I took apart a furnace once to replace an igniter. I was thinking I was going to find a series of pipes or tubes fitted together. Instead I found just two pie

I took apart a furnace once to replace an igniter. I was thinking I was going to find a series of pipes or tubes fitted together. Instead I found just two pieces of pretty thick sheet metal, close to plate thickness. They just went through a metal bender to form the pipes, and bolted together with a gasket in between. Maybe that's how it got started. A plane, wrapped around something.