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tncraft

Ventilation Qs - exterior distance, duct size, etc

tncraft
12 years ago

I didn't think about the total duct run we'll have in the house and how it should be considered when selecting the hood/blower/etc.

I believe we can go straight up from the hood. The approximate total duct run will be 14 ft. Does this mean I have to go with a powerful blower? How many cfm? Does this also mean we have to go with a remote blower because of the duct length?

We are going with a 36" induction cooktop, but will be configuring the kitchen to easily accomodate a gas rangetop (like the CC) in case I'm not totally happy with the induction and want to replace it with gas. So, we want the vent to also be able to handle a gas rangetop.

I was looking at the Kobe and Cavaliere; both has up to 1000 cfm and 8" duct. Would either of these okay considering how far the duct run needs to be? Do we need to go with a more powerful blower?

Another question, why use 8" or 10" duct? Is this based on the range hood's specification? So, if the Kobe hood says 8", I have to go with 8"? Or, can I go with 10"? Hope this is not a stupid question. LOL. Just wondering because out of the discussions I've read here, bigger seems better. ;)

Oh, one more question.... On the Kobe (and other hoods), they include heat lamps. Why would one need those heat lamps? Am I missing what they are used for? I'm thinking, they are used to keep food warm?? But we won't have a high shelf, and the food will be on the rangetop. So, I just don't see any use of it. Am I wrong? This is the only thing I don't like about the Kobe hoods.

Comments (5)

  • Emilner
    12 years ago

    I would not go smaller than 10" on your ductwork. The larger the duct the more air it will flow and the quieter it will be. 1000cfm sounds like overkill for an induction, but if you are getting a grill on a CC then it would be required. The overall duct run is not very far, as long as there are not alot of elbows. Elbows restrict airflow and increase noise. Remote blowers will decrease noise if they are far enough from the hood (usually over 5').

  • weedmeister
    12 years ago

    My mom had heat lamps on her VAH. There was a separate shelf option with fold-down wire shelves. She said it was the thing she liked most about it.

  • tncraft
    Original Author
    12 years ago

    So what is the right size blower for an induction? 400 cfm? 600 cfm?

  • live_wire_oak
    12 years ago

    This may be heresy, and it's a bit backwards, but if you are undecided about what cooktop you want to get, then just don't do a hood at all at first. Go ahead and put in the 10" ducting, because you can always slip an 8" duct inside if the hood you end up using works better with that. But as long as you have easy attic access, you can put the blower any darn where you please when you decide how strong of a hood you want to put in.

    A gas range has a lot more excess heat produced than induction so the two ventilation needs are different. I think most people could get by with a 600CFM on induction as long as they don't plan to do a lot of high heat searing and stir frying. If you do, then go up to 900CFM. For a gas range, 600 CFM is where I'd start with a 30" cooktop with decent BTU power. More BTU's and larger mean more CFMs and larger hood.

    So you really need to finalize your cooking equipment before finalizing your venting equipment.

  • kaseki
    12 years ago

    First, hood size should be selected to assure capture of the rising effluent, which expands as it rises at about 22 degrees half angle from the entire hot pan surface. While an induction surface is relatively cold, the effluent you are trying to collect comes primarily from the hot pan. So a gas surface just adds additional rising heat because some of the heat generated misses the pan.

    Second, choose a cfm that for the hood design selected assures containment. This is vastly easier said than done, but essentially you want the velocity of the air entering the baffles times the area of the baffle portion to be at least that of the rising effluent, about 3 feet per second. Even better would be 3 ft/s at the hood aperture, particularly if the aperture is a flat surface with a mesh filter.

    Or, you can guess and say that for serious cooking you will probably want at least 600 actual cfm. Note that this is the cfm achieved when all pressure losses are accounted for on the fan curve (cfm vs. pressure loss). Given baffles, duct transitions, negative house pressure without good make-up air, etc., you may need a 900 cfm rated fan to actually get 600 cfm.

    Next, size the duct so that the 600 cfm that is being extracted from the house has enough velocity to minimize particle and vapor deposition on the duct walls. This is at least 500 feet per minute for warm ducts, and at least 1000 fpm if the duct passes through a cold attic during part of the year.

    kas