Forge Chimney Questions

[Toolworks]

OK, so I'm in the planning stages of building a forge in my new shop. My old blacksmithing setup involved working at a forge that sat in my driveway, so I didn't need a chimney (and yes, my neighbors threw a block party when I moved). I'm thinking my new forge will likely be made of steel, not masonry. Here are my questions. 

 

I've read in multiples places about people using galvanized HVAC materials for their chimneys. Is it actually safe to use galvanized metal like that? Won't it get hot enough to release zinc fumes? I also have read in multiple places that forge chimneys do not get as hot as fireplace or wood stove chimneys. Is that true? 

 

When you make your chimney from single walled pipe, what sort of clearance do you shoot for to combustibles, like where the pipe passes through the ceiling or roof? 

 

I just got done installing a wood stove in our house; the regulations involved there were really rather strict. It seems like it would be very easy to create a fire hazard with a forge chimney. 

 

Thanks in advance for any thoughts. They are much appreciated.

[matto]

it would not be advised to use galv. pipe or sheet metal for the first 3 to 6 feet of your forge hood and chimney. as for clearances you need to follow local code to be safe. usually going through the roof you need 2" min. clearance from combustibles. as for design take your pick. "hoods" are all about the setup for getting the best draft whether a side draft or a traditional hood. i like both and use both. side drafts are as simple as cutting a hole in a 12" pipe and running it all the way out the roof to fabing a super sucker design. key is opening to diameter ratio. hoods on the other hand is all about hight from fire. some of the best hoods i have seen are simple and you can move them up and down for start up (having the hood close to the fire) and forging (after you have a good coke fire going you can raise your hood). and i would agree forge chimneys do not get as hot as wood stove chimneys. but you still have to use the same code going through the roof.

[LastRonin]

Is the reason forge chimneys tend to not get as hot because the size of the fire? While a forging fire gets hotter, it is usually a much smaller fire, whereas a fireplace fire usually fill the fireplace. It's larger fire heating up a lot more air. So that would equate to the hotter air created by the forge fire would be diluted by the rest of the air being drawn up the flue with it to the point of not transferring as much total energy(heat) to the chimney surface whereas basically ALL the air going up a fireplace flue is heated and transfers more energy(heat) to the chimney surface. Now understand that this is just my logic.

[ciladog]

The subject of size and type of flue pipe comes up a lot on IFI.  Most smiths will tell you that the most efficient size is at least 10 inches.

 

When you are building a smithy I guess you have to ask yourself the question am I doing this according to the building and fire codes where I live.  Most places use the National building and fire codes as standards.  Your town may have slight differences but for the most part you can go by the national codes.

 

Why are there codes important?  Because if you have a mishap and you want to collect insurance the question will be asked if the construction met the codes.  Whether you obtained permits or not when you built it.  This question will surely be asked it you did not obtain permits and did not get inspections.  If you did not obtain a certificate of occupancy for the new or altered structure you will have problems.  But if you build to code you are in a much better position to prevail.

 

When I built my smithy a few years back I decided that I would do it according to code just in case there would be any problems in the future.  The most expensive thing I had to buy, including lumber, siding, electrical, plumbing, etc. was the flue.  If the flue passes through a combustible material like a roof or a wall (where else would it go) then it must be triple walled stainless steel by today’s codes.  That’s it, no ifs, ands, or buts.  The flue must be supported at the ceiling line with an approved support.  What is below the ceiling can be just flue pipe.

 

Now for kicks if you search online for 10 inch triple walled stainless flue pipe you will have a heart attack at the cost.  And none of the suppliers carry it in stock.  It is usually made to order.

 

So to answer your question, do it by code or take your chances with what you feel comfortable doing.  I would suggest at least 6 inches on all sides that is non-combustible.

 

[Toolworks]

Our insurance is interesting because they completely cared about the wood stove we just put in the house, but they don't care at all about a wood stove or forge going into my detached shop. It struck me as kind of odd; I mean, I half expected the guy to fill his pants when I said I need to put a forge in my shop. But all they cared about was the house. 

 

I have serious reservations about using HVAC for any of the chimney; it's so thin. So I guess I wonder what people use. So many people use metal forges. Unfortunately, triple walled pipe isn't something I can afford right now. My shop building is very tall, too. The roof is easily 20' at its peak. Whateve the chimney is made of, it's going to be very tall. It would probably cost me less to build a masonry forge than to buy triple walled stainless. 

 

I'm trying to think, I've been to Touchstone and to the John Campbell Folk School; I don't remember what their forge chimneys were made of. But I don't think it was stainless; I could be wrong, but I don't recall them being shiny like stainless. Or maybe Touchstone's was. I can't remember. It's been too long.

 

I suppose you could weld up a chimney from heavier gauge steel; but man that would be an awful lot of steel to luck into at the scrap yard. Maybe I'll think more about a masonry forge. The metal forge was appealing because I can take it with me if I ever move. If I go to the trouble to build a masonry forge, I doubt I ever move. 

[Steve Sells]

you do not need all that to be tripple walled, you only need  a 10 INCH LONG section of tripple wall for where it penitrates the roof, the rest can be single walled.

[ciladog]

you do not need all that to be tripple walled, you only need  a 10 INCH LONG section of tripple wall for where it penitrates the roof, the rest can be single walled.

I agree but the problem is that the suppliers don't make a cap transition to get back to single walled pipe.

[don schad]

Perhaps going through a wall then up could be an option.   I replaced part of my exterior wall with masonry (cinder-block), and put the pipe through that (a "Hofi style" side draft).    Once outside it uses the HVAC spiral pipe (22 g I think) to go straight up.  I don't know how long it will last, but at $45/10ft section replacement cost wouldn't be terrible if it lasts a few years.

 

don

[Steve Sells]

I agree but the problem is that the suppliers don't make a cap transition to get back to single walled pipe.

 

Maybe that is why HVAC sheet metal is a lilcensed trade, just like electrical and many other specialty trades?  Why not make what you need if your supplier does not have it off the shelf?

 

Make a double walled sleeve to wrap around the single walled pipe at the transition area,  then you have the double air gap required, or hire a sheet metal guy to make it. still cheaper than making the enitre thing tripple walled.

[ciladog]

Maybe that is why HVAC sheet metal is a lilcensed trade, just like electrical and many other specialty trades?  Why not make what you need if your supplier does not have it off the shelf?

 

Make a double walled sleeve to wrap around the single walled pipe at the transition area,  then you have the double air gap required, or hire a sheet metal guy to make it. still cheaper than making the enitre thing tripple walled.

I think a good alternative would be to buy the chimney cap and remove the top cover and put the pipe into that.  But they are still not cheap. 

[Toolworks]

Thanks for the input. Most forge threads seem to be about the forge itself, but honestly, it's the chimney that gets me. A wood stove has to have double walled pipe, and they get to what, maybe 800 degrees? A forge welding fire pretty much flashes hair off the backs of my hands. I've heard it said that a forge chimney runs much cooler than a wood stove chimney, but does it? A forge chimney does have a much bigger displacement---but the fire is so much hotter.

 

A masonry forge would be cool; but that starts to be a much bigger question than just what kind of forge to make; once I make one of those, I doubt I'll ever relocate or anything again. Although who knows. I'm sure you just build another one if you ever need to. 

[DanBrassaw]

Thanks for the input. Most forge threads seem to be about the forge itself, but honestly, it's the chimney that gets me. A wood stove has to have double walled pipe, and they get to what, maybe 800 degrees? A forge welding fire pretty much flashes hair off the backs of my hands. I've heard it said that a forge chimney runs much cooler than a wood stove chimney, but does it? A forge chimney does have a much bigger displacement---but the fire is so much hotter.

 

A masonry forge would be cool; but that starts to be a much bigger question than just what kind of forge to make; once I make one of those, I doubt I'll ever relocate or anything again. Although who knows. I'm sure you just build another one if you ever need to. 

 

I can't speak for everyone's setup, obviously, but my hood is just 12 inch single walled pipe, on top of a 2' tall square sheet metal side draft hood on my table. 3' up (1 foot up the pipe), I can rest my hand comfortable when I have a ripping coal fire going. Granted, the ambient temperature in my shop doesn't get above 50 in the winter, so the air getting sucked in is going to cool things down. I built six inches of clearance into my roof when I ran this pipe, figuring it would allow me the space to sleeve the pipe at the through-point if I got worried about the single wall. Now I realize I wasted my time cutting that much clearance out! All that being said, the most important thing is to meet your insurance company's requirements. Often they can be a little stricter than code for certain things.

[Jonny Foreigner]

I had similar concerns to you having installed double wall for wood burners on several occasions but found that what Dan says above has been exactly the case for me.  I used 10 inch single wall which penetrates a steel corrugated (lacquer finish) sheet roof, the top of the hood touches the roof but the pipe goes through with 2 inch clearance all round.  I then sealed with one of these: http://www.fluesupplies.com/index.php?cPath=28_65&osCsid=71d99b31af3c1c13f0f6475446eed5cd

 

Now after using it for some time the flue really only gets hot enough to warm your hands (touching) on and I realise that I could of used the low temperature version of the flashing!!!  If your in an outbuilding I would of thought that like me, the biggest issue is what you roof is made of - sorry if I missed it but you don't mention in your post.  Do you have a ceiling as such to be concerned with?

[Grizzer]

Look for a metal roofing shop in your area, and ask them.  I talked to our local guy, and when the time comes, he's told me he can fabricate what I need and quoted me a much lower price than the local HVAC shop.  I can also have it fabbed from a heavier guage steel and still come out ahead.  A shop that does metal roofing and siding usually fabs up the corner pieces and trim so they have a press brake, etc.  Of course, once the shop is up and running it won't hurt to return the favor now and then when the odd problem comes up on a job they're doing :)