humphreymachine Posted February 9, 2011 Share Posted February 9, 2011 Hi,I’m sure this topic has been covered before but I am new here and would appreciate input. I’m putting the finishing touches on a building where I can finally get my forge in out of the weather and am ready to start planning a stove pipe chimney and wonder what the minimum diameter pipe I can get away with is? I’m hoping 8” is sufficient as I have a neat old 8” diameter sheet metal pipe and partial hood which I hope to use as the starting off point. My forge is a Buffalo pedestal base type design and I plan to use a rectangular hood suspended overhead. I will be working some larger stock so the blower will be used liberally and the fire hot. Is 8” diameter large enough or will it be overwhelmed at high heat/air volume? Also—should one use a stove pipe damper to reduce draw for smaller fires? Quote Link to comment Share on other sites More sharing options...
keithgartner Posted February 9, 2011 Share Posted February 9, 2011 Your 8" will probably work but that would be the minimum. Personally I would use 12". Length of flue ( the higher the better), height of hood over fire come into play, don't put a damper, low smokey fire is heaver thus draws less. Here look at these. http://www.beautifuliron.com/chimneys.htm Quote Link to comment Share on other sites More sharing options...
CurlyGeorge Posted February 9, 2011 Share Posted February 9, 2011 I have a hood over my forge and an 8" flex pipe going up to a 12" X 12" square roof vent. I installed a 12" fan just under the vent and the pipe dumps into the side of the fan's airflow. it works very well, for me. I tried coming right up under the fan, but the motor got too hot and shut down. http://www.iforgeiron.com/gallery/image/27937-my-shop/ All you can see in this pic is the hood. Quote Link to comment Share on other sites More sharing options...
FieryFurnace Posted February 9, 2011 Share Posted February 9, 2011 While you are going through the trouble of installing a pipe, don't try to get away with anything. Use 12-inch flue pipe. While some people have installed 8 inch pipe and it works for them, I believe the rate of failures with 8-inch is much higher. Go big! If it draws "too good," no problem, it's when it doesn't draw enough that you get into trouble! Good luck, and if you decide to use the eight inch and it works, come back and call me a liar! (That was a light hearted joke!) Quote Link to comment Share on other sites More sharing options...
ThomasPowers Posted February 9, 2011 Share Posted February 9, 2011 Look into a side draft system they tend to work a lot better than even large overhead hoods. (I tried a large overhead hood once and had to add a power assist to it as well) Quote Link to comment Share on other sites More sharing options...
humphreymachine Posted February 9, 2011 Author Share Posted February 9, 2011 Thank you for your informative replies, links, photos etc. The consensus seems to be that 8” pipe is too small which is unfortunate considering that I have a nice antique partial hood to work with which is set up for 8”. Also, for fire safety, I was hoping to use double insulated ‘metalasbestos’ type pipe where it breaches the roofline. I think this is commonly available in 8” but I don’t know about 12”. If one uses 12” uninsulated pipe how much clearance is needed between collar ties, roof rafters, sheathing etc. My collar ties and rafters are 16” OC so I’d probably need to remove one tie and a section of one rafter? One of these days I’d like to build one of the adjustable height hoods one often sees in early industrial forges. Usually a large inverted funnel shape with a pipe slightly smaller in diameter than the main chimney allowing it to telescope up and down. The funnel section being suspended from above buy fine chain, pulleys and counterweights. I was thinking that for smaller projects/fires a secondary/removable surround with a much smaller side opening could be placed directly on the forge and the hood lowered onto its pipe for better draw? Quote Link to comment Share on other sites More sharing options...
KYBOY Posted February 9, 2011 Share Posted February 9, 2011 8" will work with a good side draft set-up, but its bare minimum. Bigger is better..I have a 8" on our side draft, if it was a over hood it most likely wouldent work..Then our chimney sticks about 6' above our roof line too..Ive been meaning to change our 8" to a 10" or 12" but finally got it to working right with hearth adjustments and raising the chimney height. Quote Link to comment Share on other sites More sharing options...
thecelticforge Posted February 9, 2011 Share Posted February 9, 2011 I have a 12 inch clay flue in mine and at times I think it may have too much volume to create the velocity that I had planned for. I hear tell 8 inch is good enough, but if I had to do it again, I would go with 10 inch. Every thing will depend on every thing else too. Study a lot, and make the best decision based on your individual situation. Quote Link to comment Share on other sites More sharing options...
DanBrassaw Posted February 9, 2011 Share Posted February 9, 2011 As far as clearance is concerned, the folks I know around here told me I needed very little, as in 2 inches. I was a little skeptical, but figured I could change the framing around later if I felt uncomfortable. I use a 12 inch pipe, and have roughly 18 inches between rafters in my shop. I've had maybe a dozen fires so far, and only once has the first 2' length of pipe gotten uncomfortable to hold my hand to. I use a side draft, and that hood only gets hot when I have a really ripping fire going, or start the coal with kindling. I'll be interested to see if that changes when the ambient air that's getting sucked in and cooling things off warms up this summer. If you put sheet metal on the rafters to reflect any heat off of them, you might be OK with 1 1/4" clearance, but I'd wait to hear from more experienced folks than myself. Also, if you plan on attaching a wood stove to it at some point, then I'd definitely go with more clearance, and insulated pipe. I couldn't locate any 12 inch stove pipe (though I've read that people use vent products or other appropriately sized pipes). A friend suggesting using 2 6" snap together pipes, which worked, but if you go that route, secure them with metal banding or anything else, they don't like to stay together like that. When I was asking around about clearance requirements, my father suggested that I take a larger diameter pipe over my 12', fill with vermiculite (or ceramic wool), and weld on caps, to make my own insulated pipe. Since I didn't end up going that route, I can't recommend or denounce it, just an idea. Edit: After re-reading this, I realized that when I say "I had a dozen fires", it's not necessarily clear that I mean in my forge. :P Quote Link to comment Share on other sites More sharing options...
humphreymachine Posted February 9, 2011 Author Share Posted February 9, 2011 DANBRASSAW---thank you for your input. I see you are from Cooperstown NY. I went to school down the road in Oneonta NY. The D&HRR had a huge shop complex there of c 1865 brick buildings including a massive blacksmiths shop with anvil station after anvil station and a wide array of power and steam hammers and a huge two story punch and pretty much everything else a large industrial shop would need. The complex was mostly closed in the late 80’s and torn down in the mid 90’s. Not infrequently do I wonder if any of the blacksmithing equipment was saved or did the scrapper get it all? An auction at that shop would have been talked about on this site for decades! I highly regret not photo documenting the place. I wonder if anyone did? There was some stuff piled outside and I found a top swage stamped A&SRR which was the RR which built the line in the 1860’s and was bought by the D&H several years latter. Quote Link to comment Share on other sites More sharing options...
ThomasPowers Posted February 9, 2011 Share Posted February 9, 2011 Forge flues tend to run MUCH cooler than woodstove flues; but you may want to install against woodstove code anyway for insurance purposes. Only wood in my shop is 4 telephone poles, so popping a hole in the sheetmetal is not so much an issue out here. Quote Link to comment Share on other sites More sharing options...
jimw Posted February 10, 2011 Share Posted February 10, 2011 DANBRASSAW---thank you for your input. I see you are from Cooperstown NY. I am from binghamton. Quote Link to comment Share on other sites More sharing options...
Pat Roy Posted February 10, 2011 Share Posted February 10, 2011 Metalbestos double wall pipe is available in many sizes. It is expensive however. I used 8" because I'm cheap and that's all I could spend. It is working for me. Size might matter if you are planning to have really big fires. Google metalbestos to see all the various sizes and materials are available. You hardware store should be able to get it for you. I have used "True Value" and a local lumber yard. Quote Link to comment Share on other sites More sharing options...
Countryboy39067 Posted February 11, 2011 Share Posted February 11, 2011 Mine is 22" in dia. It's built from the core of sheet metal rolls a local company uses. I'm not limited on the size of fire I build and even with a small fire it rips the smoke up the stack!! Quote Link to comment Share on other sites More sharing options...
humphreymachine Posted February 11, 2011 Author Share Posted February 11, 2011 I like the overall aesthetic of a smaller pipe but ultimately function is more important here than looks and I do plan to run hot fires so it sounds like 12” is the way to go. I Googled metalbestos and found thousands of sites none of which were the manufacturer's giving specs for the full product line. It appears that some companies stock it in sizes up to 10” and 12” may be available by special order. I’d prefer to use only a roof breach section and one section beneath it and regular uninsulated pipe for the rest. Does metalbestos come with fittings to transition to regular pipe or do they try to trap you into using their product for the entire run? Also—If I choose to use uninsulated 12” pipe can one get a factory roof flashing kit for that size? Did anyone here make their own roof flashing kit and have it work well? Quote Link to comment Share on other sites More sharing options...
Pat Roy Posted February 11, 2011 Share Posted February 11, 2011 You can connect regular smoke pipe to the metal bestos but you have to get their adapter. That's what I did. Quote Link to comment Share on other sites More sharing options...
DanBrassaw Posted February 12, 2011 Share Posted February 12, 2011 Mine is 22" in dia. It's built from the core of sheet metal rolls a local company uses. I'm not limited on the size of fire I build and even with a small fire it rips the smoke up the stack!! Holy cow! As a bonus, you can rent it out as an apartment when work is slow! Quote Link to comment Share on other sites More sharing options...
Countryboy39067 Posted February 20, 2011 Share Posted February 20, 2011 Holy cow! As a bonus, you can rent it out as an apartment when work is slow! Yeah I'm still single but I thought ahead on the doghouse I might get put in.....lol. It really does draft well even with a small fire. I'll post pictures when this mobile app. allows me. Quote Link to comment Share on other sites More sharing options...
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