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Installing a JennAir 600cfm hood, correct size venting?

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homerowner

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Greetings,
Currently I'm finishing up a kitchen remodel, tying up the loose ends.
I was able to source a new JennAir 600cfm hood for a great price. The OEM flap vent that attaches directly to the top of the hood provides a 10" x 3.25", I found this to be available at the local Home Improvement store:

3.25-in x 10-in x 7-in Galvanized Steel Straight Stack Duct Boot

026715002689.jpg
The offset will work well for me, there is a ceiling joist almost directly over the hood, this should allow for an easy install straight up through the ceiling.

My question: This provides for 7" ducting. I'll be using 5' lengths of galvanized steel round duct. The run to outside, if I install this as I'm thinking, will be close to 30' through an unheated attic area (mid 1950's ranch, central New England) and exiting out the side of the house. There would be a single 90 degree involved. Will I want to keep the 7" size for the entire run, or could I go down to 6", or smaller. Looking for advice for best operation of the hood, not so much trying to save dollars.

Many thanks in advance for the advice.
 

homerowner

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I just remeasured the run. More like 25'.
Thanks for your expertise. Now I just need to find 7" ducting. 6" and 8" are all over the place...no 7"?
 

Snoonyb

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Home desperado used to carry it. I've usually purchased duct material from a wholesaler, because I use such a variety of sizes and shapes.
 

homerowner

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Home desperado used to carry it. I've usually purchased duct material from a wholesaler, because I use such a variety of sizes and shapes.
I've been looking at Lowes website. Will check out HD.

The 7" size makes sense. 10x3.25=32.5 square inches of area. 6" ducting only gives 28.25, 7" would provide about 38.5 square inches.
  • The area of a circle is the diameter x diameter x 0.7854
Again, thanks. The range is shipping tomorrow from PA, I should be able to pick it up next week (6+ week wait for the darned thing). I'd like to get the hood operational before then. Off to HD tonight.

****Just checked the local HD locations, lots of 7" available. This may actually go well.
 
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Snoonyb

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They also used to sell 10X3.25 ducting, or with some sheet stock, and with a piece of angle iron and a couple of clamps, you can make your own.
 

geochurchi

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Hi, just curious, if it’s a ranch, why can’t you go straight up through the roof, 25’ is asking quite alot of an exhaust fan .
JMO
Geo
 

homerowner

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I considered that. Just had a new roof put on a couple of years ago, I'd rather not risk getting leaks. If the 25' run is problematic I'll pay a professional to pierce the roof. I don't trust myself doing so.

Spent about $150 for the needed goods, I've got the 7" hole in the kitchen ceiling, but may wait until it gets a little cooler to run the ducting in the attic proper. Hot as blazes up there, and I'm not a young guy anymore.

But I know what you're saying.
 

Snoonyb

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For your area, roof pitch and material familiarity are key, also some folks worry about creating the potential for an ice damn.
 

homerowner

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I've got good under soffit venting, full length. Attic area above soffit is clear for the venting. Open vents each side of the house. When we had the roof shingles replaced, original 65 years of 2 layers, I instructed the roofers to not put in the top vent along the ridge. We've never had ice dams in the past almost 30 years. When we bought the house in '82 I added 8" of insulation to the attic. Our house is the house on the street that has snow on it long past our neighbors. Not a lot of heat escaping upward.

I was worried about putting a vent in the roof because we do get a lot of snow, some years, up here in Central Massachusetts. Also, putting a vent in the roof would have been a 15+ foot run (to get it to the back side of the roof, visuals you know). Adding another 10' didn't seem a deal breaker to go out the side. The reason for the length is we have an attached garage. Viva la 50's!!!!

Thanks much for your responses. I'll bump my way up and give you a bunch of likes.
 

Snoonyb

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Also, instead of the "general" "T" top's there are flat vents which are little more than 2+" high, which are an aesthetic choice.
 

homerowner

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Play by play? Not sure if you really want to see this happen.

Just got notification that the range is in the area. Almost 7 weeks of waiting. Of course, picking it up tomorrow is out of the question, my oldest is getting married. Or, maybe....if I can move a few things forward, .....

Few pics here.
7" hole in the ceiling.
Bad pic of the duct run needed. Currently at 110ºF. Not going up there. Maybe Sunday. Fans are in place.
Pic of the exit vent. I'll be fabbing up a bird guard out of 1/2" mesh to protect it.

And have the preliminary ductwork pushed together, riveted, taped.

Wish me luck up there in the attic.IMG_20200724_162226168.jpgIMG_20200724_162441019.jpgIMG_20200724_162330643_HDR.jpg
 

homerowner

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Time for a little update.
Got the initial through the ceiling flex set in, went well. I plan on taping the entire 4 or 5 foot run just for good measure.
Decided to throw my health to the wind and attempt the attic/crawl space install of the 7" venting. Once I had the 90º bend attached I blasted the hood on high sending 78ºf air up through...it feels like air conditioning.
I should have this stage finished today, including the vent cut into the side of the house.
 

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