Before furnaces were invented, hours of each day were devoted to chopping and stacking wood, ensuring there was fuel for a fire to keep a family warm. Today, furnaces are taken for granted that they will warm the house. Often forgotten, furnaces are left to age in the dusty confines of basements, attics and garages. According to The Learning Channel, in the article, "How to Save Money on Home Efficiency" 25% of American households own furnaces 20 years or older, and the average furnace age is about 17 years old. Determining the age of your furnace can be a challenge.
Look on the furnace surface for a paper or metal tag that contains a serial number and/or the furnace manufacturer. Don't expect the age to be listed in plain writing. According to Inspectapedia website, the age of the furnace isn't always easy to detect from the label. Other places to look for serial numbers are relays or gas valves. Typically, furnaces over 30 years old will not have serial numbers to help determine the furnace age.
Write down the serial number found on the data tag. According to Peak to Prairie Inspection Service, use the serial date to determine the furnace age. The manufacture date is sometimes encoded within the serial number, but you need a furnace chart with manufactures, serial numbers and model numbers to consult to figure out the "hidden" furnace date.
Determine the age from the serial number if possible. Some serial numbers can be interpreted by weeks and years. So a number reading 1193CA4567 would be dated as the eleventh week of 1993. According to Inspectapedia website, in the article, "How to Determine Furnace Age", serial numbers from furnaces made after the year 2000 are easier to decipher. The serial number might have a six-digit code at the end; for example, 67890-3011606 yields a furnace date of June 2006.
Consult a furnace age chart using the manufacturer, model of the furnace and serial number, if any. There are many charts and books that list the information contained on your data tag to decode the furnace age. See the resources for this article for two charts. There are books available online and through bookstores.
Look in the furnace manual. According to Home Heating Oil Prices, in the article, "Thinking of Upgrading or Replacing Your Residential Heating Oil Furnace?", if your furnace lacks a data tag with an identifying serial number and you have the original furnace manual there might be a date in the manual that dates the furnace.
Contact the furnace manufacturer if the company is still producing furnaces. Often companies are absorbed by other companies and some detective work must be accomplished in order to discover the present "name" of the company that made your furnace. You will have to have a model number or a serial number for the company to help you in any way.