Joel OF Posted January 4, 2014 Share Posted January 4, 2014 Hi guys, I've got a few questions about chimneys.. An extremely generous fabricator near to me is giving me a 3 month trial sharing his huge workshop, if I don't drive him crazy in that time, I can stay! I don't know the building dimensions but off the top of my head the building is about the size of 2 tennis courts side by side, the roof at the highest point has got to be at least 30 foot and it slopes down to something around 8 feet tall. The area I can put my coke in would be up against a wall where the roof is about 9 or 10 feet tall. I know that chimneys need to go a few feet higher than the roof but do they need to extend beyond the roof at it's highest point or just where the chimney is? Also, are there any differences between how high an overhead chimney VS a Hofi chimney need to extend? There's many air gaps around the building as it's a steel farm barn so there should be plenty of air to replace what's going up the chimney. I don't currently have a chimney in my VERY humble workshop, hence why I'm lacking in knowledge. I know - it's a 3 month trial and putting in a chimney is going to involve cutting a hole somewhere in his building, he's happy for me to do it - he's one in a million. Quote Link to comment Share on other sites More sharing options...
njanvilman Posted January 4, 2014 Share Posted January 4, 2014 1. Are you going through the wall and a 90 degree bend, or cutting a hole in the roof? 2. I use a side draft hood, with 12" diameter pipe, 16' above the forge, about 10' above my roof. Not quite to the peak, but no problems. A very knowledgeable smith once explained to me why 16' and 12" pipe is critical, but I cannot remember the engineering data. I go straight up from the hood through the roof. No smoke problems, it draws great. 3. Other factor to check is the prevailing wind direction. Bottom line....put in the chimney pipe at least 16' or more and try it. You can always add more height if it doesn't work. Quote Link to comment Share on other sites More sharing options...
arftist Posted January 4, 2014 Share Posted January 4, 2014 Wood stove code is 2 feet above peak. Quote Link to comment Share on other sites More sharing options...
Joel OF Posted January 4, 2014 Author Share Posted January 4, 2014 1. Are you going through the wall and a 90 degree bend, or cutting a hole in the roof? I think a Hofi hood through the wall would be easier. Does the distance the hood extends out of the building affect how far it needs to go up? Quote Link to comment Share on other sites More sharing options...
ianinsa Posted January 4, 2014 Share Posted January 4, 2014 Hi Joel , if you Google chimney images you'll see a theme, they work best when they go higher than the ridge, and if the internal size is too small the rising air loses heat to the sides and stops rising. People often don't think/bother to look around them and see what has worked for decades(in your case centuries) this is the type of job you hope to do once and not have to fix/redo . Sorry if I sound rude I don't mean to be. It sounds like you're getting a good bit of luck(and as is well known about luck, the harder you work at it the luckier you get)I hope this pans out for you. Ian Quote Link to comment Share on other sites More sharing options...
Joel OF Posted January 4, 2014 Author Share Posted January 4, 2014 Cheers Ian, didn't sound rude. if you Google chimney images you'll see a theme, they work best when they go higher than the ridge, Do you know if this applies regardless of how far you are away from the ridge? My issue is that there's only one position that I can occupy in his workshop and that's nowhere near the ridge, I'd say it's a good 30 feet away from the ridge, it's a very long building that's not equally split either side of the ridge. The ridge is 80% to one side of the building. If the roof line is 9 foot tall where I can position my forge and the ridge height is 30 feet, that's a chimney over 20 feet tall! It'd be such a shame if this can't work somehow. Quote Link to comment Share on other sites More sharing options...
MLMartin Posted January 4, 2014 Share Posted January 4, 2014 Keep reading this site. There are many old post dealing with this. I can not remember the exact numbers but I believe your 12 inch chimney needs to be 2 foot higher than any part of the roof with in 10 feet. If the ridge line is 30 feet from your chimney I do not believe you need to be higher than that. Where I am my metal chimney pipe came in 5 foot sections. I used 3 or them 10 foot inside and 5 foot outside. My ridge line is 16 feet away and probably the same high as my chimney. Mine works fine. I would never consider using a natural draw chimney less than 12 inch round. If your chimney pipe comes in short sections around 5 foot try putting up a section and try it out. If it is not drawing smoke well just put on another 5 foot section and try it again. Quote Link to comment Share on other sites More sharing options...
ianinsa Posted January 5, 2014 Share Posted January 5, 2014 Joel, the height thing is affected by prevailing winds, it's often thought that it's so wind can 'blow away smoke' however it has more to do with wind coming along and being forced up the slope and then ' curling over like the crest of a breaking wave' and thus it pushes down the chimney . You can often 'cure' this with a 'whirlybird' or a cowl shaped like a bit like a birds head that rotates . Think of a tube say 15mm with a small ball-bearing inside and a 1/2" rod on top, tube fixed to strap across top of chimney and the rod fixed to cowl fill tube with oil and now the cowl will rotate in the wind. Companies like jetmaster make these sorts of cowls for shops like B&Q take a trip to on near you and have a gander. You should be able to rustle one up yourself in no time. Good luck Ian Quote Link to comment Share on other sites More sharing options...
Joel OF Posted January 5, 2014 Author Share Posted January 5, 2014 Cheers. The prevailing wind is from the south west and my forge/chimney is in the north east corner of the building. The ridge is very much on the south side of the building so going by what you said I'll be in the least ideal spot but hey-ho, it's worth a shot! I measured the building using Google Earth earlier, it's 21 metres long & my forge will be 14 metres from the ridge. The wind pressure coming down off the ridge will be strong especially as it's a steep roof with a 20 foot drop between the ridge & the forge. Fingers crossed! Quote Link to comment Share on other sites More sharing options...
VaughnT Posted January 5, 2014 Share Posted January 5, 2014 It'll work. Sounds like you'll not be getting the full down-draft effect of a the wind toppling over the ridgeline, so that's very good. The distance from your ridge, in this case, isn't critical because the distance has gotten so great. You have plenty of room for winds to wick away the smoke. I would put a spinning top on the chimney just as a preventative measure. You'll need something to block out rain and birds, so you might as well go with something that moves with the winds like a weathervane does. As long as you can get a reasonable amount of pipe above the roof, you'll do fine. Quote Link to comment Share on other sites More sharing options...
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