Thanks for all of the replies - lots of good info! So it looks like I should for sure replace it - if the retrofit of the needed exhaust/supply isn't a major ordeal, I will plan to go with a 95% unit.
In regards to getting the correctly sized unit, I am wondering if you could consider the BTUs of the current (old) furnace and the way it acted in various temperature extremes, and then could gauge how much of a furnce I really need. I realize that doing a load calc get good info, but I am trying to see if more real-world / existing data would trump a hypothetical calculation.
The current furnace (1988) is 116k BTUs. I live in a 1600 ft2 tri-level house. The lower level is half finished, and half un-insulated crawl space. On the coldest winter days (-25 F), the furnace would run almost non-stop when the thermostat was set to 70.. On the other colder days, it would cycle on and off - the longest span of time that it would be off would be 15 minutes. Seemed like it would cycle on and off a lot.
Could I assume that the old furnace may have been 25% efficient? If so, could I assume that I was only getting/using 87k BTUs of the 116k BTUs, in which case, a 95k or 100k 95% furnace would be OK?
Another wrinkle in all this is that I just insulated the walls of my crawl, which I assume will lesson the need of the furnace.
In all, I am ok with getting a little bit more furnace than I need becuase we are contemplating a small addition (400 ft2) that will need to be heated.
Or, should I forget all this and do a proper load calc?
Lastly, are these install prices ($3600 for a 100k BTU 80%, $4400 for a 100k BTU 95%) out of line?
Thanks so much for all of your help!