Why no condensing tankless gas water heaters?

Hello,
I'm shopping for a gas water heater, and I'd like to get the most efficient model possible. As I understand it, when comparing efficiencies, the energy factor is the only important number. The highest energy factor that I can find is 0.85, from either a tankless water heater, or a condensing tank water heater. See, for example, <http://www.oregon.gov/ENERGY/CONS/RES/tax/appheat.shtml .
Now all the tankless units I'm aware of are non-condensing units, so their thermal efficiency is at most 85%. I guess a tankless unit has no standby losses, so the energy factor is equal to the thermal efficiency, hence the 0.85 maximum energy factor.
I'm a bit surprised that the condensing tank water heaters also have a maximum energy factor of 0.85, since their thermal efficiency can be up to 95%. I guess a 0.05 loss in energy factor is attributable to the standby losses through the tank, which is why electric water heaters have a maximum energy factor of 0.95. This suggests that the remaining 0.05 loss in energy factor is due to standby losses through the condensing flue?
Is this analysis above correct? If so, why doesn't anyone make a condensing tankless gas water heater? I would think you could use the vent gasses to preheat the incoming cold water, thereby condensing the vent gasses. It ought to allow for an energy factor of 0.90 to 0.95.
Thanks, Wayne
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To followup up on myself, apparently there is at least one condensing tankless gas water heater, the Takagi T-H1. Unfortunately, it seems to cost 2.5 times as much as a comparably sized non-condensing unit. That's a big price increased to go from a 0.85 Energy Factor to a 0.94 Energy Factor. I'm also surprised that it still requires a Category III (sealed stainless steel) vent.
Cheers, Wayne
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