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Considering, that every energy conversion process is creating losses, it follows, that the cheapest way of heating a house should be a portable nuclear reactor. Also, since no water or turbines are needed, it shouldn't be hard to make a "safe" nuclear radiator right? 
@snugglefist
I know, that there are many "small reactors", but I was just wondering, how cmall could a reactor get, if the goal is 100°c, and if goal was not electricity, but just the heat.
Bonus points for possible to turn off, and not radioactive.
But even with this talk, he said it is a 30 year reactor at the 50MWe.
But it seems, that space programs do use these things already.
https://en.wikipedia.org/wiki/Radioisotope_heater_unit
https://www.ted.com/talks/taylor_wilson_my_radical_plan_for_small_nuclear_fission_reactors