Lately I had been noticing that the temperature of the "heated" air
coming out of my gas furnace was barely above room temperature, 75
degrees by my infrared thermometer. Filter is nearly new, flame is a
perfect blue.

Turns out that the problem was I'd opened up the registers to a couple
of rooms, and the "redumbdant" register in the living room. After I
closed the registers again, the temperature of the heated air shot up
to 110+ degrees at the furnace.

Now, my gut is telling me that this furnace is undersized for the house
if I can't have airflow to more than one register in the living room,
one in the breezeway and one in the main hallway without the register
temperatures dropping to barely-above-ambient. Any expert opinions on
this? House is 1300sq ft, furnace is 64,000 BTU, location is Rochester,
NY. Former owners were snowbirds and the furnace never had to do more
than keep the house at 50 over the winter.