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18" Double wall pipe too big?


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Hello all,  I am having a dilemma.  I have read many a fine posts on chimney size here and am wanting to get some feedback from you all.  I have graciously received from my cousin (works in industrial HVAC) about 30' of brand new Metal-Fab 18" SS double wall duct for free.  It is 18" id SS duct with a 1" fire blanket wrapped around that all encased in a 20" od aluminized steel pipe.  They are in sections about 4' long.  Plus a weighted flue topper.  I am wanting to build a side blast forge and will be running the chimney about 15' total (maybe more if needed). 

Would you run with the big 18" duct, or try and sell it and use the money to get say 12-14"?  I have read that 18" can be almost too big, but height of the duct can offset that to help with draft.  Some people have said that 18" works well on their setups.  I have been struggling whether to sell this pipe (new it would cost about $3500+) or use it?  I may not be able to sell based on the size!  Just trying to make a decision.  It's gonna be a bit of weight to support but I am not too worried about that.  More worried if I will be struggling to get a good draft going?

Any thoughts to help me make a decision would be greatly appreciated.  Thanks in advance.

-Adam

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Thanks guys, that's what I was kinda leaning towards.  My plan was to actually manually control the flue cap with an extension rod (i.e. pull it down to open and up to close or something of the like) if I decided to use it.  They were going to scrap it all, so I figured maybe I could use it!  That duct work is quite rotund.  Hopefully somebody around me is in need of some heavily discounted double wall huge pipe! 

Thanks!

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