I'm building a little furnace for backyard foundry duty, so I can cast parts for some of my zillion other hobbies. I'm not happy with all the brute-force furnaces out there on the Internet, that achieve all their melting power with a MOAR PROPANE! mentality, so I want mine to be one of those virtuous circle devices that work better the harder they work. Enter the air preheater:
Those holes in the side wall are air channels, for the incoming air to soak up some heat from the furnace wall instead of just letting it sink to ambient, unused. Unfortunately the side wall didn't slip out of the bucket as cleanly as I had hoped, and some fireclay got stuck in the corner, breaking out of the bottom of the wall when I turned the bucket over:
I'm glad the channels ended up mostly where I wanted them - parallel to the inside wall right to the bottom. (I made a little jig to help me line up the steel rod, that I used to make the channels, with the inner wall.)
Next I need to build a floor for this furnace, again with internal channels to communicate with the ones in the side wall. A roof might be nice too, but that can be (has to be) a separate part. I probably won't put air channels in the roof, in order to avoid plumbing problems - and so it will be possible to run this furnace without its roof.