Please Critique Vent Hood Plans
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.