Thank you for responding George6488.
I have not completed an exhaustive search of the tankless options but it seems that the demand/flow rate necessary to fill large tub is too high. I am installing the Kohler sok tub (K-1188 ) with a 142 gal demand. Incoming h20 is around 50 degrees (groundwater). Will not be using this tub everyday but will need a full tub of hot water, plus home includes 3 bathrooms and a couple teenage girls...BTW, my energy efficiency tax credit was already depleted with other insulation, windows, etc. purchases last year.
In addition to the 63" height for existing vent other design obstacles include 73" ceiling in the area of the heater (crawl/basement). Kohler specs call for water heater capacity 70% of tub capacity, so around 100 gal. Looking at power vent models and venting horizontally out wall but >50gal capacity models have max heights pushing or exceeding 73" when elbow is added to top of blower. Considering keeping the current 40 gal atmospheric venting unit that I have and placing a 50 gallon power vent unit next to it, venting the new tank via separate horizontal line - then piping supply water to the two in series.
Any input would be welcome. Thanks,