Jura T Posted December 18, 2008 Share Posted December 18, 2008 I should get a flue/chimney for my new shop. It seems that the size most often recommended is 10". Over here they only seem to have sizes up to 8" on the shelf. A 10" flue would have be ordered (4-5 weeks). In one place they had some pieces of 12" flue, that they would sell for the price of 10". I was reading the building codes for chimneys and there was a warning that a too large chimney would cause the gasses to cool too much and thus the draft wouldn't be good enough. There was some chart on chimney sizes for oil furnaces. That chart only went up to about 10". So my question is that could 12" be too much? It is going to be a straight flue (about 4meters/13feet). I was planning to make a side draft flue but it seems that it doesn't fulfill the codes over here. You shouldn't be able to touch any part of a flue/chimney that gets over 80C (176F). I could get away with that by using insulated flue all the way from the forge up to top, but that stuff costs an arm and leg... I guess I have to call someone in building control centre tomorrow. Quote Link to comment Share on other sites More sharing options...
dablacksmith Posted December 18, 2008 Share Posted December 18, 2008 ok there is a difference between a forge flue /vent and a chimney . A chimney is usually much hotter on a stove or furnace . i can usually touch the pipe on my hood for a few seconds without getting hurt ... so temp isnt that high . i would go with the 12 inch and if draft is a problem get a flue fan .. it is hard to get to much of a flue in my opinion... it also depends on how tall you chimney is going to go generally the taller the better draw ..good luck and keep us informed! Quote Link to comment Share on other sites More sharing options...
Jura T Posted December 18, 2008 Author Share Posted December 18, 2008 i can usually touch the pipe on my hood for a few seconds without getting hurt ... so temp isnt that high . Is that a side draft hood? I would assume a side draft hood to get hotter than an overhead one as there is less room air getting into it and it is closer to the fire... If the building control decides to get nasty they can ask me to provide some calculations before installing any kind of a flue. So if any of you have any data on flue sizes and temperatures then that could potentially be really valuable to me. Quote Link to comment Share on other sites More sharing options...
Steve Sells Posted December 18, 2008 Share Posted December 18, 2008 My side draft never gets hot enough for any code issues. Never to hot to touch bare handed and leave it there. I used 12x12 flue, works great. A forge does not work the same way nor have the same issues as a stove or fireplace chimney. also My day job is in construction, so I talked to everyone before I built mine. Most do not know anything about forge flues. The local code enforcement guy (not building dept) served an apprenticeship as a blacksmith, so he did understand. Quote Link to comment Share on other sites More sharing options...
kasper Posted December 19, 2008 Share Posted December 19, 2008 I have a side draft forge with a flue 12x12,works great.I found my pipe at a scrap yard.GOOD LUCK..........best regards Quote Link to comment Share on other sites More sharing options...
Paul B Posted December 19, 2008 Share Posted December 19, 2008 Steve and Kasper, your flues are square? Are they steep pipe or flue liners? If steel pipe which I would think it is, are they just duct? Paul Quote Link to comment Share on other sites More sharing options...
Glenn Posted December 19, 2008 Share Posted December 19, 2008 do a IForgeIron search for chimney and there is a bunch of information to read. You can combine (snap together) a 4 inch and a 6 inch chimney sections to arrive at larger section. You do the math to see what you need. You can have too large a chimney pipe. When that happens, the whole system slows down. The chimney size depends on the forge set up. I have used the same set up (side draft hood with a 10 inch diameter exit at the top of the hood) with 8, 10, 12, 14 inch chimneys. The 8 inch was not quite enough, the 16 inch saw a noticeable decline in performance, and the 18 inch worked poorly. All chimneys were about the same height, and on a outside (no in a building) forge. With a different forge and chimney arrangement, I now use a 24 inch diameter chimney of about the same height that works very well. A lot depends on your specific set of circumstances. Look in the 1000 series of blueprints for how Hofi has his side draft chimney set up, very simple system and it works well. Quote Link to comment Share on other sites More sharing options...
Steve Sells Posted December 19, 2008 Share Posted December 19, 2008 Steve and Kasper, your flues are square? Are they steep pipe or flue liners? If steel pipe which I would think it is, are they just duct? Paul standard square clay flue liner tiles. Quote Link to comment Share on other sites More sharing options...
sandpile Posted December 20, 2008 Share Posted December 20, 2008 Hello folks--10" or 12" pipe, flu, chimney -- tall enough to extend a few above the tallest closeby obstruction. Check with code people if needed. That keeps their noses from getting bent out of shape. chuck Quote Link to comment Share on other sites More sharing options...
divermike Posted December 20, 2008 Share Posted December 20, 2008 you want the chimney 3 feet above the roof, 2 feet above anything within 10 feet, I went with 10 inch drainage culvert for mine, works like a charm, maintain a 1" clearance to combustibles just in case!! Quote Link to comment Share on other sites More sharing options...
Jura T Posted December 20, 2008 Author Share Posted December 20, 2008 Thanks for the replies. It seems that there shouldn't be any problems with the 12" size. The codes require that the flue is insulated where it goes through ceiling/roof. So this is what I'm planning to have at the moment: 2 meters (6.5 feet) of non-insulated 12" tube above the forge and then 2 meters of insulated tube. I want to have non-insulated tube at the bottom as it is easier to make a side draft flue out of it and also to get some heating to the shop (it is cold here for most of the year). I still have to check this with the building control on Monday (could get in contact with them on Friday). Quote Link to comment Share on other sites More sharing options...
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