Problems venting Imperial range hood

texasgal47May 13, 2014

I was estatic to discover this morning that Imperial began offering, just this month in May, baffles for their Slim Line (6" high) range hoods. Problem solved, or so I thought, until Donald at Vent4Less informed me that at least 18" straight vertical venting is required above the range hood before angling the venting in another direction to allow the vent hood to operate properly. I believe it concerns preventing blowback of the air supply. Imperial range hoods do not vent out the back until their larger models. My range hood will be installed against the middle of an internal 10 ft. wide wall of a single story house. The kitchen cabinets are already installed with an open vaulted ceiling with lovely lighting reflecting up on the ceiling immediately above. There is a 30w x 24h x 12d cabinet above the hood, cabinets on either side, and 6"h stacked crown molding on top. Plans are to use a closet in the master bath immediately behind this kitchen wall to vent back and then turn and vent up to the roof. The problem is that there is only 22.25" total vertical space available from the top of the range hood to the ceiling of the cabinet above and a 7" round vent being installed. All of these requirements are not going to fit inside the space available inside the cabinet as far as I can see. The currently installed venting is a mangled mess going up inside the wall for the OTR microwave that was previously in place. I've contracted with the roofing folks, who are already doing some minor roof maintenance, to close up the roof where the current roof vent is in place and install a new one where the 7" round vent will exit. From my perspective, here are the options:
1. Take the venting straight up to the roof like it should be done. There is just 14" height above the top of the crown molding to the ceiling and then 30" from the ceiling to the roof directly above. But no, my female brain doesn't work that way. I don't want that boxy protrusion of sheetrock around the vent messing up the looks of my open vaulted ceiling.
2. Running the venting horizontally to the left on top of the cabinets can't happen because the outside wall is only 5 ft. to the left and that's where the roof rafters are meeting the side wall.
3. See if Imperial would do a custom rectangular vent out the back of that 6" high range hood and then immediately transition to the 7" round vent. Seems to me that might invite problems with air turbulence, and I don't think I want to invite additional noise problems. I just checked with Imperial. They don't back vent until the 18" high range hoods.
4. Do an immediate 90 degree 7" round vent transition on top of the wall cabinet and then horizontal back into the closet behind. Or do a gradual 45 degree angle inside the wall cabinet, toward the top, and then vent into the back wall above the cabinets. If this works, this would eliminate going into the closet. I measured my total depth on top of the cabinets to the top of the molding and it is a little over 7". I'm afraid though this may not work as there is an 8 ft. high ceiling in that closet with horizontal roof rafters immediately above. I'll have to have my tall son with the long arms drive some test nails into the sheetrock from the kitchen side to check this out.
5. Remove my current wall cabinet in place, buy and have installed a shorter one, and buy an 18" high Imperial range hood so I could do the 7" round vent out the back.
6. Go with my second choice and buy a Kobe range hood which has a back vent. However, I really like the superior quality of the 304 Marine grade SS, the continuously variable controls, and the 7 yr. warranty of the Imperial range hoods.
Anyway, my deep gratitude for suffering through these ramblings. Any feedback you can provide would be most gratefully appreciated as I'm stuck at this point.

This post was edited by Texasgal47 on Tue, May 13, 14 at 13:45

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[I feel like I need to parse that one paragraph blob in a word processing app just to decode its content. Only having read it thrice, I'll limit my comments for now to hood construction and general requirements.]

In order to obtain relatively equal flow through the filter area, whether baffles or mesh, there has to either be some height above the baffles (probably commensurate with their airfoil length), or similarly above the mesh (narrow dimension), or the mesh has to be so restrictive that it chokes the flow equally at all points (not good). So of all the noted configurations that weren't already rejected by the OP, I would vote for narrowing her consideration to whichever ones provide enough height for the selected filter type and dimensions.

Baffles have advantages over mesh filters, but both can filter if the meshes are kept clean.

Less tortuous duct paths are preferred over more tortuous paths.

Full power actual flow rate (CFM) should be roughly 90 cfm/sq. foot of hood aperture, if possible. A rated capability of the blower of 1.5x that above should accommodate pressure losses in ducting, hood, filters, transitions, and MUA restrictions.

Duct area should allow the air velocity in the duct at full power (90 cfm/sq. ft times the aperture area in square feet divided by the duct area in square feet) to be between 1000 and 2000 feet per minute. Aim toward the lower value if all the ducting remains warm in winter, or the higher value if passing through a cold attic.


    Bookmark   May 14, 2014 at 10:05AM
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Thank you kas, I was hoping you would respond. Sorry you had to reread 3x to try to decode. I had to read your next to last paragraph 3x to understand the necessary calculations. Haven't done that yet, but will make my calculations. Further Imperial recommendations for my situation was to go up the required 18" vertical inside the cabinet, then angle 45 degrees out the top of the cabinet and to the wall behind. They then recommend at least 5 ft. straight duct run from the first 45 degree angle before angling up to the roof.
Kas, I do have a question for you. With ducting angling above the cabinets, could I get by with just painting the duct that is exposed on the kitchen side, or would you enclose with fireproof sheetrock? Other questions-- I notice some hood manufacturers recommend a T-shaped roof cap. Do you or anyone else have any thoughts or recommendations on roof cap style?

This post was edited by Texasgal47 on Wed, May 14, 14 at 15:15

    Bookmark   May 14, 2014 at 3:00PM
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Although I am a belt-suspenders-duct tape kind-o-guy, normally residential hood ducts do not pick up enough grease to support a fire, particularly if firestopping baffles or mesh filters are in place and the filters also do their job as grease collectors. Otherwise, firecode sheetrock would be required all the way to the roof.

I think what you do on the outside of the ducts is an aesthetic issue.

My roof cap is a centrifugal blower with its own housing and damper, with its air output aimed down the roofline. It is made by Broan and sold by Wolf. Although it could be buried by snow, it seems here in southern NH that it doesn't, and in any case, such snow would have to be fast falling wet snow rapidly turning cold, because anything movable can be blasted off the roof in front of the fan position, or melted by warm impinging air.

However, others may not be so lucky, and an external fan of the upblast type (what one typically sees on a restaurant roof, but smaller) may be a better choice. My secondary kitchen fan is of this type, obtained from NuTone many years ago.

If the blower is in-line or at the hood, and not so powerful, then a T-shaped cap can certainly be designed to keep out rain and provide some height above the snow. I wouldn't want to make recommendations without having some experience with them. I think some sort of damper should be present at the roof cap, and at the hood (just above is ok). Duct mounted dampers can be obtained from Fantech.


    Bookmark   May 14, 2014 at 9:34PM
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Thank you kas for your thorough reply--greatly appreciate all your help.

    Bookmark   May 15, 2014 at 9:40AM
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