Rheem's own published guidance for HVAC contractors states the rule directly: on a Rheem, Ruud or WeatherKing air conditioner, furnace or heat pump serial number, find the letter that sits near the beginning or middle of the serial, then read the two digits immediately after it as the production week, from 01 through 52, and the two digits after that as the two-digit year. A serial reading W4217 after that letter position means week 42 of 2017.
This current format is Rheem's own stated convention, not a third-party guess, and it is separate from Rheem's water-heater serial numbers, which use a different month-then-year order described on this site's water-heating pages. Rheem has used several older HVAC serial formats across past decades that independent HVAC-trade references document but Rheem's own current page does not restate, so only the current, manufacturer-confirmed format is covered here with full confidence.
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Decode your own serial number
What this covers
- Rheem, Ruud and WeatherKing residential furnaces, air conditioners and heat pumps carrying the current serial format
- Reading the week (01–52) and two-digit year from the two digit pairs immediately following the serial's leading letter
- Current-production equipment; Rheem's page does not date-bound this format
What it does not cover
- ×Rheem, Ruud or WeatherKing water heaters, which use a different month-then-year serial format covered separately on this site
- ×Older Rheem HVAC serial styles that independent references document but Rheem's own current page does not confirm
- ×A guarantee that every historical Rheem HVAC serial fits this exact position rule
Try in this order
0 of 4 completedLocate the rating plate
ReversibleOn a furnace, check inside the front access panel with the unit off. On an outdoor condenser or heat pump, check the exterior panel near the service valves.
Find the letter in the serial
ReversibleRheem's current format places one letter near the start or middle of the serial number, not necessarily as the very first character.
Read the two digit pairs after it as week then year
ReversibleThe two digits immediately after the letter are the production week (01–52); the next two digits are the two-digit year. W4217 reads as week 42 of 2017.
Confirm the product type before applying this rule
ReversibleThis rule is for AC, furnace and heat-pump units. If the product is a water heater, use this site's water-heating serial guidance instead — the digit order is different.
Evidence ledger
Sources behind this answer
Where the manufacturer's article stops. Rheem's own article states the week/year reading directly for HVAC equipment. It does not address water heaters, older serial styles, or exactly how many historical formats exist — that context comes from the independent trade source cited alongside it.
Rheem's own article states the rule directly: "the first two numbers after the letter in the middle or beginning of the serial number are the week of manufacture, and the third and fourth numbers are the year of manufacture." Verified Sept. 10, 2026.
Independent HVAC-trade reference documenting the same current-format reading plus six older, less consistently documented Rheem serial styles not covered on this page. Verified Sept. 10, 2026.
Exact questions
Questions about Decode a Rheem HVAC serial number
Is a Rheem water heater's serial number read the same way?+
No. Rheem's own water-heater serial documentation places month in the first two digits and year in the next two, ahead of a plant-code letter — the opposite emphasis from the HVAC rule on this page.
Where exactly does the letter sit in a Rheem HVAC serial?+
Rheem's own wording says "near the beginning or middle," without a single fixed position, because Rheem has used more than one HVAC serial layout across the letter's placement. Locate the letter first, then read the two digit pairs that immediately follow it.
What if my Rheem HVAC serial doesn't match this layout at all?+
Older Rheem HVAC units used other formats. Independent HVAC-trade references document several of them, but Rheem's own current page does not restate them, so this page does not present them with the same confidence.
Where do I find the Rheem unit serial number?+
Look for the rating plate or label on the unit, on the unit's data plate. Photograph it before you start, since the label can be hard to read once the appliance is installed.