How old is my Rinnai tankless water heater?

By Dana Whitlock. Updated October 2026. Official format.

Rinnai went to letters in 2009, so a serial that opens with two of them is 2009 or newer. The first letter is the year and the second is the month, which makes BH.AB-000123 an August 2010 unit. Older Rinnai serials start with digits, year then month, so 03.06-001000 was made in June 2003.

We list Rinnai as an Official format. Rinnai prints its serial table, including both layouts and the full year-letter list through 2029, in its own service literature. One caveat. The copy on file sits in a Rinnai furnace manual, and third-party guides confirm the same table on tankless water heaters.

Where to find the label on a Rinnai tankless water heater

The rating plate is on the outside of the cabinet, usually on a side panel of the heater, with the serial printed after the words SERIAL NO. Outdoor units may have it on the side facing the wall, so a flashlight and a phone camera help. Read it from outside the unit. The front cover hides the burner, fan and circuit board, and taking it off with the power and gas on is a job for a licensed technician. You don't need it to find the serial.

How to read the code

Current Rinnai serials look like two letters, a dot, two more letters, a dash and six digits. Letter one is the year: A is 2009, B 2010, C 2011, D 2012, and so on, skipping I, O, Q, U and V, which makes P 2022, R 2023, S 2024, T 2025 and W 2026. Letter two is the month, A for January through M for December with I left out (J is September). Letters three and four identify the plant and product type, and the six digits are a count that restarts each year. Older serials read YY.MM followed by a dash and six digits, or eight digits run together as YYMM plus four: 05083453 is August 2005.

Real serials in this code

When the format changed

Rinnai moved its water heaters from digits to letters in 2009; the furnace manual dates the change to June 1, 2010 for that product line. Either way, a serial starting with two letters is 2009 or later, and one starting with digits is earlier. The letter table runs out at Z for 2029, so there is no 20-year cycle to worry about yet. The one known wrinkle is 2022. Rinnai's table lists P, but distributors report that one model used O that year, so an O reads as 2022 too. Registration records with Rinnai, or the installer's invoice, settle anything that still looks odd.

Who makes Rinnai

Rinnai was founded in Japan in 1920 and remains a Japanese-headquartered gas appliance maker. Its North American operation dates to 1974 and runs a distribution, research and training center in Peachtree City, Georgia. Rinnai's serial code is its own; no other brand in our database uses it, and it covers Rinnai's tankless water heaters and its boilers.

How sure is this?

Official format. Rinnai publishes how its serial numbers encode the date. We tested the rinnai rules against 8 serials. None decoded to a wrong year; codes whose year mark repeats show every possible reading. Always confirm with Rinnai's warranty page before you spend money on the answer.

Quick notes

• Two letters at the start means 2009 or newer. Digits at the start means older.
• If the first letter is O, read it as 2022.
• Ignore letters three and four. They mark the plant and product type and say nothing about age.
• DOE assumes a gas tankless heater lasts about 20 years on average (with limited data), so a Rinnai from the early 2010s still has useful life if it has been descaled on schedule.