Methodology and sources
This page explains how every result on the site is produced. The rules were last checked against their sources on .
Primary sources only
A decoding rule is published here only if a primary source states it: a manufacturer's web page, manual, catalog or warranty document on the manufacturer's own site or document host, or a document of a U.S. government agency such as a Consumer Product Safety Commission recall notice, EPA or ENERGY STAR. Forums, home inspector blogs and lookup sites are used only as hints about where to look.
Some manufacturer documents are easiest to find on a distributor's or a manual library's server, for example Rheem parts lists kept by HVAC distributors or a Rinnai service manual kept by a manual library. We use such a copy when it is the manufacturer's own document, identified by its form number and title, and the source list then names both the document and the site that hosts the copy, so you can judge it yourself.
Five kinds of result
- Decoded: one date fits the published rule.
- Two or more possible dates: the rule allows several readings, for example a year letter that repeats every 20 years. All of them are shown, with what tells them apart.
- Not confirmed by the manufacturer: the serial has a recognisable shape, but no primary source covers that shape or that period. No date is given.
- Does not fit the format: the serial matches a layout but contains an impossible value, such as month 13 or a date after today.
- Not in our database: no documented layout matches.
Our own readings, labelled
A few steps are our reasoning rather than the manufacturer's statement, and the decoder labels them every time they are used: the century of a two-digit year, the earlier cycle of a repeating letter table (for example Bradford White before 2004), the month in which a production week begins, dates for American Standard and RunTru read with Trane's table, and dates for Rheem-built brands read with Rheem's documented formats (Ruud and Richmond water heaters, and Ruud air conditioners and furnaces where that page is published). Readings that cannot be right, such as a date in the future or one excluded by the serial length, are listed as ruled out with the reason.
Model numbers
Capacity and refrigerant codes are read only from a published nomenclature, and only the codes listed in it. Furnace input codes are translated only from the manufacturer's own values, because several manufacturers do not use the code times 1,000. Water heater model numbers are not decoded: none of our sources publishes a nomenclature that gives their size.
Keeping the rules current
Every rule carries the date it was last checked. The site will not build when a serial rule has gone unchecked for 18 months, an EPA refrigerant date for 6 months or a service-life figure for a year. Dates on the pages change only when the content does.
Your input
The decoder is a function that runs in your browser on rule data that was loaded with the page. Serial and model numbers are never sent to a server. See the privacy policy.