Although the Fedders brand is used for window units nowadays, this encoding only applies to when they manufactured unitary (split) systems years ago. The second letter of the serial number indicates the year of manufacture (from 1966 to 1977): 

A = 1966    B = 1967   C = 1968    

D = 1969   E = 1970   F = 1971

G = 1972   H = 1973   I = (not used)

J = 1974   K = 1975   L = 1976   M = 1977

    An ancient Fedders unit we recently inspected had the serial number AH11877, indicating that it was manufactured in 1973. The home was built in 1973, which provided a secondary verification of the manufacture date indicated by the serial number.

    Also, see our blog posts Why is my air conditioner not cooling enough? and How can I find out the SEER of my air conditioner?

    To find the age of other home components, go to How can I find out the age of a roof? and How do I determine the age of an electrical panel? and How do I determine the age of my water heater? For life expectancies, visit What is the average life expectancy of the components of a house? 

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Field Guide for Home Inspectors, a quick reference for finding the age of 154 brands of HVAC systems, water heaters, and electrical panels, plus 210 code standards for site-built and manufactured homes, and the life expectancy rating of 195 home components. Available at amazon.com for $19.95.
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  To learn more about heating and air conditioning systems, see these other blog posts:

• How can I find out the SEER of my air conditioner? 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning? 

• What is wrong with an air conditioner when the air flow out of the vents is low?

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while? 

• Why is my air conditioner not cooling enough? 

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.