Hi Bob, just FYI some years ago I built the original steel boiler with copper tubes to the Bill Harris design. Worked out fine, the only issue I had was getting the tubes sealed in the tube plates. I should have snuck up on the hole diameter and done some reaming to get a smoother finish. I eventually did get them sealed after three sets of copper tubes and lots of annealing. I also made a miniature three roller tube expander as the rubber washer style tool in the Bill Harris book was not moving the low quality copper tube I had. (this new but bad copper was all I could get at local plumbing supply houses. It has since been replaced with 1950's era tube salvaged from a friend's old house reno. This old copper annealed to very soft condition and installed / sealed much easier than previous attempts with modern and less ductile copper tube.) The Bill Harris steel boiler design is very strong , it is over designed actually. Of course the welding of the steel parts has to be done by someone skilled in pressure vessel welding. I fitted Marty burners for propane in mine, and had some trouble getting good firing as the firebox is too short in height for that type of burner. I lowered and shortened the burners and eventually got them working OK. If I did another such boiler I would make the firebox 3/4" or 1" taller inside to permit better combustion when gas firing. I never tried mine with sterno fuel as the designer originally intended.
Whether to do a steel or an all copper boiler depends largely on the laws and the appropriate regulatory agency in your area. If you have a steam preservation organization or a live steam train club near you I would ask them about local regulations. If you are going to operate only on your own property you can build it any way you like. You might want to check if your homeowner insurance covers it, but I have found insurance companies really don't want to know about steam equipment, so don't be surprised if they say you are not covered if something bad ever happened involving a boiler you built. Hope these comments help. Good luck with your project.
