Range Hood Ducting into Roof Vent

[aigel]

Here some home improvement questions for the brain trust, please:

I bought a hood (a hood microwave) that puts out 400 cfm and need to vent it. The current kitchen did not have a hood, but the previous one probably did, because the single story home's roof has an existing vent in the roof above the oven area.

The hood puts out into a rectangle which I adapted to a 6" diameter ducting starting in the cabinet and I am done running solid 6" duct into the attic.

The roof vent is a chimney looking job. It looks like it is 8" on the base and runs down in diameter to 6? There isn't a flange or anything to connect to.

Questions:

- It is not clear to me how to connect to the existing roof cap. Do I push a 6" solid duct into this until it "locks" / wedges into place? Or could it be an 8" cap and I need an adapter?

- Is it okay to connect two solid ducts from kitchen to roof with a flex duct to make up for the offset (about a foot offset and 2-3 feet of ducting, I guess)

- Do I need to insulate the ducting? I am in a moderate climate in the San Francisco Bay Area.

Thanks!

George

[dad911]

Don't need to insulate, but it won't hurt.

Sheet metal ducting, not critical. You can mechanically connect the ducts together, and seal them with adapters, metal foil tape (don't use duct tape) and/or duct mastic

[wdfifteen]

Originally Posted by aigel

The roof vent is a chimney looking job. It looks like it is 8" on the base and runs down in diameter to 6? There isn't a flange or anything to connect to.

Questions:

- It is not clear to me how to connect to the existing roof cap. Do I push a 6" solid duct into this until it "locks" / wedges into place? Or could it be an 8" cap and I need an adapter?

It could be 8" if the old vent was a powerful one. The only way to tell is to get up there and measure it. If it's 8" use one of these.



Hold it all together with self drilling screws.

Originally Posted by aigel

- Is it okay to connect two solid ducts from kitchen to roof with a flex duct to make up for the offset (about a foot offset and 2-3 feet of ducting, I guess)

I guess it would be OK, but I would use 2 adjustable ells and a piece of straight duct. It's cheaper and not much more work. Again, use self drilling screws to hold the pieces together.

Originally Posted by aigel

- Do I need to insulate the ducting? I am in a moderate climate in the San Francisco Bay Area.

Probably not. The only danger is moisture condensing on cold duct work and dripping back into the vent hood, but if it's a short run that shouldn't be a problem.

[A930Rocket]

My understanding is flex pipe doesn’t meet code for vent hoods. The pipe catches grease, etc.

Of course, the directions to our FridgeAir vent hoods say to use the flex pipe.

[911boost]

Zero flex pipe was used in my kitchen remodel for the vent hood. We had to go outside through the garage and I insulated and built a soffit around it.

[aigel]

Thanks all! I had a successful install. Maybe did over-analyze before getting in there hands on ...

The flex pipe went nicely into the roof cap , I was pulling the duct in and buttoning it up from the top of the roof.

I'll have a look at the grease collecting in the flex pipe next time I clean gutters.

G