Can we replace 10" duct with rectangular rough equivalent in vent hood

8 years ago

last modified: 8 years ago

Hi - I'm planning on installing a 1200 CFM vent hood (possibly ModernAire) above my 48" range - I know the recommended vent size is 10" - however, can this be replaced with a vent which has roughly equivalent cross section area but isn't 10" in diameter?

Featured Answer

opaone

8 years ago

The transition from round to rectangular or reverse can cause rather extreme static pressure. The shorter the transition the greater the SP problem you'll encounter.

Make sure to get a proper calc on your full run for exactly how it will be built (including MUA and the hood+baffles itself) and look at the impact on the fan curve for whatever blower you're using. You could easily find yourself with only 200 CFM of exhaust and a lot of air movement noise. Once you're happy with the design and calculated performance then make sure it's installed properly. A surprising number of HVAC people do not understand the impact of static pressure on blower performance.

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Comments (6)

mtvhike
8 years ago
I'll let the pros weigh in, but I think it's cross section should be somewhat greater (a round tube has less resistance than a square tube).
sktn77a
8 years ago
Are you trying to vent it through a standard 4" (actually 3.5") interior wall space?
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Kalcium
Original Author
8 years ago
For now I think the idea is we will go up to the ceiling, then straight into through the ceiling joists to the outside wall. So I don't think we will be moving vertically through walls, but if we decide to vent to the roof then we may have to do so...
PRO
Victory Range Hoods
8 years ago
If you need to use a rectangular duct for a blower that produces 1200CFM, you will need to get a duct that is either 8" x 10" or 9" x 9". These sizes aren't too common so you may need to get it custom made in your case along with a customer transition to go from 10" round to either of the two sizes above (assuming your hood starts with a 10" round duct). If you try connecting it to a rectangular duct that is small that what I just described then you will most likely cause more air resistance which will produce more noise and likely reduce the actual CFM you are able to produce.
Kalcium
Original Author
8 years ago
great - thanks for the insight!