Wood gasification boiler. Made for indoor installation. Wood gasification is the most efficient form of heating with wood. Wood is placed in the upper chamber and the heat gasifies the wood and the flame is at the bottom of the furnace. Unbelievable efficiency and only a small amount of white ash remains after the burn. Research the Olson 25 on the internet and see how it works. Computer controlled and shuts down if temp of water gets too high. Temp in furnace is over 2000 C but the chimney smoke is so cool that you can place your hand on the furnace. Was used to heat a 3000 sq foot house and is like new. Paid over $7000 so price is firm.