Morning,

I'm finally working on a proper stack for a coal forge in my 16' x 12' backyard shop and planning to use a 10' section of 12" culvert pipe for my chimney going straight up and through the steel roof, extending 4 feet above the peak.  I'm going to use 12" non galvanized black steel chimney pipe to attach my super sucker forge hood to the stack keeping the galvanized culvert pipe at least 3-4 feet above the forge & hood.  

I originally planned to just fabricate a simple cone/dish rain guard cap secured a foot above the top of the stack, but was considering the Low Loss Stack Cap concept documented on anvil fire (Mod Note: Hot link removed at the request of anvil fire) instead.  Following the published guideline, I'd end up adding an additional 4 feet of height to the stack (a 14" diameter, 4.5 foot sleeve secured 6" below the top of the 12" stack pipe) for a total of 8 feet above the highest point of  the roof (4' of stack pipe and 4' of stack cap) ... but I'm curious if the height of the stack cap should be figured into the math when figuring out how far above the roof line (normally 4') I need to be for a proper draft or "secondary draft" function of the cap interacting with the main 12" stack.

Given it's a 12" diameter stack that will extend perfectly straight for 14 feet right out of the forge hood I will probably have plenty of draft in my little shop with a typical cone/cap, but figured I'd check since I like the concept of the low loss cap particularly when it comes to minimizing dark colored smoke exhaust in my semi-suburban shop... but this would likely be offset by having such a tall eyesore-ish chimney pipe coming out of the roof.    

Ideally I'd keep the total stack sticking out of the roof around 5' so either 4' of stack pipe and 1' of cap, or 1' (18" with the 6" overlap) of stack with 4' of low loss cap pipe...  any thoughts or calculations I'm missing to help me decide which will work best?

Cheers -- J

 

I assume you are referencing the followup where they note that the joint between the inner and outer stacks need to be at least 3' above the roof level.  I'm fairly sure that this is not related to functionality of the assemblage, but is an interpretation of the typical international code requirement to have flue vent terminations 3' above roof structure (for applicable codes in your area see: https://codes.iccsafe.org/public/ ).  As with any code references, it is subject to interpretation.  In any event, if you do plan on putting up such a tall stack I would "guy" it against wind loading.  Of course, if you can tolerate some weather going down your stack the low loss cap could probably be shorter...

From he internet 

Constructed as a duct that is attached to and exhaust into a second duct that is 1" larger in diameter, the larger duct allows the air to exhaust while allowing rain water to run down its inside walls and drop outside of the inner duct. Typically, the second duct has a length that is four times the diameter (4 x d) of the inner pipe, plus a 6 inch overlap. It is attached to the inner pipe with brackets and often has a flange on the top lip in order to 'guy' wire the entire unit to the roof. 

 

Thanks for the response gents!

Any idea if the "4 foot above the highest spot on the roof" best practice for proper draft from a 12" diameter forge stack would include the additional 48" from a low loss cap?  if so I'm thinking of putting the roof penetration for the stack on the back side of my shop instead of the front as originally planned (shed roof, slants front to back) so I could run the chimney pipe 3.5 feet above the lower side of the roof deck to get even (plus 6" for the connection overlap) with the peak of the roof, and then attach the additional 4 feet of cap/secondary duct. 

Also.. reality check.  I noted the third time I read the article about the low loss cap on Anvilfire, the author indicated he needed to improve draft from his 8" stack pipe and saw the significant benefit of the low loss cap when he swapped one from his original "cone" cap. With a much larger 12" chimney pipe that runs vertically (no angles/bends) through the roof in a 12'x16' shop I'm wondering if I'm just adding unnecessary complexity and cost for my small time setup by pursuing this?

My thoughts... a 12" chimney 4' above the highest point within 10' (3' within 10' is code) is big enough to not need the addition draft provided by the extra 4' of "cap".  You can add it later if required but leave it off and see how it draws.

  Todd