The heat exchangers in Tankless/on-demand water heaters freeze and break in cold climates. For any brainiacs claiming that some of them don’t, the challenge is -40, F, with 110 mph wind gusts.
Some homebuilt, drainback water and space heating systems are the best for northern climates with sunlight, but some of the main components in them are outlawed by the florida solar heating lab racket (constituents behind the politicians). Most inspectors in CO will reject them and any build that isn’t very common and familiar to them. Rocket stove mass heaters are outlawed by the expensive wood stove racket (constituents behind the politicians). The common closed loop systems preferred by installers (plumbers) are exceedingly expensive and ineffective (huge amounts of antifreeze needing periodic replacement, too little thermal mass, glass tubes in certified collectors often break at high elevations during rapid temp changes).
Not mine (Takagi TK-2) - it has a built in anti-freeze program, where it will fire up the burner for a few moments if the temperature drops to freezing. Mine’s mounted in a garage utility room, so it would have to get pretty cold before this would even come on.
Can’t speak to other brands, or the electric type ones.