Advice needed: forge chimneys and fire safety | Problem Solving | Forum

  When I did my hood, I got a buy on triple wall pipe, which reduces your clearance to combustables. Double wall insulated pipe (metalbestos) does the same thing. 1st, whatever vent you buy, make sure it's listed for solid fuel, install it using the installation instructions, including the mounting accesories for your brand of pipe. If you start mixing various brands of product, you violate the listing for your vent pipe. If you burn your shop down, the manufacturer will tell you to pound sand, they won't pay. Your insurance company won't pay if you don't get a mechanical permit from your building codes department, something I'd suggest anyway, partly because I'm a building inspector, but partly because I don't want you to burn your shop down through ignorance (I'm not saying you're stupid, I'm saying you may be ignorant, ignorance is curable).

  The double wall pipe takes up less space, but is heavier than the triple  wall. I frankly only used the triple wall when I got near the roof of my shop. I did have to put some heat shielding up on some of the bracing for my trusses, to reduce the allowable clearance to single wall pipe. The mechanical code has a table that gives you ways to reduce minimum clearance to combustables. It typically only applies to single wall stove pipe though, if you use anything other than single wall  pipe, you go by the manufacturers' installation instructions.  I made my own hood for my forge. The mechanical code typically requires any "equipment" (your forge) to be listed and labeled, by an independant test lab. Oregon has an exception that allows antique stoves to be installed. My forge is factory made, but old enough to allow me to use the exception, I don't know if Washington has a similar exception.  You can also ask your building official for allowance, but be a bit prepared to have to make your case. If you explain up front what you're wanting to do, maybe have a photo of your forge, even some pics of what you make on it, you'll probably get issued a permit to install it. I'd call ahead first, and ask when's a good time to come in and discuss it. While the code is fairly black and white, if you catch the guy who's going to review what you want to do at a time when he can really listen and give it some thought, he'll probably do it. If you catch him when he's up to his armpits in alligators, he may just say "It's not listed, can't use it. Most guys though will actually probably show alot of interest in what you're proposing, and try to find a way for you to get your forge installed in a safe manner.