Takagi makes Bosches larger 188000 btu unit, The small 117000 btu Bosch
is what I have, a pilotless piezo ignition powered by 2 d cells, My D
cells last 2 years so far. It heats 33f incomming for a 110f shower not
being set on high so it works well. Water flow Gpm, and Temp rise per
Gpm, are how you rate all of them. so if you are in a cold area measure
incomming water temp now, but for me near Chicago with a warm winter it
is not accurate now since my incomming is 40 but can in a cold -20f
winter be 33f, so adjust your temp to calculate a lower temp.

Why not heat the whole house water with it, I figured my savings at 25
a month from removing an electric tank, my payback is substantial, apx 4
years.

A Rinnai or Takagi, or Bosch-Takagi-same thing, output in the
180000-190000 btu range, enough heat for full house Gpm and Temp rise in
cold incomming water areas. The better units have remote thermostats,
and the Rinnai and better Takagi may heat To a Temp. My bosch 120 heat
to Temp Rise and gets hotter as incomming gets warmer, so I turn down
the temp as summer comes, the better units may maintain a set point. The
cheap Bosch 120 is 83% efficient, The Takagi TH1 is 93% efficient.
The best gas sears tank has an energy factor in the 60s, Tankless go
into the 90s, there is your proof tanks are Crap for energy efficiency
and another reason to prove to junk your tank you plan to keep.

One thing you must know first is Gas supply, a 190000 btu unit takes
alot of gas, my little 117000btu unit needs 3/4 Ng line over 10ft, a
190000 btu unit at a long run will need a big big pipe. You must use a
Manometer with competing loads on to calculate flow and account for
supply pressure variances, or you will never get 100% output. If cost is
not an issue get the Takagi TH1, Rinnai is also good, reasearch all the
specs. tankless are the way to go.