An open flame, like the one in your fireplace, Moe explains, is at most only about 20 percent efficient— almost perfectly useless. "It produces hot smoke and ash, which is nothing more than potential fuel that is wasted. Masonry heating, on the other hand, approaches 80 percent efficiency."
A masonry heater is a chambered stone or brick structure, very popular in the wood-sparse tundras of Scandinavia and Russia, that burns a small amount of wood—usually a house needs just one burn per day—in a short amount of time. It takes in cool combustion air from outside, which causes the fire to burn at terrific heat (2,000 degrees Fahrenheit, compared to an open fireplace’s 1,000 to 1,200 degrees), creating very little smoke or ash. The hot combustion gases flow through a labyrinthine flue that heats the thermal mass of stone or brick, which then radiates that heat back into the house for hours. Some masonry heaters incorporate a baking oven into the design.
"By extending the masonry mass upstairs you can heat two floors at once," Moe explains. "And there are many masonry designs that have small windows to give the user a visual relationship to the fire."