It depends a lot on your particular set-up, but as a rule of thumb it is usually preferable to have as few bends as possible in your flue pipe. If you have to have them, the 15 degree would be better than the 30 degree option.
Going thru wall
Then vertical not wanting to cut out soffit
Would like to offset 18" soffit then go vertical again
Been woodstoving since i was born
Upgrading after 20 years to good double wall
Just real confused bout all the regs and stipulations. Some say ok others say don't do it
I understand that weight is a huge variable
It would probably be best if you can go out far enough to clear the soffit, then vertical all the way up. If you have a tall wall (two stories or more), this might not be practical, and then, as has already been mentioned, use the smallest angle possible for offsetting.
I just got through installing some stainless double insulated pipe, it was Supervent/Selkirk brand....as usual I read every square inch of the manual. It specifically says NOT to do what you are attempting. It states to cut a hole and go through the soffit, and to never use an offset kit to go around the soffit.....so I would have to go with do not attempt it
I agree with Eric. The flue is the engine for your stove. The guide rule is, no more than two 45º and no nearer to the horizontal than 45º, though this often gets pushed a bit. However, the idea of going round the soffit is not a good one.
As a former plumber I read a lot of code books. That is usually first place to start. Then go to installation instructions. Then look at whether whatever you've come up with actually works the way you want it to.
Usually the code rules limit the number of total degrees of bends in the whole flu. Manufacturer's install instructions also spec some piping (vents) this way. If your flu comes out the back of the stove and you go out the wall horizontal and then straight up, that's 90degee of bend. If you then (somewhere up near the soffit) offset 18" or so to go up around the soffit and use two 15degree bends, you now have 120degrees of bend in that flu. If it's a new stove, the install instructions probably have something say about this.
If the flu comes out the _top_ of the stove and you use a 90degree bend to send the flu out out the wall horizontal, you've just add 90degrees to the total bends in the flu so now you've got 210degrees of bends. That may or may not be legal (code and/or mftr's instructions).
But I'm actually not clear from what you said whether you're trying to do an _additional_ offset after going vertical outside the wall...
You should not be going out and then up an outside wall at all. Not because it is not allowed by code, but because you will not get a very good wood burning stove.
The problem is, the 2 wall pipe. By going outside, and not keeping the pipe inside (not to mention a straight rise), the outside air will chill the smoke reducing heat. This causes two things, increased creosote, and it will lower the stack temperature so that draft will be greatly diminished.
The only reasons I could NOT see putting in an inside chimney is if the building is two story, and something is in the way on the second floor, or the chimney will exit out through a valley in the roof which would be incredibly problematic, otherwise there is no good excuse. It is harder to cut through a cieling, then roof and do everything properly, BUT the stove will draft so much better and not create nearly as much cresote.
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