On 2005-11-10, 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?

Perhaps using colder combustion air results in colder flue gasses?

Personally, I'm in favor of direct vent gas appliances from a safety
point of view, to eliminate the possibility of backdrafting. Likewise
having a good source of combustion air is critical to the efficiency
of the burner.

Cheers, Wayne