Hi Tom,
What type of furnace do you have that died? I had a working oil burning furnace that i hadn't used for a couple years since I've been heating primarily with wood. I decided to clear out all of my ducting and the furnace to make space in my ancient basement for other projects like a rmh. After removing the ducting, cold air returns and the furnace shell i was left with a very substantial steel heat exchanger. Removal of the injector revealed an intact refractory core! Seeing that, i just had to convert it to a wood fired heat exchanger!
I used fire brick to build a J tube feed and burn chamber feeding directly into the refractory core and fired it up. It works great! The design isn't perfect but the surface area is truly substantial and because it is attached to the chimney, there is always draw through the system. I extended the exhaust down to the floor and buried it in sand (temporarily) to steal as much heat as I could from the flue gases.
As far as the heat exchanger design goes, it consists of what is essentially a barrel with a vertical 3" wide opening to box surround. It is designed for air flow and creates good convective current on it's own. Built into a shell with a proper air gap it has massive surface area and could again be used as a heat exchanger for forced air. Since it is still in the middle of my space, I intend to move it to a new location. Next, I will build a masonry chamber around it essentially mimicking the old steel shell's inner dimensions, but with masonry, and finally using a
cob mass bench to extract whatever heat is left in the gases.
I plan to document the build when i relocate it, but I have a feeling there are many of these old furnaces going to the scrap
yard.