Adding masonry to increase wood stove efficiency
By adding masonry and mud to an old cast-iron wood stove, I greatly increased its efficiency — and it even has an oven! (note the wooden door on the right, just above the iron stove door). The wood that used to over-heat me, briefly, in the morning, now keeps me comfortably warm all day, and into the next morning (depending on how long I fire it and how cold it is). And, unlike most iron stoves, it no longer generates that fierce, dry heat that you can only moderate by burning wet wood, or by damping the fire down to a messy, smoldering smoke generator (both strategies will cause dangerous buildups of creosote in a chimney — a major fire hazard).
Masonry heaters, and similar adaptations like this one, burn much cleaner, and don’t typically generate creosote. A few minutes after lighting it (depending on the size and dryness of the wood), no visible smoke comes out of the chimney. I typically burn it for less than an hour and have heat all day (western Oregon is rarely sub-zero). In my under-insulated cabin, the “heater hat” usually holds heat until the next morning. And (if I start a fire early) I can bake breakfast potatoes in it…). The surface temperatures of the heater portion are also much lower than hot iron and (except for the tile, which gets hotter), very huggable.
Old stoves can burn pretty clean — if you open the dampers up wide, and if you can line the firebox with firebrick, which raises combustion temperatures so more of the fuel burns, and burns more completely. When you line the firebox with brick, it also keeps the metal surfaces cooler and safer. Inside the “hat” itself, combustion gases vent out the back of the stove, up and around the oven, then zig-zag through the narrow top sections, and head out. Much more of the heat of the fire is captured in the masonry, so less escapes out the chimney.
For more design details, including a new diagram, see the following post.