Increasing wood-stove efficiency can be as simple as adding just a layer of bricks on top of the stove: more mass stores more heat — as will a low brick wall next to the stove (it can be 1, 2, or 3-sided). However, you can also effectively upgrade your old (or new!) woodstove by re-directing the hot gases from your stove fire thru masonry channels that will directly absorb even more heat. The following construction details & videos show how I added 300 pounds of mass to my stove, not only increasing comfort, but significantly reducing fuel consumption.
Care and caution are called for. Containing fire in the home — however you do it — exposes you to real risks, no matter the stove design. But it’s not rocket science: masonry heaters were developed and designed by people who worked for love, not money (the whole story is in David Lyle’s Book of Masonry Stoves: Rediscovering an Old Way of Warming).
The videos below show (roughly) how it goes together. The (free) plans are available thru the bookstore (link below). In addition, I’d recommend you to the Masonry Heater Association, and Alex Chernow’s website. Alex has been developing bell stove designs, and has links to a Russian heater builder who makes brilliant sense of the theory, which is really just flow — think funnels full of water, then turn ’em upside down and re-envision the water as hot gases. Everything goes from there.
It helps to get the bricks and mortar right, especially if you’re only building a heater that’s a single brick thick (code requires double layers of brick). The goal is to combine similar materials (mortar made of the same clay as the bricks, etc.) to limit cracks that could compromise function and/or safety (I did have some cracks in mine, which didn’t seem to greatly affect performance. Also, it’s worth noting that it lived in a renovated, but still drafty shack — which is why more tightly sealed spaces built to current code require additional precautionary measures like CO detectors.)
I was reluctant to publish formal plans or how-to info as I consider this to be an experimental prototype, but rocket goddess (and Sketchup artist) Erica Wisner was inspired to draw up some basic plans, and convinced me that they should be available for other experimenters. So, if you’re inspired to work on increasing wood-stove efficiency, please do download the drawings (free! through the bookstore) — but note that there are important detail photos at the bottom of this post which will clarify some important points that aren’t covered in the plans). I’ve also been answering questions in the replies below the post, so read through those, too. And take good care! That means build and test your design somewhere outside before you do your final installation! Also design and build-in a better clean-out, and please do send me news and photos so others benefit from your experience!
I would be happy to publish any stories of successes — and failures! — as that’s authoritative in a much more important way — taking responsibility for one’s own experiments, as well as one’s own heat…
NEW, ADDITIONAL NOTES TO THE PDF SCHEMATIC:
After poring over photos and dredging up old memories, I’m fairly certain that the oven chamber is functioning as a “bell,” with the middle vertical brick in course 3 functioning as a dam to hold the hot gasses at the level of the top of the oven. You could also design it so the gasses flow through the oven chamber. Your choice. Whatever you do. test it out before you install! And follow the above links to learn more about bell theory and gas flow.
At fig. 7, there is a damper made out of stainless sheet metal. See the detail photos for a good view: the hinge is a bit of wire running L to R along the upper side. The sheet metal wraps around the wire. The L end of the wire makes a pin which sits on the brick. The R side goes through the brick to make the handle, which is bent into a spiral.
What would be fig 8 (see detail pix) is just a final bit of square tile to make the final connection to the metal chimney. The end of the metal pipe is wrapped in a kaowool gasket mudded in place (visible in the photo as a 1/4 round collar).
On cracking: the cracks were minimal, and mostly vertical. Three of them extend from level 2 to level 4, one each at the corners, and one in the middle. The middle crack continues up through the next level — that’s the widest one. The horizontal crack is at level 4, from the bottom L up to the bottom of the flat red tile. There are heat-proof fabrics that can be used under the final plaster to further limit cracking.