16tonBEAN Posted October 11, 2009 Share Posted October 11, 2009 Hi. I've read many threads concerning chimney pipe diameter for a coal forge. I am new at blacksmithing and am currently in the middle of shop construction and still am unsure what chimney diameter to use. My roof will be asphalt shingles on a half-inch deck - no ceiling. The problem is that I already have a forge hood with an 8" opening, but from what I've read, a 12" chimney pipe is preferable. Now is the time to decide on the 12" chimney pipe, but hate to waste the hood. Any ideas? (At this point I have to stick with the steel pipe chimney instead of masonry). Quote Link to comment Share on other sites More sharing options...
David Einhorn Posted October 11, 2009 Share Posted October 11, 2009 12 inches is much preferred. Can you cut out the opening and make it bigger to adapt it to a 12 inch opening? Quote Link to comment Share on other sites More sharing options...
Glenn Posted October 11, 2009 Share Posted October 11, 2009 Side draft is a point of use system where as a open hood suspended above a fire tries to gather air (smoke) from the entire room and is less efficient. Depending on the type of forging you will be doing, size of the fire, and heat from that fire will influence the chimney size needed. 8 inch chimney has a cross sectional area of 50 square inches. 10 inch chimney has a cross sectional area of 78 square inches or 1.56 times larger 12 inch chimney has a cross sectional area of 113 square inches or 2.26 times larger It is generally accepted that bigger is better. I have used a side draft hood with a 10 inch exhaust opening and an 8, 10, 12, 14, 16, and 18 inch chimney. Side draft hood height was 3 feet with about 10 feet of chimney. Each increase in size increased efficiency up to 14 inches diameter. 14 inch was about the same but 16, and 18 each decreased in efficiency. This could have been fixed with a taller stack. (all other items were held constant, only the chimney size was changed) No matter which method you use, you MUST replace the amount of exhaust leaving the chimney with the same or greater volume of fresh air entering the shop. Otherwise you starve the chimney, resulting in lack of draft, and smoke in the room. Maybe others can report about real life experiences with different size chimneys. Quote Link to comment Share on other sites More sharing options...
kevan Posted October 11, 2009 Share Posted October 11, 2009 12 to 14 inches otherwise expect smoke in the shop. 8 inch pipe , a fair amount of smoke up your nostrils, 6 inch, alot of smoke up your snout and you will feel it in your chest as well. I'm with Glenn on all he says. Quote Link to comment Share on other sites More sharing options...
16tonBEAN Posted October 11, 2009 Author Share Posted October 11, 2009 Ok great, thanks for the replies! Yes, I see now that side draft is better and I should make the 12" chimney, side draft work somehow. Thing is I bought a forge unit from Centaur forge awhile back, and that design has the open hood with 8" pipe opening. None of it is installed yet, though, and the framing for the shop is happening this week, so I have a chance to make it better. I will try to figure it all out today - hopefully with some modifications, I can make the forge work with side draft? I don't have much metal work experience yet. Quote Link to comment Share on other sites More sharing options...
thecelticforge Posted October 12, 2009 Share Posted October 12, 2009 I have had the same experience as Glen. When I was at the living history museum, they had 3 12" flues for one firepot. It was a major task to get a draft going in that monster. I finally put stone on two of the flues and it worked much bettter. Quote Link to comment Share on other sites More sharing options...
16tonBEAN Posted October 12, 2009 Author Share Posted October 12, 2009 Well I think I've worked out a solution to use a side draft hood and my existing forge unit - appreciate the advice. I'm having issues locating chimney pipe in my area. Any opinions on 12" Simpson Dura-Vent double-wall class-A chimney pipe the whole way up - from forge to cap? (except the expense). I really only need 7' of pipe to get 4' above the ridge pole, but a lot of people seem to think that 10' of chimney is kind of the magic number for a large side draft hood. Quote Link to comment Share on other sites More sharing options...
Glenn Posted October 13, 2009 Share Posted October 13, 2009 The amount of chimney suggested is 3-4 feet above anything within 10 feet of the top of the chimney to overcome any cross draft or down draft situations from neighboring obstructions. This may need adjusted for your situation and location, trees, buildings, etc, to make the chimney draw properly. I may be mis-reading your post so help me out. 7 feet of chimney to get 4 feet above the ridge pole, would mean you would have only 3 feet of chimney below the ridge pole. This does not sound right somehow. If the chimney is connected to the side draft and is a straight shot to the sky, then it should work. If not add another section of chimney. Any bends or elbows will decrease the draft. Please remember that putting a rain cap on the top of the chimney will also reduce the draft, but will keep out some of the rain in the process. Build a small fire in a forge outside the building and set up a chimney to get the smoke to approximately the same height as your inside chimney. Watch the smoke on several different days and several different types of weather to observe what the smoke does. Here (my location) the smoke blows in 3 different directions as well as straight up on cool clear nights. One direction is influenced by a large tree, another direction is influenced by a building and the third is favorable most times. Still in heavy weather I have seen the smoke just clear the top of the chimney and fall back down directly onto the roof of the smithy, over the edge of the room and flow across the yard like a heavy fog. This same pattern also will sometimes generate a lack of draft or a backdraft situation and allow smoke into the smithy. I have learned those are the days to gather in firewood, clean up the yard and otherwise prepare for bad weather. You know where you live. You should know which way the wind blows and how the air acts and reacts in most weather. Use that knowledge to your advantage when choosing a chimney height. Quote Link to comment Share on other sites More sharing options...
DennisG Posted October 13, 2009 Share Posted October 13, 2009 I have a hood being made by my brother, he has a heating and AC company. The hood is 30"X40"X24" high, about 6" skirt, the back is flat on the wall, 3 sides angled up to a 15" sqr top. He has figured an 8" pipe for the flow will be good enough. Reasoning?, he is installing an in-pipe exhaust fan. I just hope that the hood at 2' from the forge surface is high enough. Pics to follow. Quote Link to comment Share on other sites More sharing options...
Glenn Posted October 13, 2009 Share Posted October 13, 2009 the back is flat on the wall, I can put my hand on the chimney of my side draft hood 3 feet from the entrance and it is touchable, but uncomfortable warm. This is due to all the air being drawn in by the draft. The flame will turn 90* to enter my side draft hood and HIT THE BACK WALL of the hood before being redirected upward. This back wall was actually the first part of my 1/8 inch plate steel hood that deteriorated and later perforated first. Please move that hood out from the wall and put a heat shield and 2 (two) air spaces between the hood and the wall. That is to say wall, air space, heat shield, air space, back of the hood. DO NOT encumber the air space but let it move all the air possible by natural convection. Quote Link to comment Share on other sites More sharing options...
16tonBEAN Posted October 13, 2009 Author Share Posted October 13, 2009 Glen, Thanks for the info, and I will definitely test the wind first as you suggest. The distance I'm referring to is from the top of the side draft hood to the chimney cap. If I use one of the hood designs I have found on the net, that would mean I would have only 3 feet of pipe on the inside, then 5 feet more chimney pipe on the outside to get to a point where I'm 4 feet above the ridge. Maybe the tall hood is not a good choice, I don't know. I've been looking at several side draft hood designs and am trying to decide which to use, but my biggest issue is where to get pre-fab double wall chimney pipe or for that matter single wall interior stove pipe 12" in diameter. Or do you make your own? I have found Dura-Vent pipe, but it's expensive. Thanks Quote Link to comment Share on other sites More sharing options...
16tonBEAN Posted October 13, 2009 Author Share Posted October 13, 2009 DennisG, your hood design with 24" height seems more reasonable than the one I was considering - 22"w x 16"d x 48" h. I need to look into hood design some more. By the way, where did your chimney pipe come from? Quote Link to comment Share on other sites More sharing options...
Glenn Posted October 13, 2009 Share Posted October 13, 2009 You can get regular stove pipe and put two pieces together to get a one larger diameter pipe. You may be able to get by with only a double walled vent pipe where it goes through the roof. Always check with the building code in your area. Quote Link to comment Share on other sites More sharing options...
Glenn Posted October 13, 2009 Share Posted October 13, 2009 I Forge Iron - BP1048 Side Draft Chimney Quote Link to comment Share on other sites More sharing options...
16tonBEAN Posted October 13, 2009 Author Share Posted October 13, 2009 Much obliged, Glenn! Quote Link to comment Share on other sites More sharing options...
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