🔍 How Old Is Your Furnace? Here's How to Find Out! Check the Serial Number! The easiest way to find your furnace's age is by locating the serial number on the manufacturer's label, typically found on the inside of the furnace cabinet door or on the unit itself. The serial number contains a date code, though the format varies by manufacturer. Common Date Code Formats: - The first four digits may represent the year and week of manufacture (e.g., 0815 = 15th week of 2008) - Some manufacturers use