Age by Serial Number decodes manufacture age from serial numbers for water heaters (Bradford White, A. O. Smith, State, American, Whirlpool, Rheem, Ruud, Lochinvar) and for furnaces, air conditioners and heat pumps (Goodman, Amana, Lennox, Carrier, Trane, American Standard).
It also has model-specific status and flash-code pages for Rheem, Goodman and Rinnai, and a water-heater sizing calculator based on DOE and ENERGY STAR guidance. It decodes manufacture date, not install date, and Trane and American Standard publish no residential decoder. The site is independent and not affiliated with the brands listed.
The goal is to make these references easier to find and keep coverage limits visible. The site links to source material instead of guessing unsupported answers.