Carrier: Age is incorporated into the serial number. Through 1969, the first digit of the serial number indicates the year of manufacture. Example: 3xxxxxx = 1963, 4xxxxxx = 1964, etc. 1970 and later, a letter followed by a number indicates the month and year of manufacture. Example: A1 = January 1971, B1 = February 1971, C4 = March 1974 etc. A = Jan B = Feb C = Mar D = Apr E = May F = Jun G = Jul H = Aug J = Sep K = Oct L = Nov M = Dec I am assuming the unit is likely from the 80's and installed with the furnace in 1988. According to your post from Home Tech if it is indeed from the 80's my serial number should begin with a letter and mine didn't, it begins with a zero. So I still don't know.