I'm watching this space carefully too. Every heating project I look at one of the bigger challenges is finding a way to hit my desired BTU output.

Right now your easily available options are 30w from a bitaxe then nothing until 600w from a single s19 board.

S9 boards are 100w to 500w a piece but their TH output is so low you'll never cover costs so they are only good for prototypes and proof of concepts.

Stuff like the nerdqaxe++ and other open source multi chip hashboards that fill that W gap at a modern w/th are needed to go wider with using mining waste heat.