"Rob Kiz" wrote
The furnace is a Carrier Weathermaker Infinity (58SXi) and has an
Annula Fuel Utilization Efficiency rating of 94%.

The furnace runs fine. Outside of a rather expensice blower motor
replacement 10 years ago, I have had no problems.

I can't see getting a new furnace that would be much more efficient
than the current one, so no savings there. On the other hand, it is
pretty old and will eventually have to be replaced and I hate to miss
out on the $1500 tax credit.

Invest $100 and have a reliable, honest, heater guy check it out. You won't
save from efficiency if you are at 94%, but if there are expensive repairs
ahead, it may pay to do it now. At that age, it could dies next week,
could go another 10 years. The AC side may be batter replaced now though,
if that qualifies.