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10' duct = too big??

15 years ago

Aargh.

So, just when I thought we had landed on hood (Independent, 42x27, 1200 cfm, chimney style), DH brings up a concern re. size of duct. He notes that 10" is a v. big hole and will lead to cold air coming in.

Researched extensively before choosing this one (Prestige and MA also contenders.) Stove = 36" BS, all burners, and I cook heavily. So, lots of smoke and smells with which to deal.

In our application, venting is to outside wall directly, after relatively short run up wall (to 9' ceiling).

Thoughts? Help!

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Comments (6)

venice_2008
Original Author
15 years ago
Umm. That should be 10"!
clinresga
15 years ago
That's what a backdraft damper is for: it prevents cold air from blowing backwards down your vent. As long as you have that, it's no problem. And, in every other way, 10'' duct is superior to smaller duct.

Sounds like a great choice of hoods and installation. Stick with it!
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Alright, I spoke to the roofer to get more information. I should have done this in the first place but wanted to get this post up here quickly as the work is being done today. Roofers are being great. The issue appears to be that the HVAC supply places they went to are telling them that a 6" gooseneck is the largest they can supply and it is the supply store telling them to connect the 10" duct to 6". After explaining what I want and sending some examples of 8" and 10" roof caps (from Broan) they are rechecking today what they can get. I also provided them the spec sheet of the range hood they can show the HVAC place. The issue of fitting the vent between seams isn't an issue. I've found the 8" Broan vent is in stock locally but 10" could be difficult to find in time. So the question somewhat remains about connecting 10" duct to 8" vent cap. I'm looking at 42" 900cfm hood liners for over a 36" gas rangetop (Hestan). Duct run would be upwards at a slight angle maybe 8-10' from hood to vent cap.
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sombreuil_mongrel
15 years ago
I have a 10" duct, but it is horizontal, running about 6 feet. I can imagine if a large duct with a long vertical run were to be installed, it could set up a plunge of cooled air falling through by convection. The damper on my duct flaps in the wind occasionally, but seems to exclude cold pretty well.
Casey
grendal_fly
15 years ago
Venice_2008
I have a similar setup with a Wolf hood and don't have any issues with the cold here in Chicago. I have never had issues with the backdraft damper flapping either.
venice_2008
Original Author
15 years ago
Thanks to everyone for your reassurance -- just what I was hoping to hear. I didn't see how it could be an issue, as it didn't come up in any of the extensive, near-obsessional research I did, but one never knows ...

Will share with DH, and I think we'll be sticking to the plan!
kaseki
15 years ago
Duct diameter should be sized so that the cfm that are actually pulled (pushed) by the fan keeps the effluent at a velocity that aids removal rather than condensation of grease in the duct. Historically, the preferred range per ASHRAE and others was 1000 to 2000 feet per minute. I recently came across, but haven't had time to read, a paper at www.Greenheck.com that asserted (IIRC) that new evidence points to 500 feet per minute as being better for kitchens. The older ASHRAE flow range was claimed to be optimal for industrial dusts. I can't recommend the new value without more investigation, but it should be considered.

This drop in desired velocity implies using larger diameter ducts than needed before, given that cfm is determined more by the hood and cooking requirements. Bottom line is that if a 10-inch duct will physically fit, and you can move air at at least 500 fpm on maximum through it, then use it. As noted above, the damper will handle the backdraft, and insulating the duct up to the damper will help reduce any moisture condensation at sub freezing temperatures if the duct is subjected to them.

kas

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