Input on duct diameter affecting CFM in range hood installation

[Adam Finkelstein]

Greetings.

I have this scenario. I have a 1500 CFM fan that requires a 10" duct. My floor joists are 9.5 inches wide. If I go with the 10" duct specs, I'll have to build a chase to cover the duct, and this will cause several design problems within the room as well as being more costly.

However, if I use a 9" duct, I can install the ductwork between the joists and not have to build a chaste.

What will decreasing the 10" rated duct to 9" do to the functionality of the fan? If I have 1500 CFM with 10", what performance loss will I realize with 9"? Are there any other concerns going with 9" over 10"

Thanks in advance for your responses. Sincerely, Adam

[ELAhrens]

The reduction from 10" to 9" will increase the velocity pressure and noise in the duct but I doubt the overall performance will be affected. Make sure you do NOT use flexable spiral duct. A few things you can do is ovalize the 10" or get square/oval duct to fit between the joists (assuming you were going to use round duct). Keep the bends to a minimum. A 10" round duct gives you 78.5 sq.in, a 9x8 duct gives you 72 sq.in.

Hope this helps,

Erik

[Jimmy]

Dare I ask, what do you need 1500cfm of ventilation for ? I know of commercial grills that don't have that much draft.

FWIW, the 9" should work, with a little more noise due to increased velocity - assuming this is not a code violation (required device installed as speced by the manufacturer). What's less forgiving is length and bends. Each 45 or 90 reduces airflow significantly.

[Adam Finkelstein]

Home lead foundry.

Thanks. I appreciate your input. Adam