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Sources and testing
Manufacturers rarely publish their serial codes, and the guides that do often contradict each other. So we don't just copy a chart: we write each code as rules and test them against serial numbers whose dates are known.
How we build the rules
- Collect each brand's code from manufacturer documents, service notes and the published guides used by home inspectors, and compare them. Where they disagree, we note it.
- Write the code as rules the decoder can run.
- Test the rules against serial numbers with known dates (below). A rule that fails is fixed or marked as an estimate.
What we test against
The best free proof is a government safety recall. US Consumer Product Safety Commission (CPSC) notices often list the serial numbers affected next to the dates the units were made or sold:
| Brands | Record | What it proves |
|---|---|---|
| A.O. Smith, State, Reliance (gas water heaters) | CPSC recall 06-002, 2005 | serial prefixes by month, January 2004 to July 2005 |
| A.O. Smith family (Ultra-Low NOx water heaters) | CPSC recall 19-029, 2018 | first four digits 1115 to 1631 = April 2011 to August 2016 |
| York, Luxaire, Coleman (gas furnaces) | CPSC recall 01-523, 2001 | WFKM and WGKM serials made May to July 2001 |
| Amana (packaged terminal units) | CPSC recall 04-229, 2004 | serials starting 0301 to 0303 = January to March 2003 |
| Rheem (electric water heaters) | CPSC recall 16-177, 2016 | A0114 to A5214 = 2014, A1015 to A1615 = March to April 2015 |
| Goodman, Trane, American Standard, State, American, Bradford White, Lennox | CPSC recalls 17-145, 17-103, 14-263, 17-709, 08-241, 02-020, 02-553, 11-197 | serial prefixes checked against the dates the units were sold |
We also use data plates that print a manufacture date next to the serial number, and cases where several home inspectors agreed on a date. In our November 2026 test, all 34 serials backed by recall notices or printed dates decoded correctly, and 64 of 66 overall.
What we found along the way
- One widely used guide says digits 3 and 4 of an American Water Heater serial are the month. A 2001 recall notice (serials 0124 to 0127, sold June to September 2001) shows they are the week.
- Two published Trane letter tables for 1983 to 2001 swap the letters S and Z. Until we find an official answer, the decoder shows both dates.
- Rheem water heaters use two codes: older ones start with the month and year, newer ones with the week and year.
How sure is each brand?
- Tested against official records: Checked against official records (recall notices or data plates with printed dates).
- Sources agree: Published guides agree on this code, but we have not yet checked it against an official record.
- Estimate: Based on a single published source. Treat the date as an estimate and check the label for a printed date.
Tested: Lennox, Trane, American Standard, Ameristar, Goodman, Amana, Janitrol, Rheem, Ruud, York, Luxaire, Coleman, Bradford White, A.O. Smith, State, Reliance, American Water Heater, U.S. Craftmaster, Richmond. Sources agree: Carrier, Bryant, Payne, Whirlpool, Kenmore. Estimates: Heil, Tempstar, Comfortmaker, Arcoaire, Nordyne, Intertherm, Miller, GE, Rinnai.
Have a unit with the manufacture date printed on the label? Send us the brand, serial number and date. Each one makes the decoder more reliable.