*PLUMBERSSTOCK EXPERT* responded:
Thank you for posting an interesting question under this Ducane 4AC13L42P 13-Seer Louvered Split System Air Conditioner. A Ducane serial number has 10 digits and has two types. The first type looks like this "2246780515" (sample serial number). You can determine the month and the years from the last four digits of the serial number. "05" is the year the unit was made and "15" is the week the unit was made (15th week of the year is in April). Hence, the sample serial number shows that the unit was in in April, 2005. The second type, however, looks different. Example is 46 09E48972 (an actual serial number). To decode this serial number, you will only read the first five digits of the serial number, including the letter E. "46" means the plant location, "09" is the year the unit was made, and "E" means the month the unit was made (A=January, B = February, C=March, D=April, E=May, F=June, G=July, H=August, J=September, K=October, L=November, M=December). Hence, the sample serial number shows a manufacturing date of May, 2009.