Run numbers for her, yours when new was 80%, it is now likely 70- 75%
efficient, dirty burner and clogged-dirty coils. New 94.5% efficient
units with VSDC blower could save you maybe 25% on Ng and 10-20% on
electric. Figure in maintanance, things braking at -20f or a complete
exchanger failure which at 30 yrs is going to happen sooner or later,
and the excessive cost you will pay when the heating season is raising
furnace prices, Now double the price of the utilities 5 yrs out, and
figure a 20% increase this winter, if she is blind to the benefits pull
a fuse, remove a wire.