I searched this forum for the keyword "age" and couldn't find a description of how to determine the age of my system. I have a Lennox furnace but don't have the model and serial # handy. It may be approaching 20 years old, but how can I learn what year it was manufactured? I bought the house in 2001, and the sellers said they thought it might be 13-14 then, but they'd only lived there 4 years. Thanks.