How to Tell How Old Your Water Heater Is (Serial Number Decoder)
How to tell how old a water heater is: read the serial number on the rating plate, where most brands code the build year plus the month or week in the first two to four characters.
Key Takeaways
- Rheem and Ruud put month then year first. Palo Alto Utilities decodes serial 1209D01234 as December 2009.
- A.O. Smith uses year then week on current tanks. Serial 203707002601001 was built in week 37 of 2020, per Palo Alto Utilities.
- Bradford White uses two letters, year then month, and Palo Alto Utilities shows the year letters repeat every 20 years, so H means 2011 or 1991.
- The U.S. Department of Energy rates storage tanks at 10 to 15 years and tankless units at more than 20 years.
- ENERGY STAR says to start planning a replacement once a water heater passes 10 years old.
- GE tanks were built by Rheem and Midea, per the Building Intelligence Center, so they decode month then year like a Rheem.
Where is the serial number on a water heater?
The serial number is on the rating plate, a printed sticker or metal tag on the outside of the tank jacket, usually on the side or near the top of the tank. It lists the brand, model number, serial number, capacity and fuel. On a tankless unit, check the side of the cabinet. ENERGY STAR notes that some labels also print the manufacture date outright.
You need two things from that plate: the brand name and the full serial number. The brand tells you which decoding rule applies, and the serial holds the date. City of Palo Alto Utilities, which asks residents to report water heater age, tells them to copy both the model and serial number exactly as printed.
- Walk around the tank with a flashlight and find the largest label on the jacket.
- If an insulation blanket covers the tank, loosen its tape and fold it back. Labels are often hidden under it.
- Photograph the plate with your phone flash angled to one side so the glare does not wash out stamped digits.
- Write the serial number down character by character, including any letters at the start.
- Match the brand to the decoder table below.
Leave the burner access cover and any electric element panels closed. The serial is never behind them, and electric models carry live line voltage there.
How do you read a water heater serial number by brand?
Each manufacturer codes the date its own way, so read the brand first and then the first two to four characters of the serial. Rheem family tanks lead with month and year, the A.O. Smith family leads with year and week, and Bradford White uses a two letter year and month code. The table below uses only example serials published by Palo Alto Utilities and the Building Intelligence Center.
| Brand | Serial format | Example serial | Decoded date |
|---|---|---|---|
| Rheem / Ruud | Digits 1-2 month, 3-4 year | 1209D01234 | December 2009 |
| GE (Rheem built tanks) | Skip letter prefix, then month and year | 1108903259 | November 2008 |
| A.O. Smith (current) | Digits 1-2 year, 3-4 week | 203707002601001 | Week 37 of 2020 (early September) |
| A.O. Smith (pre-2008) | Digit, month letter, two digit year | 2C062938405 | March 2006 |
| State | Digits 1-2 year, 3-4 week | 1735107397129 | Week 35 of 2017 (late August) |
| Whirlpool | Digits 1-2 year, 3-4 week | 1210A002243 | Week 10 of 2012 (early March) |
| Kenmore (A.O. Smith built) | Month letter, then two digit year | A03307353 | January 2003 |
| Bradford White | Letter 1 year, letter 2 month | HK16433183 | October 2011 (or 1991) |
Sources: Palo Alto Utilities (Rheem, A.O. Smith, Bradford White), the Building Intelligence Center (GE, State, Kenmore, Whirlpool example) and Waypoint Inspection (Whirlpool rule).
How do you read a Rheem, Ruud or GE water heater serial number?
Read the first four digits as MMYY. On Palo Alto Utilities’ decoder, serial 1209D01234 reads as month 12, year 09, which is December 2009. If the serial opens with letters such as R, RH or RHNG, skip them and start counting at the first digit. The same month then year order covers Ruud, Rheem’s sister brand.
GE tank heaters were built under license by Rheem and Midea, according to the Building Intelligence Center, which also notes Rinnai built some GE tankless units starting in 2006. Its GE example 1108903259 decodes to November 2008. This rule covers GE tanks only, so date a GE tankless unit through its maker.
How do you read an A.O. Smith, State, American or Whirlpool serial number?
Current A.O. Smith family tanks lead with a two digit year and a two digit week, so the first four digits read YYWW. Palo Alto Utilities decodes 203707002601001 as week 37 of 2020, which falls in early September. Older A.O. Smith serials, mostly before 2008, put a month letter in the second position followed by the year.
The brand family matters because A.O. Smith bought most of its rivals. The Building Intelligence Center records that A.O. Smith acquired State Industries in 2001 and American Water Heater (with the Whirlpool, GSW and John Wood brands) in 2006. State tanks have used A.O. Smith style serials since, so State serial 1735107397129 reads as week 35 of 2017.
- A.O. Smith pre-2008: 2C062938405 decodes as March 2006. The letter C is the month (A January, B February, C March) and 06 is the year.
- Whirlpool: Waypoint Inspection says the serial always starts with four digits, year then week. Serial 1210A002243 reads as week 10 of 2012.
- American: the first two digits are the year, so a serial opening 05 was built in 2005.
- Kenmore: Sears sourced Kenmore tanks from A.O. Smith, Rheem, State and Whirlpool, per the Building Intelligence Center. Its A.O. Smith style example A03307353 reads as January 2003. Identify which pattern your serial matches before you decode it.
How do you decode a Bradford White water heater serial number?
Bradford White serials start with two letters: the first is the year and the second is the month. Month letters run A for January through M for December, skipping I. Year letters skip I, O, Q, R, U and V and repeat every 20 years, so Palo Alto Utilities decodes HK16433183 as October 2011. A letter H could also mean 1991, so confirm the decade from the tank’s condition.
| Year letter | Years | Year letter | Years |
|---|---|---|---|
| A | 1984, 2004, 2024 | L | 1994, 2014, 2034 |
| B | 1985, 2005, 2025 | M | 1995, 2015, 2035 |
| C | 1986, 2006, 2026 | N | 1996, 2016, 2036 |
| D | 1987, 2007, 2027 | P | 1997, 2017, 2037 |
| E | 1988, 2008, 2028 | S | 1998, 2018, 2038 |
| F | 1989, 2009, 2029 | T | 1999, 2019, 2039 |
| G | 1990, 2010, 2030 | W | 2000, 2020, 2040 |
| H | 1991, 2011, 2031 | X | 2001, 2021, 2041 |
| J | 1992, 2012, 2032 | Y | 2002, 2022, 2042 |
| K | 1993, 2013, 2033 | Z | 2003, 2023, 2043 |
The 2004 to 2043 pairs come straight from Palo Alto Utilities; the 1984 to 2003 column applies the same 20 year cycle. To pick the right decade, compare against what you know. A tank with a C prefix and a clean, bright label in a home you bought in 2019 is a 2026 replacement or a 2006 original, and permit records settle it in one search.
What if the rating plate is missing or unreadable?
Check five fallbacks in order: a printed manufacture date elsewhere on the label, the owner’s manual or warranty card, your local building department’s permit history, the inspection report from your home purchase, and the installer’s invoice. Permits in many areas list the install date. Inspection reports often record the brand and the inspector’s age estimate for the water heater.
- Clean the plate. Wipe it with a dry cloth and shine the light flat across it. Stamped metal tags often stay readable after the ink fades.
- Read the partial serial. On Rheem and A.O. Smith family tanks, only the first four digits carry the date. On Bradford White, only the first two letters do.
- Search permit records. Many cities post permit history online by street address. Water heater replacements commonly need a permit, though rules vary by jurisdiction.
- Pull your closing documents. The home inspection report usually photographs the water heater and its label.
- Use the decoder sites ENERGY STAR names. ENERGY STAR lists the Building Intelligence Center, InterNACHI, A.O. Smith, Bradford White and Rheem as serial lookup resources.
How long do water heaters last by type?
A conventional storage tank lasts 10 to 15 years and most tankless units last more than 20, according to the U.S. Department of Energy’s Energy Saver guide. Tankless units last longer because they hold no standing water to corrode the tank, and DOE says their easily replaced parts can stretch life further. ENERGY STAR models heat pump units on a 13 year lifespan.
Your decoded date tells you where a tank sits on that curve. A 2014 tank is about 12 years old in 2026, in the back half of the DOE range. Our guide to how long water heaters last covers what speeds up or slows that clock, and the overview of the main types of water heaters explains why tankless and heat pump units age differently.
Maintenance moves the number. The DOE storage water heater page suggests flushing a quart of water from the tank every three months, testing the temperature and pressure relief valve every six months and inspecting the anode rod every three to four years.
Does water heater age matter more than condition, and when does replacement beat repair?
Age sets the odds and condition sets the deadline. ENERGY STAR says to start planning replacement past 10 years, and Palo Alto Utilities flags gas tanks over 10 for replacement. A leaking tank body, rusty hot water or heavy corrosion means replace now at any age. A failed part on a dry, younger tank is usually a repair.
| What you see | Likely cause | Repair or replace |
|---|---|---|
| Water pooling from the tank seam or bottom | Corroded inner tank | Replace. A leaking tank cannot be patched. |
| Rusty water from the hot tap only | Tank interior corroding | Replace if past 10 years; check the anode if younger |
| Rumbling or popping while heating | Sediment on the tank bottom | Flush first; replace if older and still noisy |
| Hot water runs out sooner than it used to | Sediment taking up capacity, or a failed element | Repair on a younger tank |
| Drip at a fitting or relief valve | Loose connection or worn valve | Repair |
ENERGY STAR lists those same red flags (visible corrosion, leaks, rust in the water, shrinking hot water supply and rumbling) as warning signs of failure. For part by part fixes, see our water heater repair guide.
Planning ahead also protects your choices. ENERGY STAR notes an emergency swap often limits you to whatever standard tank is on the truck. Price the job early with our water heater replacement cost breakdown. Warranty length gives a rough floor on design life, since ENERGY STAR certified gas storage and heat pump models must carry at least a 6 year warranty.
How we sourced this
We consulted 12 sources between October 17 and October 19, 2026, and cited 9. Decoding rules and every example serial come from City of Palo Alto Utilities and the Building Intelligence Center (Rheem/Ruud, GE, A.O. Smith, State, American and Kenmore pages), plus Waypoint Inspection for Whirlpool. ENERGY STAR names both decoder sites as serial lookup resources. Lifespans come from DOE Energy Saver (live pages returned errors in October 2026, so we cite the January 2026 archived copies) and ENERGY STAR. We left out decoding rules we could not confirm, including newer numeric Bradford White formats and tankless serial formats.
Frequently asked questions
How can I tell how old my water heater is without the serial number?
Look for a printed manufacture date on the tank label first, since ENERGY STAR notes some labels print it beside the serial number. If the label is gone, check the owner’s manual or warranty card, the permit history at your local building department, and the home inspection report from your purchase. A plumber can also estimate age from the label style, venting and fittings.
How long does a water heater last?
The U.S. Department of Energy puts a conventional storage tank at 10 to 15 years and a tankless unit at more than 20 years. ENERGY STAR uses a 13 year lifespan when it models heat pump water heater savings. Hard water, skipped flushing and a spent anode rod push a tank toward the short end of that range.
Should I replace a 10 year old water heater?
Start planning for it. ENERGY STAR says a water heater more than 10 years old is due for replacement planning so you are not forced into an emergency purchase. A dry, quiet tank with clean hot water can keep running, but price the replacement now. Replace right away if the tank itself leaks or the hot water runs rusty.
Does the model number tell you the age of a water heater?
No. The model number describes the product line, such as fuel type, capacity and efficiency, and the same model can be built for many years. The manufacture date lives in the serial number, which sits on the same rating plate. Write both down, because the manufacturer asks for the model and serial together on any warranty or parts call.
How do I read a Bradford White serial number?
Read the first two letters. The first letter is the year and the second is the month, from A for January to M for December with I skipped. Year letters repeat every 20 years, so H means 2011 or 1991. Palo Alto Utilities decodes serial HK16433183 as October 2011. A unit starting with C was built in 1986, 2006 or 2026.
How do I read a Rheem serial number?
On most Rheem and Ruud tanks the first four digits are the month and the two digit year. Palo Alto Utilities decodes 1209D01234 as December 2009. If the serial starts with letters such as RH or RHNG, skip the letters and read the first four digits after them. GE tanks built by Rheem follow the same month and year order.
Can a tank water heater last 20 years?
Some do, but it is not the norm. The Department of Energy rates storage tanks at 10 to 15 years and gives the 20 year plus figure to tankless units, which hold no standing water and corrode less. A 20 year old tank is running on borrowed time, so check for rust at the base and fittings and keep a replacement quote ready.
