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