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I know I may be coming in a little late on this one, but if you are going to mount a tankless unit in a basement, it needs to be a direct-vent model to pass code (and more importantly work correctly). You would be much better off to mount the unit on an exterior wall and use and outdoor unit. It's a little more involved since you have to move your piping and provide a 120V outlet, but you'll save a ton of money on venting materials.
As far as pipe size goes, the diameter required is determined by the distance from the source (most likely as gas meter for residential units) and the BTU demand of the unit. Most tankless heaters for a whole house will be in the neighborhood of 175,000 to 200,000 BTU's, quite a jump from 35,000-40,000 BTU's which standard tanked units demand.
A little off-topic, but most of Bosch's units use galv. Type B vent pipe, most other units from Takagi, Noritz, and Rheem require 4" Cat 3 S/S vent pipe. Bosch is actually making a line of tankless heaters that can be vented with 3" PVC which will be something to see. Hope this info helps.
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