There are so numerous ways to build a furnace.
The first furnace I built as a young girl was made from a hydronic heating system and electric furnace combined.
It didn’t run absolutely well, however it did work, and as a novice Heating, Ventilation, and A/C serviceman it was pretty well built. When I was a teenager, I built my own portable space boiler out of scrap metal, then many of my friends who were interested in interests and girls and such, made fun of my interest in heating and cooling units. I wanted to be just like my Grandpa who was a cooling expert for numerous years, and then moved on to learn about heaters and heating systems, and then a few years after that she became an expert in media air cleaners. I wanted to never stop learning about Heating, Ventilation, and A/C and I wanted to start young. At every science fair in our school I would bring in a mini a/c, smart temperature control, or oil furnace as my project and I would often get first place for my work. I carried this motivation into my adult life, and took a task cleaning air ducts. I enjoyed cleaning air ducts, but I just wanted more. I wanted to get my hands into a central a/c or hydronic heating system and get to repair it myself. I didn’t want to settle just being an air duct cleaner. I started taking classes on the side to learn more about central a/c and furnaces. I then entered the Heating, Ventilation, and A/C field and began fixing Heating, Ventilation, and A/C units at a heating and cooling business.