This furnace was installed in our house Dec, 1999. The technician who serviced it today claims it's 20 years old. The people who installed it say it's only 15 years old. Which is correct?

Thanks,

Model numbers are pretty meaningless since the particular model can be any number of years. A Trane 80 was made for many years. Serial Numbers are what count. I would suspect a 1999 would have a 9 as starting serial number, perhaps the letter I. What is the serial number?

We need a serial number to tell you how old the furnace is.