I don't know about the other mfg's, but Takagi was highly recommended to me by the folks I bought my Pex and circulators from, and I'm using one of their largest tankless propane-fired units to heat the concrete floor in a 2800 square foot shop. I raised the default temp to its max via dipswitches on the PC board.
I see no reason why these couldn't handle all the needs of a residence; they're pretty awesome units.
