The short version
Find the serial number on the rating plate. Ignore any letters at the front, then read the first four digits:
| Digits | Meaning |
|---|---|
| first two | month, 01 to 12 |
| next two | year |
Two worked examples from two independent sources:
- InspectApedia: RN 1292A29282 is 12 and 92, so December 1992.
- Fast Water Heater: 0720 F12345 is 07 and 20, so July 2020.
Fast Water Heater on the prefixes: "Many Rheem-family serials start with a brand or fuel prefix such as RH, RN or GE. Ignore those and read the first four digits."
The exception that catches people out
Some Rheem lines use a week code instead of a month. The tell is arithmetic: there is no month 52.
Fast Water Heater: "on some Rheem lines, heat-pump and certain electric models, a single plant letter is followed by a week code instead, so a serial like A5214 is week 52 of 2014, not month 52. If the first two digits are larger than 12, you are looking at a week."
InspectApedia records the same variant as a separate format, Z WWYY 12345, where WW is a week from 1 to 52.
So the rule in order: if the first pair is 01 to 12, it is a month. If it is 13 to 53, it is a week. Above 53, the plate uses a format not covered here.
The nine formats
InspectApedia's explanation for the number: "Because Rheem water heaters or calorifiers or geysers were sold under quite a few brands and over many years, there is a plethora of Rheem water heater serial number formats." They have found at least nine.
| Pattern | Where it appears | Example |
|---|---|---|
| MMYY P12345, 10 digits with plant code | current | prefixed R, RN, RNG, RHNG |
| ABCD MMYY 2345, 9 digits with letter prefix | older tanks | RN 1292A29282 is December 1992 |
| Z WWYY 12345, week code | some lines | week then year |
| MMYY 123456, 10 digits, no plant code | older tanks | 09815101633 is September 1981 |
| MM.YY -123456 | some tankless | month, dot, year |
| MMYY P123456, 11 characters | certain tankless | with plant code |
| Z MMYY P12345689X, 16 characters | with plant code | month, year, plant |
| 1234 WWYY, 8 digits | older Rheemglas | InspectApedia marks this one uncertain |
| B950-3138 RCY-5, 12 characters | pre-1978 | InspectApedia states it is not precisely decoded |
Two things worth taking from that table. First, InspectApedia labels its own uncertainty on two of the nine formats rather than guessing, and we are repeating that rather than smoothing it over. Second, an older plate can still be read: InspectApedia's photo caption for a Rheemglas Standard records serial R 0884B10488 as August 1988.
Where the serial number is
InspectApedia, by unit type:
- Tank units. On the data tag or label on the exterior.
- Tankless. Beneath a QR code on a sticker on the right side of the exterior on modern units, and on some models inside the heater body on the right side. Tankless units fail differently from tanks and cost differently to fix, which the tankless repair page covers.
- Heat pump units. On a label on the upper left side.
Rheem's own checker, and its limit
Rheem runs a checker at How Old Is My Equipment. It is the authoritative answer when it works, and worth using to confirm whatever you calculate by hand.
It has a documented limit. InspectApedia: "Rheem provides their own Rheem water heater age decoder at https://www.rheem.com/how-old-is-my-water-heater however we found that this web page does not work on older Rheem water heater serial numbers."
Our advice, marked as ours: use Rheem's tool first. If it returns nothing, your plate is one of the older formats, and the table above is then the only route. Neither source frames it as a sequence; that ordering is ours.
Which brands follow the Rheem rule
Rheem builds for several badges. Fast Water Heater and InspectApedia both list Rheem, Ruud, Richmond and GE, with InspectApedia adding Hotpoint under GE and noting that Lowe's own-brand heaters are Rheem-built.
One brand that does not belong here, despite being widely claimed to: Kenmore. Both of our sources place Kenmore in the A.O. Smith family instead, so a Kenmore plate reads year first. Our own decoder had this wrong until 8 October 2026 and we corrected it; the detail is on the age decoder.
The mix-up to avoid
| Plate reads 1219 | Reading | Result |
|---|---|---|
| on a Rheem | month 12, year 19 | December 2019 |
| on an A.O. Smith | year 12, week 19 | May 2012 |
Over seven years apart from the same four digits. Confirm the brand before you decode.
What the age tells you
Once you know the year, the repair or replace page turns it into a decision, the expected life page covers the bands by type, and the replacement cost calculator gives the figure to hold a repair quote against.
Sources
- InspectApedia, Rheem Water Heater Age Decoder (nine serial formats with worked examples)
- Fast Water Heater Company, How Old Is My Water Heater? (Rheem month-year rule, prefix and week-code exception)
- Rheem, How Old Is My Equipment (the manufacturer's own checker)
- InspectApedia, Water Heater Age and Manuals (brand index, Hotpoint filed under GE and Lowe's under Rheem)
Price figures are national US ranges from the sources above as of the date shown. Your local price depends on labor rates, permits and site conditions. Read how we get our numbers.