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BP1048 Side Draft Chimney
by Uri Hofi
This is the forge station in my school. The side draft chimney works very well.
The square tunnel is 13 inches x 13 inches with a 4 degrees down slope to prevent the rain water from flowing back into the fire. The tunnel is some 4 feet long, but it can be 1-2 feet more if needed without any problems. The tunnel is fabricated from 3/16 inch regular sheet metal (black), and painted twice with silver paint. After 15 years it is not rusted yet.
The stainless steel pipes are 12 inches inside diameter. The height of the chimney is 10 feet. In this case I wanted the chimney to go up as near as possible to the end of the roof. You can clearly see it.
On the tunnel there is a 2 inch high coller that the chimney tube is seated on. There is a collar band holding the chimney with straps bolted to the wall. They can be seen in the photo.
There is no rain hood on top of the chimney as it would slow down the air flow. This is the reason for the 4 degrees down slope of the tunnel so that any rain water flows out a drain hole and not into the forge.
In the school we have 11 chimneys, and 3 more in the shop, for a total of 14 chimneys of this design.
Because of the large amount of air being moved up the chimney, you should allow for the same amount air to enter the building by opening a window or door. The large amount of air being moved up the chimney also has the benefit of keeping the horizontal and vertical chimney pipe cool. Hofi has zero clearance in his set up. Some folks using this set up have reported the horizontal section of chimney at the wall, being cool to warm to the touch.
The forge should be of sufficient distance from the wall so the heat from the forge does not affect the wall. You can mount a thin sheet of metal (or other nonflammable material) with a air gap between the material and the wall in order to protect the wall if needed.