30 years is a good amount of time for a furnace. (Our drummer sold heating & cooling hardware, so I found out a lot about this stuff. More than I needed to know!
We replaced our circulating hot water system about 7 years ago. The American Standard system that was in there was installed in 1972. I guess those systems last even longer that hot air furnaces.
When he & his stepson installed it, the music was DVDs of the band, because his stepson wanted to hear them. (The stepson was, no surprise, a drummer, too.)

It fits in the space available and can still heat the house.

New windows last fall and now a new furnace. Whoever buys this house should have pretty low utility bills.

Back in the mid 80s, we were renting a big old house (1890s).
During an especially cold winter, we had a gas bill of over $300 one month. That's NOT adjusted to today's dollars!
Our drummer looked inside it and saw a golf ball size hole in the heat exchanger. The efficiency of the furnace was probably under 20%.
I also found put that the flue damper was rusted OPEN.
After the fireplace was sealed & with the new furnace, our highest gas bill the next year was $75.
That places needed new windows, too.