Range hood ductings!!!

[rcjian]

Hi there, newbee here, I'm glad I found this site coz I need an opinion on Range hood ductings. I recently bought a new Range hood and it requires a 7 inch duct size, my old range hood uses a 4 inch duct size, in short, 4 inches duct size is already built inside the walls and ceiling of my house. And to change it to 7 inch duct requires detaching a part of the ceiling and of course I'm trying to find solutions to avoid that. My question is, would it be alright if i just let the new range hood uses the 4 inch duct size which is already built inside the walls or I really need to change it to 7 inch? thanks a lot!!

[LazyPup]

When comparing different sizes of pipe or ducting we must consider the cross sectional area. The formula is Large diameter squared divided by the small diameter squared.

Let us now consider your 4" compared to the required 7".

N=(7x7) / (4x4)
N=49/16
N= 3.06

This means that a 4" duct only has 1/3 the capacity of the required 7" duct.

In actual operation it will work, however in order to do so the internal working air pressure and velocity of flow will be much higher resulting in vibration, excessive noise and probably overheating the fan motor.

[HayZee518]

Just check the cfm's of the old motor against the new motor. If the old, say, is 1050 and the new is 1200, you won't overheat the motor. air velocity may be partially restricted but I don't see any problems.

[rcjian]

Thanks for the quick reply guys.

The CFM of my old Range hood is 120 and the new one is 300. They are both made by Broan. My kitchen is in the middle of the house and the 4" duct goes to the side of the house not the roof. Thats why replacing the duct will be cumbersome for a me because of the location of the kitchen.

[HayZee518]

question - why did you go with a higher cfm fan?

[mrcaptainbob]

Stove vent....

Would a second in-duct fan mounted near the exit point help ? Maybe even one that's a higher cfm?