I am buying a house with two 20 year old furnaces. They are Carrier Weathermakers furnaces. I haven't had them inspected yet, but should I be concerned about the amount of life left in them? What could be the extreme longest period I should expect out of them?
Thank you.

20 years is on the long side of reliability and efficiency for a furnice, so my recommendation would be to
a) make sure that you get a good inspection done prior to purchasing the home, and
b) consider asking the seller to pay for the 1st year of a homeowners warranty that will cover the furnace(s).

I agree they're sure to be an the backside of efficiency...