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Old 01-04-2018, 09:48 AM   #1
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Best way to vent range hood


Currently I have an OTR microwave that isn't ducted. I am taking that out and putting in a 900 cfm range hood under the upper cabinet. It has an 8 in duct. Kitchen's on the top floor, and house is bi-level. I was going to take the duct straight through the attic and to the roof with a gooseneck vent. The roof is angled but not steep. This would involve no turns and a total of about 6-7 feet of ducting. I like this option because it would maintain the suction power. However, I live in Wisconsin and I worry about snow on my roof. I can't find any roof exhaust vents that are very tall, maybe 8" at most from the roof to the bottom of the vent opening.

The other option is to take the duct into the attic with a 90 deg turn and then straight out the gable end. Since it would be exiting the side of the house, snow wouldn't be an issue. However it would need to travel about 18 feet in the attic. So with the 90 deg turn and that length of duct, I'm worried I'm going to lose a lot of suction.

So what do you think?

(I'm not too worried about make-up fresh air return. The house was built in the 70s and isn't very air-tight. We do also have a deck off our kitchen that we can open a sliding door from if need-be.)
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Old 01-04-2018, 10:02 AM   #2
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Re: Best way to vent range hood


Roof penetrations should be avoided when ever possible.
I would take it out the gable end.
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Old 01-04-2018, 10:13 AM   #3
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Re: Best way to vent range hood


Just FWIW....

My experience has been when dealing with the commercial kitchen hoods at work years ago, is that it is best to keep any horizontal runs of hood exhaust ducting as short as possible. I know residential hood applications in no way handle the amount of vapors that commercial kitchens do, but the same exhaust principles apply ...over time.

As soon as the warm grease vapors enter in to the duct, they start cooling down and dropping down on the ducts cool metal. Forming grease build up in the ducts. Making fire hazards. Just why our state requires all commercial kitchen exhaust hoods to be professionally cleaned every 6 months. JMO
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Old 01-04-2018, 08:45 PM   #4
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Re: Best way to vent range hood


I have often wondered about roof vents in snow country but quite frankly have never heard anyone complain about them (I currently have an unducted vent). A problem with extending the vent about the snow line is that it will ice up and I am not familiar with an insulated extended vent cap.

What does the manufacturer say about horizontal vent limits?
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Old 01-04-2018, 08:58 PM   #5
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Re: Best way to vent range hood


I've had one contractor tell me it's best not to put holes your roof if you don't have to. To me this sounds like it's just out of his comfort zone. Another contractor agreed with what I was thinking, which is that the shortest route is the best. I will have pretty easy access to this part of the roof, so I could keep a snow rake-type device nearby. The other thing is that I will be sending warm air through the vent, so the snow probably won't last too long right outside it. (I had another contractor tell me that range hoods aren't blowing warm moist air, unlike bathroom fans, so I could just have the air let out in my attic. Needless to say, I am definitely not going with him).

So I'm definitively going straight up through the roof. Now the question is: should I go straight up or put a bend in the duct going up so condensation has somewhere to cling to? I don't like the idea of having multiple 90 degree bends (even if gradual) because it will reduce suction. But I guess I don't want condensation falling straight back down on the hood. Any advice here?
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Old 01-04-2018, 09:14 PM   #6
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Re: Best way to vent range hood


Yes, contractor #3 sounds like someone to be avoided. I would go straight up but will defer to the pros here. Make sure you use an insulated duct. If you are planning on doing this now, I wouldn't think you will get a good seal at the roof.
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Old 01-04-2018, 10:28 PM   #7
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Re: Best way to vent range hood


Two comments.
1. 900 cfm is a lot of air and noise and I don't think you will use that capacity very often.
2. In regards to make-up air the concern relates to naturally drafted appliances and to sucking tons of summer moisture into your air conditioned walls. The second one may not be a concern but I don't know where you are. However, the first one, the naturally drafted appliances definitely is, IF you have any, primarily a gas or oil water heater or heating unit. A high efficiency direct vent furnace/boiler and an electric or power vented water heater and you are fine. But back drafting needs to be tested and avoided. And a source of make-up air may be required by code.

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