I know. I have a vintage 1980 gas furnace. I can maybe save 30% of
the fuel portion of my gas bill during the winter. Doing that math at
last year's prices that's about ~$110-$125/year in savings! Like I'm
going to spend $3000 for that. Even if you do get a new furnace I'd go
with the more fixable 80% model or get a heat pump if you have cheap
electricity like a lot of places where most is generated by nuclear or
coal.