The Takagi requires special stainless venting, probably the largest
expense in an interior installation. I seem to recall wholesale
materials cost of our exhaust stack components was over $400 for a
somewhat long run from an interior room.

Larger tankless heaters require a lot of gas. The gas guys mistakenly
ran a 1/2" line when plumbing for ours, it needed a 1" line because of
the length of the run, otherwise 3/4" would have been OK.

--
is Joshua Putnam
http://www.phred.org/~josh/
Updated Bicycle Touring Books List:
http://www.phred.org/~josh/bike/tourbooks.html