I think we can all agree its simple and easy to have '240V' appliances, the setup the OP is proposing could result in me having to pull a very large 4 wire SER cable to a 100A boiler when a two wire SE cable would do.
All the electric boilers / hydronic and even forced air heating systems I see are pretty much 208-240V with a 24V control transformer. Its not uncommon though to have a 120V circ pump for hydronic such as a grundfos but those are also available as a 200-240V. No need to drag a huge neutral for a tiny control circuit.
In the UK/ Ireland its not uncommon to see a
400V resistance electric boiler with a 230V pump so thats not entirely accurate either.