There are two serial number formats:

1) 2002  to present - If the serial number is a 9-digit combination of letters and numbers that starts with a number, then the first number is the year. Beginning in 2010, the first two digits are the year. The serial number below indicates the furnace was manufactured in 2002. 

    Check the year of ANSI (or sometimes ANS) rating elsewhere on the data plate to confirm. The ANSI standard gets updated every several years, so the year of the ANSI should be the same or a just few years earlier that what the serial number says. The serial number on the example below indicates it was manufactured in 2006, and the 2004 ANSI confirms it.

2) 1983 to 2001 - If the serial number starts with a letter, then that is the encoded year. W = 83, X = 84, Y = 85, S = 86, B = 87, C = 88, D = 89, E = 90, F = 91, G = 92, H = 93, J = 94, K = 95, L = 96, M = 97, N = 98, P = 99, R = 00, Z = 01. So the serial number shown below indicates that the furnace was manufactured  in 1992. Notice that the year that the letter indicates is corroborated by the 1990 year of the ANSI rating above it, which should be the same year or just a few years earlier.

    Many, but not all, furnace data plates will also state the month and year of manufacture somewhere on the data plate, like the examples below.

    To determine the age of a Trane air conditioner or heat pump, go to our blog post How do I determine the age of a Trane air conditioner or heat pump?

    For other key specs of your HVAC system, see one of these other blog posts: 

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

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

• How can I find out the size 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: 

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

 • 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.