I live in the Chicago area. My Trane furnace is now 23 years old and I've never had to have a service call, however, currently although off, there is a humming sound now and then apparently caused by an electrical connection.
Because of it's age I have decided to replace the furnace and air conditioner. I live in a condo town house which limits me to using only an 80 rather than a 90 or 95% unit because of the venting.
I mentioned where I live because despite the cold winters, I keep my thermostat between 62 and 65. Knowing that, should I purchase a single or double stage furnace. My current furnace is single stage.
If I were to go to a 2 stage, how many years would it take to make up the cost difference between a single and 2 stage?
Please help

Just buy the single stage unit BUT havt the 2 or 3 contractors that are going to give you a estimate do a manual J. This is to make sure the size of the equiptment is correct. In days of old they oversized equipment and that is what you DO NOT WANT. If the equipment is oversized your utilitys will be higher and the comfort level especially with A/C will not be as good as a properly sized unit. In my world the professional install is EVERYTHING.

See link below for more info about choosing a contractor.