OUR $100 WOOD-BURNING FURNACE SAVES US $1,200 A YEAR!
(Page 4 of 5)
November/December 1975
By the Mother Earth News editors
This is necessary because, unlike most furnaces, our woodburner doesn't have its main flue located on the top or side of the firebox (where it draws so well that it frequently pulls great quantities of heat right up the chimney).
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Instead, our wood-burner's main smoke escape is a firebox-wide slot approximately 3 inches high that runs across the lower edge of the back of the combustion chamber. This gap opens into the sealed firebox-wide-and-3-inch-deep cavity between the rear face of the combustion chamber and the 36" X 36" sheet of 20-gauge steel used for the outside back panel of the furnace.
An outlet for a 6" stovepipe is cut into the top end of the furnace's back panel, and it's this pipe which acts as the wood-burner's main flue. To get to this stovepipe, however, smoke from the blaze in the firebox must first be drawn down through the slot at the bottom of the combustion chamber's back and must then pass up through the cavity between the rear of the firebox and the furnace's back panel.
This keeps the fumes in the combustion chamber longer and forces them to burn more efficiently. It also—as the smoke which does get out is pulled up through the furnace-wide cavity on the back of the heater—allows a certain amount of warmth from even the escaping fumes to radiate into the basement and into the stove's air jacket. In short, more heat is extracted from the wood that is burned and more of that extracted heat is used in the house instead of going out the chimney.
The two flues—the little one on the front of the furnace and the main one on the back—are connected into a single stovepipe which, in turn, runs to the chimney. And the pipes are pitched up as steeply as possible from the heater to that chimney to keep soot from building up in them.
We cut only one other opening (in addition to the loading door and the two smoke vents) into our wood-burner's firebox. This is the 4" X 10" ash removal door located on the front of the furnace and one inch up from its bottom edge. The removable cover is simply a piece of sheet steel a little larger than the hole it covers. Two pieces of angle welded to the back of the plate rub against the sides of the opening and act as a friction lock to hold the door in place.
And that's our wood-burner! The unit sits in the basement next to the old gas furnace and rests on lengths of three-inch channel iron to keep it up off the damp floor (so the bottom of its air jacket and ashpit won't rust out).
We checked with our local tinbender—he's experienced in the design and installation of heating systems and knows how to match a properly sized air duct to the rated capacity of a blower—before we began hooking our wood-burner to the old gas furnace.
Next, armed with tips and calculations from our consultant, Harvey and I cut a 12" X 13" hole in the cold air return of the gas heater and a matching hole in the air jacket of our new furnace. The two openings, as you've probably guessed, were then connected by a length of duct and the cold air return fan on the old LP furnace was turned around so that it would blow into the wood-burner's jacket instead of into the gas heater.
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