If the serial number on a Coleman furnace contains letters, the first four digits represent the manufacturing plant, date and possibly the item type; however, if the serial number only contains numbers, the first four digits represent the manufacture date. Any remaining digits are a sequence number.
Models with alternating letters and numbers represent the manufacturing plant with the first letter and the year of manufacture with the first two numbers. The second letter represents the month the item was made. If the serial number begins with four letters, these symbolize the plant, month, year and item type, in that order. Models made before 1992 with numerical serial numbers begin with the month and then the year, but those made in or after 1992 begin with the year and then the month.