What I don't understand is what the Pis are doing at the moment to get hot as they say that they don't have any clients to do any work yet? So presumably they must be doing aimless processing to get hot at the moment.

You would've thought that it could be made more efficient by centralising these into hubs and piping the waste heat in the community as an array of Raspberry Pis doesn't sound great.

What would happen if they run out of things to process or, the inverse, have too much demand for processing and no demand for heat in the summer?