If you size it right and install it right it will do the job, it just
takes research. I have a small 117000 btu Bosch that heats 34f incomming
for a hot shower not even being set to high. For a family you will need
one sized , it will im sure be 180000 btu +. Proper gas supply must be
measured since these pull more gas than most furnaces and when
everything is running-pulling gas a test must be done with a Manometer.
So install can cost to upsize Ng lines. The upside is even Sears for
example their best Ng tanks have an energy factor rating of only in the
60s, tankless go from 82-90+ and have efficiencies of up to 94 for a
Talagi tk1. An upside is you will save on gas and considering 1/3 of Ng
usage can be HW, it can be alot. The downside is Endless hot water,
that premotes waste. Look at Takagi and Rinnai 188000+ btu units with
remote thermostat. Measure incomming water temp, Im near Chicago and
incomming is not as cold this year as we are having a warm winter, so
take off 10f on your temp , or if incoming is 40f figure at -15 - 20f it
will be 33f for your calculations. You also need to measure Gpm that you
have.