Perhaps you folks in California can enlighten me, just curious.

I looked over that pdf the OP posted, and noted a basement and wall construction insulated with R19 and ceiling of R30, some double and some single glazed windows, solid core exterior doors and "tight" construction for a total home area of 13,945 sq. ft. The load calculation for this area with a design temperature of 38F (rise to 70F) is 60,385 BTU/hr, so that means the average new home in California only requires about 4.33 BTU/hr per sq. ft. on the coldest day?

The carrier furnace appears to be oversized based on the figures published, but man, that is one big area for only one furnace...

Just thinking out loud...