chimney questions

[FieryFurnace]

Ok so, I just finished pouring concrete in the shop. Once the casters arrive via UPS, I'll be moving equipment back into place and reorganizing. I'm wanting to install a second forge. My current primary forge is beside the outer wall and uses a Hofi hood style chimney arrangement. The second forge is going to be farther in the shop, and I have no choice but to go straight up through the roof.

Here is what I need to know! Where do I get a 12-inch rubber high-temp boot to seal the pipe where it goes through the roof? Is this the best option? If not, what is?

I don't really care for the "cut a ragged hole and silicone it to death" option!

And P.S. I'm going through a metal roof with 2x6 trusses and no insulation. The area going through the roof is at least 8 feet above the fire, so heat is not a problem, although I will be taking proper heat-deterent precautions.

Thanks for your input!

[Glenn]

You will want to put some sort of plug in the chimney(s) when not in use. Otherwise it will continue to draft and remove heated air from the building, even when there is not a fire.

Consider an outside air source for the forge to replace the air sent up the chimney. If not, it will not draw correctly. This outside cold air can be piped in and fed straight into the blower with additional cold air fed to the top of the fire to be drawn up the chimney.

You could get really high tech and put a double wall pipe as the chimney. The inside pipe could carry the hot exhaust, the outside pipe the incoming cold air from above the roof, pre-warmed by the inside pipe.

I will let others discuss the arrangement of the pipe through the metal roof.

[MLMartin]

Not a rubber boot, but this may work. Its hard to find rubber flanges for duct that large. A bulb seal could be crimped onto the hole cut in the roof and the duct would just pass through it. You would need to cut a very tight clean hole for this to work.

http://www.mcmaster.com/#bulb-seals/=jwm094

[petere76]

Dave,

2 forges....serious production.

10 in will suffice for the pipe if the rise is straight and you go above the ridge line. If you are at least 10 foot off the ridge read up and see what the recommended stack height is. The issues with inadequate up draft all occur when you are close to the peak and don't extend the stack high enough. You can save some bucks and use stove pipe for the long run inside the shop and convert to metal-bestos for the roof penetration and the exterior stack . The cost per foot on the metal-bestos sections is pricey but it holds up well in the weather. Stove pipe will not last long outside. Also the metal-bestos roof (jack) penetration packages have real a good flashing kit and they don't generally leak. You will get tired of climbing up on the roof with silicon sealer fixing the leaks if you just go with a no frills stove pipe penetration.

Peter

[dablacksmith]

the stuff made to go thru the roof is fairly spendy ... what i ended up using is items for ducting (air conditioning or venting) a local scrounge shop (dont know what else to call it) had a bunch of it and i managed to come up with a piece that did what was needed ... it was made from aluminum and had a built in flange that fit well ...

[swedefiddle]

Check out your local building code!!!! There are rules for placement and height, rules not built on guess-work.

Talk to a local chimney brick-person, they will tell you what will work and the set-backs required. :)

Neil

[Frosty]

Dave: NO rubber or silicone! Neither is safe where heat is a factor, sure a forge doesn't typically exhaust high temps up the stack but it can sure happen and rubber burns as will silicone under the right conditions.

What you want is a "roof jack." The jack passes through the roof's top chord, sheeting and sheathing, steel in your case. A roof jack provides proper standoff distance between the stack and wood so BAD things don't happen. The jack will fit the steel well enough it won't be a "blow hole", it won't be water tight but it won't whistle in the wind. The jack provides a secure connection between the stack and the roof structure. If insulation is a factor, not in your shop but FYI you can wrap the stack with Kaowool or similar refractory wool without increasing the fire hazard.

The next is a "skirt" (IIRC it's called a skirt) This is a little conic section that slips down over the stack and rests on the roof to shed water away from the roof penetration. You can safely seal the skirt with silicone as the closest the sealant is going to get to the stack is several inches and you need something to keep water out from under it on the high side. Do NOT try sealing it all the way around, water WILL get past it and it WILL go somewhere so leave the low side unsealed so water can escape without running down the stack indoors.

Lastly a good cap is important, if code or conditions require a spark arrester cap is what you want. Here I'd use a "China cap" but you can't find the darned things anymore so I have a steel gold pan on the house's stack and a SS salad bowl on the shop. Heck, you can't find steel gold pans anymore either hence the salad bowl. <sigh> This kind of cap helps the stack draw in a wind. As a wind crosses it it's divided; the part passing over the top creates a vacuum just like a wing so the part passing under the cap draws air up the stack. It works a charm but if you live where sparks are a hazard use a spark arrester cap.

For the legal particulars look up a heating supplier or service company, they'll happily give you all the info you ever wanted. The guys at Gold Star Heating and Plumbing up the road from me all but know me by name, I bring folk in pretty regularly to get components to build burners. Heck, I've never been in such a shop that had unfriendly or unhelpful folk, Lots of folk are trying to figure out problems or how to do things so they're just plain GOOD at being that kind of helpful.

Frosty The Lucky.

[FieryFurnace]

Thanks for the input guys! This will give me something to work with!

[george m.]

Dear Frosty,

I'm surprised that you can't get stove pipe caps any more in Alaska. Both hardware/lumberyards in our small town in NE Colorado carry them, both galvanized and black iron. The big box places might be more problematic.

I share your regret about steel gold pans. We use ours more often for making BIG apple pies that separating out the heavy mineral suite but they still serve a useful purpose.

General chimney questio to everyone: Isn't there such a thing a too large a diameter or too tall a chimney? I envision a chimney where the hot gases eventually cool and no longer rise causing the smoke at the bottom to back up into the shop. I imagine that there is a maximum height and diameter for any given source of smoke.

Draftily,
George M.

[Frosty]

You think you're surprised George? You should've felt my angst when I couldn't find anyone carrying them anymore. Not the hardware stores, lumber yards, stove stores, heating/plumbing suppliers, box stores, Fred Meyers, nobody. Even supermarkets used to carry china caps and steel gold pans. Oh what is this world coming too? <old coot sniff>

Yeah, it's been my experience you can put too large a stack on a stove or forge. Too large makes for slow exhaust gas rise so it not only cools and soots/creosotes up the stack it can develop inversion layer blockage. Cold air can form a plug in the stack warm/hot air can't break through so it draws poorly to not at all. A shelf diverts the warm/hot air to one side and allows cool/cold air room to descend on the other side. To temps passing in the stack so to say.

Frosty The Lucky.

[K. Bryan Morgan]

Frosty I can get steel gold pans here at the local hardware store. I saw some yesterday.