" You might be OK with a commercial test and a commercial insurer as long as you can do the numbers and get a design approved. Could be complicated and expensive though! "
I have just completed that route for a conventional steel vertical firetube boiler. 18 pages of calculations, many pages of risk assessments, installation and operation instructions etc. etc. later, plus over £1500 lighter in the wallet department just for design approval and independant testing. I cover some of this in an article coming up in the next issue of EIM.
In short, that sort of money will buy you a hell of a lot of copper and silver solder.
Martin