

kitchen remodeling.

Broan Range hood with 100 to 450 CFM exhaust rate (4 settings).

Contractor wants to use 4" flexible ducting for a 7 foot run to the outside wall.

Is 4" duct too small?

My math says that the air speed in the duct would be 60 MPH at max CPM.

Anyone have any ideas?

Check the hood specs. you will never get 450 cfm of air through a small 4" round or a small 2" by 8" duct. paul

[HayZee518]

Broan recommends a 7 inch duct outside to a wall cap. alternative is a 3 1/4" X 10" sheet metal trunk line