http://www.ho****er.com/products/residential/gas.html

AO SMITH they have condensing water heaters with efficencies of mid
90s.

if you call them they can give you the phone number of a local
stocking dealer, and direct vent models use outside air for
combustion, saving more energy.......

I went thru this same research some years ago,

the FHR is largely based on BTU of burner.

I went from a 40 gallon 34,000 BTU tank, to a 50 gallon 75,000 BTU
tank and about doubled the first hour rating.

because of space issues i couldnt go larger.

its served me very well