How about invent a hot water system that can be used on Shabbos. We have urns for coffee. Why not make the actual boiler work in a similar way so that you can turn on the hot water in the sink/shower to wash dishes etc.
Much of England is plumbed with that type of system (if you turn off the supply). The downside is that the hot water there is not potable, that’s why they have separate faucets for hot and cold.