I fail to understand why model-engineering in the UK – or its insurers – are so terrified of using stainless-steel for boilers.
Yes, it needs be the correct grade and welded correctly to the appropriate standard so would probably have to be work for the commercial boiler-makers… but “we” are doing that now with mild-steel boilers, and stainless-steel is used industrially for much more critical pressure-vessels than ours.
So that covers the PER side and the PER do not dictate designs and materials or who shall build them, but in effect states only that the plant shall be designed and built properly by people who provably know what they are doing so, in the law’s own words, it “is, in fact, safe”.
Similarly with Silicon-bronze, which would seem eminently useable by home as well as commercial boiler-makers; though I understand this does come in different flavours, some unsuitable, so you’d need use the correct one. (Its basic 98% Cu, 2% Si alloy, according to the metals catalogue I learnt this from, can be fabricated and jointed as if just copper but is considerably stronger.)
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Whether that price on the video ad would be realistic in the UK, is another matter; but even if the materials could be bought for not much more, we should consider the “beginner” also needs spend a lot of the hard-earned on equipment before the project materials.
Many experienced model-engineers, let alone novices, would not consider making injectors and pressure-gauges even if making all the other small fittings. (The railway companies were a bit more self-sufficient but many of the “full-size” engine and plant builders bought items like boiler and pipe fittings from specialist manufacturers, too.)
As far as I can determine, very few amateurs go as far as their own foundry-work either. Rather, they fabricate or carve from solid, and where appropriate, finish those parts to resemble castings; or might send patterns to a professional foundry. Nothing wrong with either approach, of course.