I don't think a door knob is an example of a simple initiative designed. It's a really common object but it's actually quite complex and it likely not something most people could describe much less draw. I bet if you asked 100 engineers to draw a scketch of a door knob working mechanism you would get 90-95 different designs that look nothing like a door knob does
Your backyard furnace would be the kind of thing that would get you hired over another candidate.
If it were me, it's a story I'd remember for the interview. You're right that it might not fit for a programmer, but it might. It shows problem solving skills and a personal interest in learning new things. Sorry about bringing up a difficult topic.
That's pretty badass. How'd you keep the oxygen out?