wrote in message
One thing that bothers me is, with the system "professionally"
balanced, the temperature at the registers was in the low 70's using
an infrared thermometer. I've since closed off the vents to all
unoccupied/unused rooms in the house, and even with just the living
room and main hallway heated, the vent temperature is still only in
the upper 80's. My understanding is that it should be around 100
degrees.

I think all the heat is going up the chimney. The exhaust pipe on the
furnace is RIPPING hot, can't even put my hand near it. It's putting
out a clean blue flame. Changing the filter didn't help. Pros say the
furnace is in perfect working order.

Has it always been like that? It does not sound right that the chimney
would be that hot. Perhaps you need a new pro to look at it.