How Old Is Your Miller Water Heater?
Miller water heaters are built by Rheem Manufacturing, so they use Rheem's date code: month, then year, in the first four digits after any letter prefix. Enter your serial in the free decoder above for an instant answer, or follow the guide below to read it yourself.
Where to Find the Serial Number
Check the rating plate on the side of the tank — a silver or white sticker, usually in the upper half, that lists the model number and serial number together. The date lives in the serial, not the model number. On older tanks the sticker can fade; a phone photo with the contrast bumped usually makes the digits readable.
How the Miller Serial Format Works
Rheem encodes the date as MMYY — month first, then year — in the first four digits after any letter prefix:
Worked example — RN1009A22108:
RN= plant code (ignore)10= October (month)09= 2009 (year)Aand the remaining digits = production sequence- Result: manufactured in October 2009
Letter prefixes are plant codes. Serials often start with R, RH, RN, RHNG, or a single letter — none of these are part of the date. Skip them and read the first four digits you find. Some old units used week-based formats; if the digits don't decode as a valid month, the decoder says so rather than guessing.
Why Your Miller Water Heater's Age Matters
- Lifespan. Tank water heaters average 8–12 years of service. Past that, internal corrosion makes tank failure — and a flooded floor — far more likely.
- Warranty. Miller tanks carried standard warranty terms counted from the manufacture date, not the install date. The serial tells you whether a repair might still be covered.
- Replacement planning. A unit past 10 years is on borrowed time. Knowing the date lets you shop for a replacement calmly instead of during an emergency.
Sources
Brand ownership and serial format verified against the InterNACHI water heater dating chart and InspectApedia's Rheem age decoder.