Chimney/roof penetration design

[ClevelandGuy]

Thanks in advance for checking this thread out.

I built a single-car garage onto the side of my house to be used as a forge area.  I've got a nice large table-style forge, purchased allegedly from the deceased forgemaster of a NY blacksmith group.  It originally came with an overhanging vent hood and 6-ft of 12" single-wall steel chimney.  I read that the overhanging vent hood styles of flues don't work especially well, since there's so much open space between the fire and the actual flue entrance, and the resulting dilution causes you to lose too much heat for effective convection, so I am converting it to a Super Sucker hood which will vent to a 12" round chimney.  I'm torn between exhausting out the side of the building or going up through the roof, but I'm swaying toward the roof penetration; less exposed pipe for my neighbors to complain about, more direct draft path, etc.

Whenever I google around, it seems like DuraVent is the product that keeps coming up.  I see specifications for 1,000F nominal operating temperature, with the ability to handle three 10-second sessions at 2,100F without damage.  Has anyone ever figured out what kind of EGT a good-sized coal furnace is capable of putting out?  I realize that a huge amount of room-temp air will be sucked in and dilute the heat.  Some heat will also be lost on the way up toward the roof, but not a ton, I expect, especially with the Class A insulated product.

I have the option to install some kind of chimney support box in the trusses and use that as a point to change the type/composition of the chimney pipe, if that would be useful.  Ultimately, I just want to avoid starting my house on fire and/or turning the chimney into molten slag.  I know there are website calculators for wood stove installs and such, but I have to think a coal fire exhaust is MUCH hotter than anything like that, and so I turn to you guys for help.

[ThomasPowers]

And your location is?  Out here in the desert I just used a convenient hole in my side wall to stick a 10" x 10' pipe up at a steep angle. I'm also in a steel building so no issues with being close to  framing.   The devil is in the details!

[ClevelandGuy]

Hi, Thomas.  I'm in northeast Ohio.  The garage is a stick-built framed design, all wood.  It mimics the original 2-car attached garage on the other side of the house.    Google Maps is behind the times and doesn't show it, so imagine a 10-ft wide single-car garage in front of my white SUV in this pic.  The chimney would penetrate on the back side of the garage roof:

Edited October 13, 2021 by Mod30
Remove excessive quote.

[Frazer]

Your forge and general approach sound very similar to how my build went. I also removed the overhead hood and added a modified super sucker hood in its place. I ended up going straight up and out rather than going through the wall.

To meet my local code I had to put a DuraTech 10" system in (the DuraVent is for enclosed masonry fireplaces and apparently isn't intended to be run exposed). I have no idea what temperature the flue gasses are, but I can say after running the forge for several hours I can touch the first few inches of pipe somewhat comfortably. By the top of the first 3' section the pipe it's pretty much room temp to slightly warm. The nice thing about a coal forge and 10" pipe is the flue gasses are significantly diluted by all of the ambient air. This is something you should make clear to to your AHJ (if you're doing this legally, which personally I recommend you do).

Most of the weight of the stack is supported by the frame I fabbricobbled up for the hood. There is also a support kit on the roof, but that doesn't bear any load. The stack is 19' tall, extends 7' beyond the peak of the roof and will suck the hammer out of your hand if you aren't holding it tight.... ok maybe the last bit is a slight exaggeration, but you get the idea.

My setup is inspected and legal in my area, but it should not be used as a blueprint for you... Local laws vary widely by location.

I just saw your pictures... You're going to have a hard time installing a solid fuel burning appliance in an attached garage. Not only might there be some legal hurdles, but your insurance company is going to fight you on that... Not to rain on your parade; it's worth a shot. However, don't expect it to be as easy as buying some pipe and slapping it up... it's not.

I don't know anything about Ohio fire code so I may be way off base on the attached garage thing, but in my area that's essentially a do not pass go, do not collect $200.

Ok, I actually read the post with your pictures this time... Sorry, it sounds like it is a detached garage, but you don't have a more recent picture. My bad. 

[Latticino]

Inside the open space of the garage it isn't all that important what you run the vent with.  Ideally I'd like to see a black steel duct (not galvanized) at least 10" in diameter, though you can usually get away with galvanized spiral duct after the first 3' or so.  The issue is when you are closer than 18" from any combustible construction (wood framing, plywood or flakeboard sheathing...).  That is where you need to upgrade your materials to one that is rated by the manufacturer for lower clearance to combustibles, typically double wall duct with an inner liner of something like mineral wool.  This is usually rated for either zero or 2" clearance (Selkirk, DuraVent, DuraTech...).  The real issue is at the penetration.  You have to maintain the correct clearance there as well, which may take an engineered "Thimble" with double wall duct and an air gap covered by a rain shield (probably what Fraser has) and can be quite expensive (I got one quoted at over $800).  Most code mandates 3' above the nearby roof peak, but more doesn't' hurt except for addressing wind loading.

[Frazer]

Agreed to everything above. However, I needed 36" of clearance from the appliance (aka forge/hood assembly), then a 2" clearance at the penetration.

My stack was originally ~3' above the peak, but at the request of a neighbor who reported smoke traveling down the roof toward his house I added another 4' section.

6 hours ago, Frazer said:

The stack is 19' tall

For the sake of clarity, this is incorrect. It's 15' tall...

[ClevelandGuy]

On 10/11/2021 at 2:32 PM, Frazer said:

Your forge and general approach sound very similar to how my build went. 

Thanks for your reply, Frazer.  You're right, we're thinking along similar lines.

The garage IS attached.  I hadn't done any research on applicable codes/laws.  While I prefer to be legal, in my life I've often found that nice guys often finish last, so I have mixed feelings on this subject.  I do know that I can't build a usable outbuilding for my needs...the city forbids anything larger than 80SF.

Edited October 13, 2021 by Mod30
Trim quote.

[Frazer]

While I understand it can be a headache I can't advocate for putting in an illegal setup. It might seem unnecessarily restrictive, but the fire code does exist for a reason and there are potential downstream effects of choosing to ignore it. If you ever need to file a claim for your home insurance, even if it's not related to the chimney, it's hard to hide a 10-12" pipe sticking out of a garage. They will almost certainly ask when and how it was installed. If you don't the right answers they might make you take it down anyway or drop you altogether. If it is related to the forge/chimney (in any way) you have given them a really good reason not to cover the damages.

I think the logic behind not putting a solid fuel-burning appliance in an attached garage is pretty self explanatory, so I won't belabor the point.. However, like I mentioned before the local laws can vary pretty widely. If you don't want to go directly to the town and ask questions then I would highly recommend going to a local chimney installer and asking them to come in and take a look at the space and explain what your options are. They will be familiar with your local laws and might be able to provide you with some guidance on how to get yourself setup properly. Just be prepared for them not to know what a forge is or have a clear picture of what you're trying to do without a pretty clear explanation.

If you're interested in familiarizing yourself with the national code, NFPA 211 is the national standard and is available online. If you would like a link PM me and I can share a previous release. It's not the most current version (you have to pay for that), but all of the relevant sections are in there. An important caveat is that not every town has adopted every section of the national code. A local company will be able to provide you with a lot more information on what applies to you.

My intent isn't to make this sound more daunting than it needs to be. I'm just a guy online and I don't claim to be an expert (or even especially knowledgeable) in this by any means. Latticino is much more experienced than I am. Call up a local company and pick their brain. Then you can make a more informed decision on how to proceed.

Keep us posted on how things are going.

[ClevelandGuy]

I appreciate the time you took to explain further.  I understand all the ramifications...I just have to figure out how much I care about them.  I'm an HVAC guy from one of my previous careers, and after I installed a forced induction hanging unit heater in my big garage, the city guy told me I couldn't do it the way I did.  I explained that I most certainly could and flexed my knowledge a bit and he backed down without much fuss.  But you're right, a forge would raise eyebrows a LOT higher, although I expect Forged in Fire has probably spawned more attempted garage installs in the past five years than in the previous 50 combined.

I've waited most of my life to have the time, space and money to really get into this hobby seriously and it would be very frustrating to be thwarted at this stage.  And talking my wife into moving probably wouldn't go over especially well.  My only other option would be to make the forge portable and move it out of the garage when I want to use it, but then there's the headache of moving a freaking anvil, not to mention dealing with working in direct sunlight, and the smoke likely heading right for my neighbor's house 30-ft away...
 

Edited October 13, 2021 by Mod30
Remove excessive quote.

[Frazer]

Just an FYI, no need to quote me if my post is the one just above yours. In general, if there is a specific part you're responding to you can highlight that sentence/phrase and click the little "quote selection" button that appears. No big deal, it just saves people from reading the same thing twice and eliminates some clutter.

I hear you, and you are probably right that FIF (etc.) has resulted in more people putting together some improvised solutions than ever before. It took me a couple of years to be able to move my shop indoors and get myself setup and that included a move into my first house. I was also worried that all my plans would be foiled right when they were starting to come together, but since I knew I was serious about the hobby I didn't want to take any chances.

I'm not telling you what to do with your shop. It seems like you're capable of making your own decisions. I would just recommend calling around and getting a few opinions on what would be required. They might say it's impossible to do legally OR they might save you a few bucks in parts by telling you what you do and don't need. Either way you'll know more than you do now.

[Yanni Rockitz]

The more I think about having the ultimate forge/workshop, the more I want to live out in the middle of nowhere, in an unorganized territory with zero code enforcement. That said, the real issue (apart from obviously not building anything unsafe that might start fires) is neighbors. My own are a good stone's throw away and are mostly friendly, but there's always someone who has to mind everyone else's business. Every neighborhood has one... Hopefully yours aren't right next door. If I ever move, there won't be any neighbors within two hundred yards -- preferably more like a mile. Until then, the coal forge only gets used when it's cold enough that the neighbors are all indoors, with windows closed. Luckily, that's a good third of the year, here. If only green coal didn't produce such nasty smoke! I recommend a 1/4" hardware cloth spark arrestor wrap around the top of the flu(s) so no burning embers bigger than that can escape prior to burning themselves up. Anything smaller should wink out before landing anywhere, if your stack is high up. Smaller mesh, like window screen, will just clog up with ash and gunk too fast. Larger mesh keeps large critters out, but doesn't do much for spark arrest. Clean it off as needed... Bottom line, my gas forge is "plan B" for clean burning when the neighbors are outside. The best dispute is the one you never get into.

[ClevelandGuy]

Obviously living out in the middle of nowhere has its advantages.  A coworker is always telling me how much better it is living out in 'the township'.  But my wife and I enjoy many of the aspects of suburban/city living as well and wouldn't want to lose them.  Unfortunately, having an amazing blacksmith shop and also living the urban life are somewhat mutually exclusive goals, which means a compromise will always be required.  And as everyone knows, a good compromise just xxxxxx BOTH parties off.