Range hood ducting question

16 years ago

We are purchasing a 30" Broan Elite 6400 series under cabinet 600CFM range hood to be installed over a 30" Bosch induction cooktop. It will be installed against an interior wall under a short 30" wide cabinet with about a 50" horizontal run to the outside wall. It requires 3.25x10 rectangular ducting vertically and transition to 7" round for the elbow to horizontal run. (We currently have an over the range microwave hood in the same position with 6" round duct running through the soffit between the top of our current 30" cabinets and 92" high ceiling.)

We are trying to fit in slightly taller cabinets (plus found a plumbing obstruction when we opened the soffit) and wondered if the following is possible:

1. Running the duct between the floor joists above. I know some people do this, but am a bit concerned about putting a hole through the rim joist (right term?) to the outside.

2. After the elbow, converting back to the 3.25x10 rectangular duct for the horizontal run to the outside wall. The appliance store didnÂt think this would be a problem. IÂm not an engineer, but I do understand that the volume of a 7" cylinder is somewhat greater than a 3.25 x 10 box. But given the turn, the short run and the fact that we probably donÂt need a full 600cfm for most our cooking didnÂt think that would have a massive impact on the usability of the hood.

Any advice would be appreciated.

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Comments (3)

kaseki
16 years ago
On number 1, my thought would be that the hole would only be a potential problem if a higher level beam were supported by a column that was supported at that point.

To really answer number 2, the actual pressure loss of these transitions would have to be determined as a function of flow rate, and the fan's flow rate versus pressure drop curve consulted to find the final flow rate. (The house leakage and hood losses would also be needed.)

But if you have little choice to make the ducting fit, the only variable might be the fan -- ok as is, upgrade to one better suited for the pressure loss, or add a supplemental outside fan. These possible actions can probably wait until and unless you demonstrate that the hood is inadequate.

If inadequate, then what is needed is a fan that pulls the desired air flow rate at a higher pressure loss. A fan that is just rated at more cfm may or may not do this. Broan publishes fan curves for several of its fans, and I would hope that if you did run into a problem, someone there could recommend the appropriate action.

Expect a bit more noise with more duct transitions.

kas
skit19
Original Author
16 years ago
Thank you for the information. ItÂs nice to know that we should have some workable options. IÂm not so concerned about noise. The micro hood sounds like a 747 landing and barely vents, so anything would be an improvement!
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kaseki
16 years ago
:) So long as the replacement doesn't sound like an F4 taking off.

kas