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12" to 8" = 8"?


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Here's the issue. My old cobbled together chimney is finally dead. It was an assortment of 12"x8" hot air duct and where it went through the roof it converted to 8" round.

It never worked all that great and so I thought I'd go with 12" dia pipe. But then it occurred to me that since I can't get a chimney boot / flashing for 12" I'd still have to reduce down to 8" to get through the roof.

SO, is it even worth it to use 12" below the roof if I have to reduce down to 8" to get through the roof?

If it IS worth it, where should the conversion take place? Just before the passthrough or can it be further down?

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12 inch x 8 inch rectangular duct is 96 sq inches.
8 inch diameter = 50.2654 sq inches. Area = pi x r2 or pi x r x r so 3.14159 x 4 x 4 = 50.2654 sq inches.
From 96 sq inches to 50 sq inches, you are reducing the area by one half.

Industrial applications put both round and square through roofs, check with a local HVAC dealer for suggestions.

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Industrial applications put both round and square through roofs, check with a local HVAC dealer for suggestions.


Unfortunately, that won't work in this case. The roof uses corrugated asphalt panels for the roofing and the flashing that matches the corrugation ripples are only available through the roofing manufacturer. And they don't have 12" dia flashing.

So, back to the original question. Is it worth bothering to use 12" diameter pipe if it's just going to get reduced down to 8"?

Edited to add. The 12" x 8" air duct is what is getting THROWN OUT. I'm considering replacing it with 12" round.
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Gobae,

I don't have any documentation to back me up, but I'd just go 8", but go taller.

Seems like the reduction might only cause problems. I'm thinking in terms of a choke in a shotgun barrel, as opposed to , say, a temporary reduction in a water pipe.

You need to get the smoke moving, so it seems like the smallest port would define your efficiency.

... but I've been wrong before...

Don

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Gobae
Go to the top of the forum page and click on user CP
Click on edit profile
Go to the bottom of the page, enter your location and save.

We would like to know where in the world your located.


No problem, done. But "where in the world" are you located? I'm sure it must seem like you live at IForge, but I bet you have a physical location too.
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There was discussion about chimney stacks on the the Forge mailing list recently. A lot of people seem to be of the opinion that 8" is too small and that you need 10" or 12". I would think (don't know for 100% sure) that reducing from 12" to 8" will only cause you problems. If you want to stick with 8" I would do what Don suggests, and make the stack tall.

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You will want at least 12" round all the way out - 12" square is better. I have tried smaller sizes and size matters with regard to good smoke exhaust. An 8" reducer will act as a choke.

HVAC contractors make flashings all the time - I'd contact someone and have them build it. You may not want them out to the shop if it's a potential permitting issue, but you should be able to give them a short section of pipe and maybe even a piece of roof. They will hand back a piece that your chimney should easily slip thru - maybe 12-1/2" ID. Of course, this will let rain inside so after your chimney is installed in the right location, go to Home Depot and get a roll of 6" flashing. Roll this around the chimney just above the manufactured flashing and fasten with pop rivets, then finish with roof tar. I did this on my corrugated tin roof and it does not leak.

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Is there any chance you can go out the back of the building you are in?


One of the first chimneys we installed did go out the side of the building. But that meant there was about a 4' horizontal run to get from the forge, out through the wall, and past the roof overhang. That one rotted out along the bottom of horizontal and had virtually no draft.

It's beginning to look like the best way to deal with this is to suck it up and find a decent method of getting 12" through the roof. Or get a bigger exhaust fan :)
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Is it a possibility to put an in line fan inside the 8" pipe to assist it?


employing forced draft you can run the flue gas through any maze your particular dementia can devise :P

for "hot" applications inline is generally pricey (if you want it to last)
but then you can get around that, you could cool the fue gas first with a heat exchanger and scavenge the heat for use, or you can indirectly force the draft with a venturi
Injector

the other consideration for any inline fan would be tars, creosotes and soot that would condense out or foul it, so indirect is generally a better bet
natural draft on the other hand has far fewer single points of failure
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According to code (years ago) if you have to run exhaust pipe horizontally you must have a 1/4" rise. 1/4" per running foot of horizontal pipe. Of course the more the better. So if the horizontal run is 4' then the pipe must rise 1" from the start of the run to the end where it turns vertical. That will help the draft, the corrosion would be something different. The acid created would then run down to the elbow or whatever the lowest point is and you would still have the corrosion problem. You can slow the corrosion if you have an efficient fire. One that is hot enough to burn clean.

Fred

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