Please Critique Vent Hood Plans

Texasgal47February 6, 2013

Let me apologize in advance for all the detail that follows, but I don't know how else to present my situation. My single story home is small but has a very open floor plan, with vaulted ceilings throughout, so noise from a vent hood is a major issue for me. I was going to use an OTR microwave but have changed my mind at the end of a kitchen remodel and want to install a hood insert with a silencer and in-line ventilator. There is a 30" induction cooktop with a 30w x 24h x 12d cabinet mounted 35" above the cooktop -- 42"h cabinets connect on either side. A vaulted ceiling is above this wall of cabinets with above cabinet lighting already installed. An exterior wall is 5 ft. to the left of the center of the cooktop. There is ducting already in place in the inside wall above the cooktop from the OTR microwave which was in place prior to the remodel. A 5" duct emerges into the middle of the wall cabinet above the cooktop, but it feels like a 3.5 x 4 duct is inside the wall rather than the standard 10 x 3.5 duct. I know this doesn't make sense but that is what I can feel without tearing the wall out. There is 10" inside the cabinet above the 5" vent, plus 21" from the top of the cabinet to the ceiling, and 24" above that in the attic for a total of 55" height immediately above the vent hole. Here are my plans:
1. Keep the cabinet above the cooktop in place with a 6" high custom hood surround built to house a Trade-wind VSL430BF Designer Series 28.5w x 4h x 18d hood liner. Trade-wind states that this liner is also available 22" deep, but I have yet to find a hood liner insert listed anywhere (will need to call the manufacturer). Although I know the deeper the better for capture of the cooking effluents, I'm not sure I would like that sudden change in depth of 22" for such a narrow kitchen. Any thoughts from the kitchen design folks? I chose this liner because it was within my budget and was one of the few with baffles.
2. Have a transition 8" to 6" duct which will make either a 90 or 45 degree turn through the inner wall to the closet on the other side. Even though "S" or back-to-back elbows are to be avoided if possible, the duct would then make an immediate 90 degree turn upward so that the damper and then a Fantech 6" duct silencer installed vertically with a Fantech 6" in-line blower above and then the roof cap. Because of the lack of height, a part of the silencer would be inside the closet and then rise through a hole cut into the ceiling. (I should add that the closet does have an 8 ft. ceiling.) The silencer is 24" long and the height of the blower is 8". I believe the roof cap already in place is in the roof above the in wall vent. I would prefer to run the silencer above the closet ceiling and have the duct work follow the roof rafters toward the center of the house to add the blower but don't want to deal with closing one hole in the roof and then adding another for the roof cap. My questions are as follows:
1. Based on the dimensions given, do you think I have enought height to do this installation?
2. With the duct transition and the immediate S-curve in the ducting, how bad will the noise be?
The Fantech 6" blower is rated at 483 cfm under optimal conditions and would be obviously reduced with all the contortions of the duct work. Fantech's exterior fan for 6" duct is 227 cfm at 7.5 sones and for an 8" duct is 401 cfm rated at 8.9 sones. Since I have a patio home, I do not think the noise level of an exterior fan would be acceptable for this application. Any thoughts or suggestions would be greatly appreciated. I just don't know if separate components are workable given my current parameters.

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Since posting, I have discovered the Zephyr AK9428AS which is their hood advertised as having the DCBL Suppression System due to having a DC motor. I'm wondering if this would be my best option. This hood allows for horizontal venting with a 3.25 x 10 duct. I believe the info said there was also the option of a horizontal 8" vent but am unclear regarding this. If it does, I'm assuming that going through to my closet with the 8" duct would be the better option. Is this correct? Lowest speed is 250 cfm at 0.8 sones with highest speed at 715 cfm. It has 6 speed settings. There is a discussion of this hood on gardenweb with subject title: Any Experience with Zephyr DC motor hoods? of June 28'11 with a response by kaseki for those who would like to google this topic. I have found with induction that I cook in the low to mid-range so the first two speeds should suit my cooking requirements. In some photos of this hood, it appears to have a slight bonnet effect with baffles only in the back half. I plan to call Zephyr regarding that aspect. The Trade-Wind liner has baffles almost across the entire surface which is flat. One thing I forgot to mention is that I would need to have an access panel installed in the ceiling of my master bath closet for maintenance and am wondering if I could avoid that by purchasing this hood with an internal motor. Are there hood inserts with a more bonnet type effect from reliable dealers which would fit my 30" width and approximate $1100 budget for parts? Also, is enlarging an existing roof opening in going from a 6" to a 8" roof cap much of a problem and would I need the roof guys out for that?

    Bookmark   February 7, 2013 at 7:15AM
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Running the fan at low speed will obviously reduce noise. The Fantech silencer can reduce blade tip noise for fans above it, but not below it. Some additional turbulence noise would be induced by an S-bend, but maybe not that much. If the fan is in the hood, it is unlikely that any noise created above it will dominate the fan noise.


    Bookmark   February 7, 2013 at 9:15PM
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Kas, thank you for your response. I already had a clear understanding that a silencer has no effect on noise below in a vent system. I believe your last sentence is saying that noise created in the "S" curve venting would be unnoticed due to the noise made by the motor within the vent hood. However, efficiency of the system would be negatively impacted. Your reassurance regarding the "S" curve venting is quite helpful, and I greatly appreciate the time you took to reply. Regarding your June, 2011 comments related to the direct current motor in the Zephyr, I believe your were impressed that the manufacturer could offer this type of system for the price point listed. I'm assuming this motor should offer greater reliability than the in-line Fantech fan vent I'm considering, is this a correct assumption?

    Bookmark   February 7, 2013 at 10:06PM
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