Help with downdraft vent ductwork

17 years ago

I am wanting to replace the downdraft vent for our island cooktop. Our current system is a Dacor with a 600 cfm remote fan with a 7" diameter round duct and I'm trying to figure out if we'll have to change it. My husband said we can fit 8" if we have to but he'd obviously rather not have to change it out. He said 10" is huge.

I am looking at an Electrolux Icon downdraft with 1600 CFM remote blower. Their installation manuals says it recommends round duct to maximize performance and then it says 3 1/4" x 10" duct has a maximum equivalent straight length of 60' and 10" round duct has MESL of 70'.

The ductwork will have a 3' drop, a 90 degree elbow and a 21' straight run to the exterior. There is a 3 1/4' x 10" opening out the bottom of the downdraft and a 10" round opening at the remote fan.

A 7" round duct has greater cross sectional area and less surface area than the 3 1/4" x 10" rectangular duct that they give as an option so it seems like it should work but why would they also give 10" round.

Any thoughts?

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Comments (3)

fandlil
17 years ago
You have the benefit of your experience with your present system. I assume you find it not up to snuff, or you would not be looking to change it.

You probably already know that downdraft is not the optimal way to go, but in an island system an understandable option. Generally you should go with duct specs recommended by the manufacturer of your fan blower, or you will get less than optimal exhaust. Your ducting is not that long and has few trists and turns that would slow down air flow. 1600 cfm is a lot, and may necessitate make up air, which will raise the cost and complexity of installation. Do you really need that much exhaust capacity . . . almost like what they have in a small restaurant kitchen that's going 8-10 hours a day.
mdod
Original Author
17 years ago
Haus_proud: Yes, I know downdraft is not the preferred method of venting but it has been fine for our purposes. We seldom use it anyway. We're redoing the entire kitchen and it's 18 years old so we figured we should go ahead and replace it too. I'm pretty sure it is a 600 CFM fan. I asked about just replacing the downdraft portion and keeping the remote fan but the sales rep said he thought they changed the wiring. I'm not sure if that is something that we can get around. The current unit has 3 speeds and an 8" rise and the Electrolux that I am looking at has variable speed, a 10" rise and costs less so that was the incentive for going with it if I had to replace both the downdraft and remote blower. I figured the variable speed would let me use lower CFM if I wanted to and I preferred the dial over the multiple speed buttons that collect gunk and seem like the weak link in the system.

The instruction manual lists 3 1/4" x 10" and 10" round. And it says not to use duct with less cross-sectional area than what is recommended. 7" round has more area than the 3 1/4" x 10" so that seems like it should work, no? My concern is that the area of a 7" round is half that of a 10" round so why would you want/need to use 10" round?
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fandlil
17 years ago
If you have crunched the numbers and found that the area of a 7" diameter circle is comparable to the area of a 3 1/4" by 10" rectangle, then you should be okay. (I forgot my high school math.) I agree that it's probably a good idea to replace the existing duct work. Who knows how much gunk is stuck in there after all those years?