This is a typical 40 gal install throughout most homes in america within a garage area. You can see the FAU on the left, and the heater on the right.  ( All these photos may be inlarged by Left clicking on the photo )

This is the the RTG74 .82 Eff tankless that was installed in place of the 40 Gal. Wtr Htr. This customer can now qualify for the $1,500.00 tax credit rebate at the end of the year on tax form 5965, and the gas company rebate.