Kendall P Posted January 9, 2012 Share Posted January 9, 2012 So - before im hit with 1001 safety firsts - This is a professionally built forge - with a professionally made flu installed. - The only issue is that their is not enough updraft from the chimney to pull out all the smoke - subsequantly smokeing out my forge. I am aware that an extra blower mounted to the flu can resolve this issue - can any one link / post some pictures / information that i can bring back to my expert who fitted the chimney so we can make the necessary modifications. Many many many thanks I also promise i will do a huge photo dump of the build and my shop when weve crossed this last hurdle of the forge to shop smoke infusion. Quote Link to comment Share on other sites More sharing options...
David Einhorn Posted January 9, 2012 Share Posted January 9, 2012 A few questions may make it easier for folks to assist you: - the width of your flue... a standard 8" wide flue could possibly be your problem. - how high the top of the chimney is above the highest point of your roof. - how far is the firepot from your flue opeining Quote Link to comment Share on other sites More sharing options...
Randy Posted January 9, 2012 Share Posted January 9, 2012 Like was mentioned, if the flue isn't big enough it won't draw for a forge fire. In brick terms it is 9" square. In pipe terms a minimum of 12" round. Also the height above the roof is important. See attached drawing. The center of your fire pot should be about 2" to 4" past the front of the chimney. This helps with access to the forge for heating various sizes and shapes of metal and in tire making allows you to lean the tire against the chimney front so you don't have to constantly hold it. Also if you have any bends in your chimney, especially any over 30 degress, you are restricting your flow by up to 60%. Now here's a strange set up: at Sturbridge Village they have a regular chimney, but the face is bricked up. All that is open is about a face of a brick opening up on the face of the wall. See photo. At first this made no sense to me. How could this possibly work? Then I figured out that it works like a crevice cleaner on your vacume cleaner. By having a smaller opening they actually increased the initial flow of air coming into the chimney. Hey, it works! Some side draft chimney designs have a smoke shelf in them. They say that helps with the flow of air by it circling around before it goes up. I know fireplaces have that system, but if your opening doesn't get wider than the chimney, like on a fireplace, you are making a "crevice cleaner" attachment for your forge chimney. I'm looking into adding a piece of 1/8" plate across the inside of my chimney leaving a few inch gap to see if it improves my draft. I expect it will. Quote Link to comment Share on other sites More sharing options...
Kendall P Posted January 9, 2012 Author Share Posted January 9, 2012 A few questions may make it easier for folks to assist you: - the width of your flue... a standard 8" wide flue could possibly be your problem. - how high the top of the chimney is above the highest point of your roof. - how far is the firepot from your flue opeining The flu is a standard 8inches yes the chimney is roughly 2.m from the top of my roof the firepot`s hood sits 1/2 m above Quote Link to comment Share on other sites More sharing options...
Rich Hale Posted January 9, 2012 Share Posted January 9, 2012 You did not give us a lot to go on to be of real help, but I do have one thought: if the smoke is a big problem as soon as you light your fire, I wonder if the chimney full of cold air is a problem,,,I have a wood stove in a two story house with a correct size chimney. If I light the kindling and wood it smokes a lot..Unless I light a rolled up newspaper and put it inside up near the chimney opening,. that provides enough heat to move the cold air out of the long pipe and start a draft, then I light the kindling and the fire does great. Your post with pics came in as I was typing, Nice shop, i have a couple more questions: You give the distance from the outlet to the peak of the roof. Is it on the lee side of the house? are there any large trees nearby that will cause the normal winds to go around? Is the pipe taller then the peak of the roof? I bet you are real anxious to get this up and working. I hope someone on here can help you with this. Quote Link to comment Share on other sites More sharing options...
knots Posted January 9, 2012 Share Posted January 9, 2012 Nice looking forge. Looks like it may be one with a rear blast water jacketed tweir. It should work unless there is a damper hidden up in the hood some where. Riche's suggestion of burning news papers to start the draft is standard proceedure for many forges. if the hood has an accessable smoke shelf that would be the ideal place to burn the news papers. Quote Link to comment Share on other sites More sharing options...
Kendall P Posted January 9, 2012 Author Share Posted January 9, 2012 Your post with pics came in as I was typing, Nice shop, i have a couple more questions: You give the distance from the outlet to the peak of the roof. Is it on the lee side of the house? are there any large trees nearby that will cause the normal winds to go around? Is the pipe taller then the peak of the roof? I bet you are real anxious to get this up and working. I hope someone on here can help you with this. The roof is on a very slight slope - but their is only one slope - as in its pitched to one side - but the 2m measurement is taken from the highest point. - their is some hedging nearby that isnt helping too much with crosswinds but shouldnt interfear too much.. The most part of the build as been by myself over the course of last year - weve hit a few snags along the way, I was expecting something to go wrong last minute so this wasnt too much of a set back for me, just a kink to iron out. Nice looking forge. Looks like it may be one with a rear blast water jacketed tweir. It should work unless there is a damper hidden up in the hood some where. Riche's suggestion of burning news papers to start the draft is standard proceedure for many forges. if the hood has an accessable smoke shelf that would be the ideal place to burn the news papers. You might have hit opon something sir - ill try that tomorrow and see if it changes things noticably. Quote Link to comment Share on other sites More sharing options...
Glenn Posted January 9, 2012 Share Posted January 9, 2012 Try a couple of sheets of newspaper to get the draft going and a couple more as you light the fire in the forge. You have a door or window open so that the air going up the chimney can be replaced? You can not exhaust any more air than what is allowed back into the room. Add a couple of sections of chimney to what you have now. You may be surprised how much it increases the draft. Please show us photos of the forge, the entry into the wall, the outside exit from the wall, and the outside of the building. We need to see what we are trying to get to work for you. Quote Link to comment Share on other sites More sharing options...
SmoothBore Posted January 9, 2012 Share Posted January 9, 2012 A properly designed, naturally aspirating chimney, is a wonderful thing ..... But in some locations, the outside air flow, ... or lack of same, ... can be a problem to resolve. Unless you're going for your "Chimney Designer Merit Badge", I suspect that a "pragmatic" solution might serve you better, than an "elegant" one. As is currently being discussed in another thread, ... I favor the use of a small "draft inducer" fan, to provide a positive draft, under ALL conditions. . Quote Link to comment Share on other sites More sharing options...
Glenn Posted January 9, 2012 Share Posted January 9, 2012 There was a thread on this somewhere, 6 inch round = 28 square inches - when you move from 6 inches to 8 inches 8 inch round = 50 square inches - 178% increase in area 10 inch round = 78 square inches - 156% increase in area 12 inch round = 113 square inches - 145% increase in area 14 inch round = 153 square inches - 135% increase in area 16 inch round = 201 square inches - 131% increase in area 18 inch round = 254 square inches - 126% increase in area 24 inch round = 452 square inches - 177% increase in area 8 to 10 = 156% increase in area 8 to 12 =226 % increase in area There is a difference in the fluid dynamics between round and square pipe. Nothing wrong with using square pipe, just that I am told it flows differently on the inside. I have used all of the above sizes of pipe on my side draft hood and with each move up there was an increase in performance until you reached 16 inches. The 16 inch diameter stack did not work quite as well as the 14 and the 18 inch worked poorly at best. I currently use a 24 inch diameter stack but with a different hood set up and it will suck up small furry animals if they are close enough. Quote Link to comment Share on other sites More sharing options...
Kendall P Posted January 9, 2012 Author Share Posted January 9, 2012 I favor the use of a small "draft inducer" fan, to provide a positive draft, under ALL conditions. I dont surpose you could link that thread or perhaps post a picture of this draft inducing fan youve used? - it's exactly the thing im looking for. Quote Link to comment Share on other sites More sharing options...
SmoothBore Posted January 10, 2012 Share Posted January 10, 2012 I dont surpose you could link that thread or perhaps post a picture of this draft inducing fan youve used? - it's exactly the thing im looking for. I'm not the cleverist guy, when it comes to hocus-pocus with the computer. But if you enter "TJERNLUND DJ-3" into your favorite search engine, the one I've been referencing will come up, and answer all your questions. Apparently, "Tjernlund" makes MANY styles and types of draft inducers. . Quote Link to comment Share on other sites More sharing options...
southernforge Posted January 10, 2012 Share Posted January 10, 2012 Unfortunately, the time to ask about flue size is before the chimney is built. As others have said, your chimney flue is too small, all other suggestions is the attempt to get around that. Quote Link to comment Share on other sites More sharing options...
Kendall P Posted January 10, 2012 Author Share Posted January 10, 2012 Im sure the company i bought the forge from would love to know they had been building the hoods 8inch flu collar wrong for 20 years.. Quote Link to comment Share on other sites More sharing options...
Steve Sells Posted January 10, 2012 Share Posted January 10, 2012 the big problem with most modern chimney experts is that they are clueless about the needs of forges. A neighbor is a pro, told me 2 ft above roof line is plenty and the 12 inch flue is over kill 8 is fine... go figure... but Smithing is a different application than most chimney people do now-a-days. For what its worth, I used 12 inch flue tile, and 4 foot above roof line (2 foot above the shops roofs room exhaust) Quote Link to comment Share on other sites More sharing options...
jkmas Posted January 10, 2012 Share Posted January 10, 2012 Kendall I have over 30 years of experience with the construction of chimneys. However 99% of these chimneys are of masonry construction. I have a home built forge table with a centaur forge firepot. My side draft hood is constructed from a dissambled water tank and re-shaped into a cone with a 8 " round stove pipe. I do rarely get some light puffs of smoke when starting a coal fire but most of the time i have no problem with a good strong draft. For the size of my forge the 8 " will suffice. It does not hurt to have a larger diameter pipe. However cost and availlability at the time was an issue. My garage is kept at a minimum 62 degrees (northern Illiniois winter). Even very strong winds hardly have and effect on the draft. Some questions? Is the spark screen that terminates on top of the flue too low to the top of the last pipe? In other words is there too much restriction at the top. Make sure that the termination is at least the same diameter of the pipe (It can be wider but never narrower never reduce diameteter below what you start with.). Is there a high efficient furnace nearby in the same structure as your forge? On some of my fireplace jobs over the years, I made a point to install a wider pipe or at least a higher pipe than the architects planned called for because of the "stack effect" of existing taller chimneys and the negative pressure created by cold air returns , tight insulated walls and new windows. In other words, some of these factors can pull air down form the smaller or lower flue. It is Ok to have slight bends in the flue as long as they are not tighter than 45degrees. I have approx. 14 lin, feet of 8" pipe . with two 45degree fittings. The only other question I have for you is if the side blast tuyer is to strong. It may be blowing the smoke out beyond the hoods capability. Quote Link to comment Share on other sites More sharing options...
jkmas Posted January 10, 2012 Share Posted January 10, 2012 I also forgot to add that the chimney height rule of thumb: 2' higher than 10' level away from where touching roof or 3 ' higher when intersecting roof ridge does not guarantee a successful chimney draw. this rule of thumb is to satisfy the building codes. For instance many people have a one story addition added to a two story structure. Then the new fireplace chimney is constructed according to code. The next problem is a smoking fireplace. Everything was built according to the plans and the building code However, they did not take into condsideration the the hieght of the existing 2 story chimney as well as large open staircases that can effect the dinamics of air flow. Quote Link to comment Share on other sites More sharing options...
southernforge Posted January 10, 2012 Share Posted January 10, 2012 Im sure the company i bought the forge from would love to know they had been building the hoods 8inch flu collar wrong for 20 years.. As Stewart, Glenn, Steve, myself, etc have said forges need larger than 8". A forge is not a furnace and a forge is not a decorative fireplace. Eight inches is standard for standard applications, your forge is not a standard application. Just because they build house chimneys does not make them knowledgeable about forge chimneys. Bottom line, Sorry to say but Stewart, Steve, and Glenn are right and your contractor wasn't. I am truely Sorry. Quote Link to comment Share on other sites More sharing options...
Kendall P Posted January 11, 2012 Author Share Posted January 11, 2012 Thankyou for all the kind advice gents - Ive had much more luck today with a carefull heating of the flu with some paper to get that warm air moveing , it seemed to do the trick and for the most part all the smoke went up the chimney - Im still getting a little excess smoke running off into the forge so ive decided to lengthen the chimney on the roof by another meter which should give me the updraft i need to keep this sucker working properly. Quote Link to comment Share on other sites More sharing options...
SmoothBore Posted January 13, 2012 Share Posted January 13, 2012 In this situation, it really doesn't matter who's right, and who's wrong. The chimney is already built, ... and the question is how to make it work. In-so-far as the above information applies to any future constriction, ... I think we would all "stipulate" to the notion, that a 12" flue is ideal. . Quote Link to comment Share on other sites More sharing options...
Kendall P Posted January 13, 2012 Author Share Posted January 13, 2012 Who would of thought there would of been such an uproar about 8 inches being too small.. >.> Quote Link to comment Share on other sites More sharing options...
Pat Roy Posted January 13, 2012 Share Posted January 13, 2012 For what it's worth, I operate with an 8" flue. It is about 16 feet tall and has a small offset bend. I have tall trees all around the building so wind has little effect. I wanted to use a 10" flue but the double wall insulated stuff was just too expensive in that size so I went with 8". It works in my shop. Randy, that crevice tool you refer to increases the air velocity but it is a restriction and the overall aif flow is reduced. Quote Link to comment Share on other sites More sharing options...
Old N Rusty Posted January 13, 2012 Share Posted January 13, 2012 My stack is a 12" pipe suspended directly over the forge. Straight up 20' with 6' above the roof. 12 " above the firepot, this chimney draws dust out of the shop on a hot day with no fire. When there is a fire ,it DRAWS! This might be the IDEAL stack as it is 316 stainless steel and will never rust. Quote Link to comment Share on other sites More sharing options...
knots Posted January 14, 2012 Share Posted January 14, 2012 In this situation, it really doesn't matter who's right, and who's wrong. The chimney is already built, ... and the question is how to make it work. In-so-far as the above information applies to any future constriction, ... I think we would all "stipulate" to the notion, that a 12" flue is ideal. . Well put. This would have been the gold standard for the first reply. Still it is as good a summary for this thread as I can imagine. Quote Link to comment Share on other sites More sharing options...
Woodskevin Posted January 20, 2012 Share Posted January 20, 2012 My forge stack is 21" dia. I have no smoke issues as long as the local wind stays below 20mph. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.