Bud in PA Posted April 7, 2022 Share Posted April 7, 2022 I have made my chimney as close to the Hofi style as I could. 14" x14" side draft of 3/16 steel plate. 12" diameter air handling tube 10' high. I would like to put a cap of some sort on the chimney top, about 12" above the top of the chimney, will this cap restrict the flow of the chimney? Quote Link to comment Share on other sites More sharing options...
Irondragon Forge ClayWorks Posted April 7, 2022 Share Posted April 7, 2022 It shouldn't at that height above the top. There is probably a formula to figure it out, I just don't know what it is. Quote Link to comment Share on other sites More sharing options...
Daswulf Posted April 7, 2022 Share Posted April 7, 2022 For the cap, make sure there is larger spacing than they offer on store bought caps. You don't want to restrict the flow. Quote Link to comment Share on other sites More sharing options...
Frazer Posted April 8, 2022 Share Posted April 8, 2022 According to my somewhat out of date version of NFPA 211 (2015) for unlisted caps the minimum distance between the underside of the rain cap and the top of the flue should be 0.7 times the long dimension (it assumes a rectangular cross section) of the covered flue. Applying this formula to a round cross section the pipe ID would be the long dimension. As an example my chimney has a 10" ID. So 10 * 0.7 = 7" which, coincidentally, is the same height as my store bought cap. -- Full disclosure, this number (0.7) is only specified for masonry fireplaces and my applying it to a round cross section might be misuse of the formula. Also, all of the caps for pipe ranging from 10-16" ID have a 7" gap (at least from the company that makes my particular stovepipe) so this formula, understandably, doesn't always hold up for listed parts. Just remember the higher you make the cap, the less precipitation it's going to stop (unless you make it bigger). 12" might be unnecessarily high, an 18-20" cap with an 8-9" gap seems reasonable IMHO. Just my 2c. Quote Link to comment Share on other sites More sharing options...
Irondragon Forge ClayWorks Posted April 8, 2022 Share Posted April 8, 2022 When I installed my chimney I made the cap out of a piece of scrap stainless steel. I sorta guessed on the opening and had the ends extend out. It has been working just fine for 30 or so years. Quote Link to comment Share on other sites More sharing options...
Bud in PA Posted April 8, 2022 Author Share Posted April 8, 2022 thanks to all who replied. I have an old stainless kitchen trash can that I intend to use similar to Irondragon's. Quote Link to comment Share on other sites More sharing options...
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