wrote:

If 100 cfm of 130 F air leaves the flue in either case, you can either
a) heat 100 cfm of house air from 30 to 70 F with about 4K Btu/h from
the furnace, then heat it from 70 to 130 with 6K Btu/h, total 10K Btu/h, or
b) heat 100 cfm of 30 F outdoor air from 30 to 130 with 10K Btu/h.

Where is the energy savings?

The outside air is "dry" so you'd have to add moisture to get it within
the "comfort zone", which requires additional energy. Using outside air
for combustion reduces or eliminates this energy sink...

Agreed, altho you don't have to add moisture to be in the comfort zone.
You can also raise the air temp slightly. The energy needed to evaporate
water is several times greater than the energy saved with a lower thermostat
setting, unless you live in an extremely airtight house.

It also decreases the potential for air infiltration in certain areas which
would make those areas "feel" cold, inciting the occupants to increase
the thermostat setpoint.

And if exterior walls are warmer, with less inward infiltration, they lose
more heat to the outdoors. And uniform infiltration through wind barriers
could make them act like Scandinavian "breathing walls" with no heat loss
to the outdoors.

Nick