I had to replace my furnace the last year I was teaching, and DR MBarnum is right: it's an expensive proposition. I was also in SCROOGE during the time it died on me, and I remember coming in one Friday night and sleeping fully clothed and under an electric blanket to keep warm that night. Thankfully, I had some space heaters I could sit in front of during the day as I waited for the men to install the new one.
And wouldn't you know it, the following summer, my central air unit died, and I had to replcae it, too. So two MAJOR outlays of money in less than a year.