I live in central New York and will be installing a furnace and coil in my unfinished attic with an A/C unit outside to condition 800 sq/ft of upstairs living space.

The attic a standard overhang (no trusses) with a 6 foot peak and is not insulated except for the 2x6 floor which also has plywood on it. Summers can have 2 week stretches of 90+ degrees and the winter can have single digit temps for 1-2 weeks.

During the summer the attic is unbearably hot and quite cold in the winter.

I am planning to do a 40k furnace and 2 ton A/C unit with sealed and insulated ductwork.

Do I have to take these temperature extremes into consideration? Should I do a 2.5 or 3 ton A/C instead?