How to Read a Water Heater Rating Plate

The rating plate is how you identify your water heater. Every unit has one. It tells you the model, manufacture date, capacity, efficiency, and electrical requirements.

What is a rating plate?

A rating plate (also called a data plate, nameplate, or rating label) is a permanently attached metal plate or adhesive label on every water heater that displays the unit's specifications, certifications, and regulatory information. It is required by federal law (10 CFR Part 430) and safety standards (UL/CSA).

Where to find it

Gas Tank

Side of the tank near the bottom, above the burner access panel. Often visible without moving the unit.

Electric Tank

Side of the tank, typically near the upper thermostat access panel. May be behind insulation panels on some models.

Tankless

Front panel or side of the unit. On indoor models, it is usually visible after removing the front cover. On outdoor models, it is on the exterior casing.

Heat Pump

Side of the tank or on the heat pump compressor housing at the top of the unit. Some models have two labels: one for the tank and one for the heat pump system.

What each field means

Field	What it tells you
Model, Model No., Mod.	Manufacturer's model number identifying the exact product line, capacity, fuel type, and feature set. This is the most important field for finding a replacement. Model numbers encode specifications. For example, A.O. Smith model GNRH50 100 indicates Gas, Natural, Residential, High-efficiency, 50-gallon, 100 series.
Serial, Serial No., S/N	Unique identifier for that specific unit. Encodes the manufacturing date on most brands. Required for warranty claims and parts orders. Serial number formats vary by manufacturer. See the brand_serial_number_decoding section for how to extract the manufacture date.
MFG Date, Date of Manufacture, Date MFG	The date the unit was manufactured. Some brands print this explicitly; others encode it in the serial number. Not all brands print a separate manufacture date. Rheem typically does; others require serial number decoding.
Rated Storage Volume, Capacity, Tank Size, Nominal Capacity	The volume of hot water the tank holds, in U.S. gallons. Tankless units do not have this field. The actual usable hot water is measured by First Hour Rating (FHR), not tank capacity. A 50-gallon tank may deliver 60-80 gallons in the first hour.
Input, BTU Input, Rated Input, Input Rating	The energy input rate. Measured in BTU/h for gas units and kilowatts (kW) for electric units. Higher input means faster recovery. Gas units typically range from 30,000 to 199,000 BTU/h for residential models. Electric tank units are commonly 4.5 kW per element.
UEF, Uniform Energy Factor	The federally mandated efficiency rating. Higher UEF means lower operating cost. Replaced the older Energy Factor (EF) metric in 2017. UEF values are tested under one of four draw patterns (very small, low, medium, high) depending on the unit's first hour rating or GPM. Units with UEF values that meet ENERGY STAR thresholds earn the certification.
First Hour Rating, FHR, 1st Hour Rating, First Hr. Rating	The number of gallons of hot water a storage tank unit can deliver in the first hour of use, starting with a full tank of hot water. This is the primary sizing metric for tank water heaters. Applies only to storage tank water heaters. Tankless units use GPM (gallons per minute) instead.
GPM, Flow Rate, Max GPM, Gallons Per Minute	Maximum flow rate in gallons per minute. The primary performance metric for tankless water heaters. GPM ratings are tested at a specific temperature rise (typically 35 or 77 degrees F). Real-world GPM depends on incoming water temperature.
Max. Working Pressure, Maximum Pressure, Working Pressure	The maximum water pressure the tank is designed to handle safely, measured in PSI (pounds per square inch). Most residential water heaters are rated for 150 PSI. If your home's water pressure exceeds 80 PSI, a pressure-reducing valve is recommended.
T&P Valve Rating, Temperature and Pressure Relief, Relief Valve	The temperature and pressure at which the safety relief valve will open. Required by code on all tank water heaters. The T&P valve is a critical safety device. It opens if water temperature exceeds 210 degrees F or pressure exceeds 150 PSI to prevent tank rupture.
ENERGY STAR, ES	The EPA ENERGY STAR certification mark, present if the unit meets or exceeds efficiency thresholds set by the EPA. ENERGY STAR certified water heaters may qualify for utility rebates and federal tax credits under the Inflation Reduction Act (25C credit).
Manufacturer, MFG by, Manufactured by	The manufacturer's legal name and address. Required by federal labeling law. Some brands are manufactured by the same parent company. A.O. Smith also manufactures State, American, and other private-label brands. Bradford White does not sell through retail channels.
UL Listed, CSA Certified, UL/CSA, cULus	The safety certification mark from an accredited testing laboratory. UL (Underwriters Laboratories) and CSA (Canadian Standards Association) are the most common. Building codes require water heaters to bear a listing mark from an accredited lab. A unit without this mark cannot legally be installed in most jurisdictions.
AHRI Ref, AHRI Certified Ref No., AHRI No.	The AHRI (Air-Conditioning, Heating, and Refrigeration Institute) certified reference number. Used to look up independently verified performance data. AHRI certification verifies that the manufacturer's efficiency claims have been independently tested. The number can be looked up at ahridirectory.org.
Estimated Yearly Energy Cost, Est. Annual Operating Cost	The FTC-required estimated annual operating cost based on national average energy prices and the DOE test procedure. This figure appears on the yellow EnergyGuide label, which is a separate label from the rating plate but often located nearby. It uses standardized energy prices that may not match your local rates.
Recovery, Recovery Rate, Recovery GPH	The number of gallons per hour the unit can heat from inlet temperature to the thermostat setpoint. Indicates how quickly hot water replenishes after use. Gas units recover significantly faster than electric (typically 40+ GPH vs. 20 GPH for comparable tank sizes).

Serial number date decoders by brand

Most manufacturers encode the manufacture date in the serial number. Use these to determine how old your water heater is.

A.O. Smith

Also applies to: State, American, Whirlpool (A.O. Smith manufactured), Reliance

Current (2008+) YYWW######
First two digits are the 2-digit year, next two digits are the week of manufacture. Remaining digits are a production sequence number.
Example: 1604123456 → Year 2016, Week 04 (late January 2016)

Older numeric (1990s-2000s) MMYY#######
First two digits are the month (01-12), next two digits are the 2-digit year.
Example: 0399A123456 → March 1999

Letter-month prefix [Letter]YY#######
A single letter represents the month (A=Jan, B=Feb, C=Mar, D=Apr, E=May, F=Jun, G=Jul, H=Aug, J=Sep, K=Oct, L=Nov, M=Dec; letter I is skipped). Followed by 2-digit year.
Example: B01A123456 → February 2001

Serial number is on the rating plate label on the side of the tank near the bottom.
Skip any leading letter prefixes (factory codes) and look for the first digits.
When in doubt, call A.O. Smith at 800-527-1953 with the full serial number.

Bradford White

All years (rotating 20-year letter cycle) [Year Letter][Month Letter]#######
First character is a letter representing the year (20-year cycle). Second character is a letter representing the month. Remaining digits are a sequential production number (7 digits before December 2007, 8 digits after).
Example: MF1234567 → M = 1995 or 2015, F = June. Use the ANSI compliance date on the label to determine which 20-year cycle.

Letters I, O, and Q are not used (to avoid confusion with 1, 0, and 2).
To distinguish between the two possible years in the 20-year cycle, check the ANSI standards date also printed on the rating plate.
Bradford White offers a serial number lookup tool at bradfordwhite.com/my-water-heater.

Rheem

Also applies to: Ruud, Richmond, GE (Rheem manufactured)

Current (10-digit format) [Prefix]MMYY[Plant]#####
After any letter prefixes (R, RN, RNG, RHNG, etc.), the first two digits are the month (01-12) and the next two digits are the 2-digit year. The fifth character is a plant code. Remaining digits are a sequential production number.
Example: R 0884B10488 → Month 08 (August), Year 84 (1984). Plant code B.

Strip any letter prefixes (R, RN, RNG, RHNG) before decoding.
Many Rheem units also print the manufacture date explicitly on the rating plate as 'MFG. DATE', so check for that first.
Rheem, Ruud, and Richmond use the same serial number format (same parent company).

Rinnai

2009 and later (letter-based) [Year Letter][Month Code]########
First letter is the year. Second character represents the month. Remaining digits are production sequence.
Example: NK12345678 → N = 2021, K = October. Manufactured October 2021.

Before 2009 (numeric) YYMM########
First two digits are the 2-digit year. Next two digits are the month (01-12).
Example: 06074363 → Year 06 (2006), Month 07 (July). Manufactured July 2006.

Letters I, O, and U are skipped in the year and month codes to avoid confusion with numbers.
Serial number is on the rating plate on the front of the unit (behind the front cover on indoor models).

Navien

Current format [Model Code 5 chars]YYMD#########
After a 5-character model code, the next 2 digits are the year, followed by 1 character for the month (1-9 for Jan-Sep, X=Oct, Y=Nov, Z=Dec), then 2 digits for the day of manufacture.
Example: ABCDE14X17001234 → 14 = 2014, X = October, 17 = 17th day. Manufactured October 17, 2014.

Navien offers a serial number lookup tool at navieninc.com/serial.
The serial number sticker is on the side of the unit body.
Older Navien serials may use a different format. Use the online tool for older units.

Noritz

All years YYYYMM########
First four digits are the full 4-digit year. Fifth and sixth digits are the month (01-12). Remaining digits are a production sequence number.
Example: 200606123456 → Year 2006, Month 06 (June). Manufactured June 2006.

Noritz uses a straightforward 4-digit year format, making it one of the easiest brands to decode.
Serial number is at the bottom of the data plate on the unit.

Takagi

2010 and later MMYY########
First two digits are the month (01-12), next two digits are the 2-digit year.
Example: 0318XXXXXXXX → Month 03 (March), Year 18 (2018). Manufactured March 2018.

Before 2010 No date encoding
Takagi units manufactured before 2010 (before the acquisition by Noritz's North American operations) do not encode the manufacture date in the serial number.

Takagi was acquired by A.O. Smith in 2023. Newer units may follow A.O. Smith serial number conventions in the future.
Pre-2010 units require calling customer service for exact manufacture date.

What to do next

Write down the model number and serial number from your rating plate.
Search the model number in the search bar above to find your unit's full specs.
If your unit has died, check the replacement page for compatible options with projected savings.
If you need parts, use the serial number for warranty claims and the model number for parts orders.

Sources. Manufacturer documentation, 10 CFR Part 430, UL/CSA certification standards. Methodology

How to read your rating plate