Righto Stan!
Sometimes these are knows as "Russian" fireplaces around here. It took a long time to get them right, but they are amazing. You can buy engineered cores, and hire a local mason to incorporate that into the middle of your house or living room. It wants a good foundation. Last time I checked, they were also 5-10 grand installed. There's a lot of info on the web, so conceivably, you could design and build one yourself that works well and efficiently.
The big trick is secondary combustion for all of these products, outdoor furnaces, indoor stoves, wood boilers, masonary heaters, etc. Primary combustion is easy. We've been doing that for thousands of years. Light wood on fire, provide enough air. A chimney helps. As I recall, the upper limit on efficiency with primary combustion only is about 40%, and most are much lower. A "standard" masonary fireplace here in the US actually has negative efficiency. You lose more heat than you gain if you measure the whole house. Awful terrible things. Exposed, exterior masonary chimneys would be against the law if I were the king of the world.
Secondary combustion is where you design an insulated secondary combustion chamber with it's own fresh air/02 supply. When you get the smoke from primary combustion up around 1100*F and shoot some more oxygen in, the smoke itself still has lots of fuel value and will burn again. With secondary combustion, you can approach efficiencies of 80% and no visible smoke comes out the chimney. Much less pollution, burn literally half the fuel, etc.
It's not a trivial job to engineer that. Scads of woodstove companies went out of business when the EPA started testing and rating stoves. They just couldn't get that secondary combustion thing to work reliably.
Some stoves even have a catalytic converter to reduce the ignition temperature of the smoke to make it happen more reliably.
Finest regards,
troy
