How old is my Bosch tankless water heater or boiler?

By Dana Whitlock. Updated October 2026. Official format.

Bosch shifts its month numbers every decade, so May shows up as 53 in 2014 and 73 in 2021. The date code is the three digits right after the first four characters. In 399A-720-000031-8733942691, that's 720, which Bosch's own table translates to April 2017.

Bosch publishes a Serial Number Reference Guide with the table, so this is an Official format, and the decoder above uses it directly. The month codes look odd at first. Once you see how the table is built, a hand reading takes a minute.

Where to find the label on a Bosch tankless water heater

On Bosch gas tankless units, the rating label is on the right side of the unit cover, and the same label is on the box. On Bosch boilers, look for the rating plate on the jacket. Wall-mounted units are often above head height in a garage or utility room; a flashlight and a phone photo save climbing. Keep the cover on. Inside it are the burner, gas valve and electronics, which are for a licensed technician.

How to read the code

The first digit of the three-digit code is the last digit of the year. The last two digits are a month code, and the month codes change by decade. For 2010 to 2013, months are plain 01 to 12, so 312 is December 2013. For 2014 to 2019, January to April are 17, 18, 19 and 20, and May to December are 53 through 60: 453 is May 2014 and 853 is May 2018. For 2020 to 2029, January to April are 37 to 40 and May to December are 73 to 80: 173 is May 2021, 180 is December 2021 and 278 is October 2022. Ignore the dashes and count characters.

Real serials in this code

When the format changed

Bosch adopted this date code in 2010. Because each decade uses its own set of month codes (01 to 12, then 17 to 20 and 53 to 60, then 37 to 40 and 73 to 80), the same first digit never points to two different years within the table. Bosch says 2030 will switch to letters in the month position (0A1 to 0AC). Units made before 2010 used a different, older code, but inspectors note that since 2010 Bosch has also printed the manufacture date on the rating tag, so on most units still in service you can read the date directly and use the serial as a check.

Who makes Bosch

Bosch home heating and water heating products in the US are sold by Bosch Thermotechnology Corp. under the Bosch Home Comfort name. The serial system covers its gas tankless water heaters and its condensing and non-condensing boilers. No other brand in our database shares the Bosch date code. The same reference guide also locates serials on Bosch heat pumps, furnaces and electric water heaters.

How sure is this?

Official format. Bosch publishes how its serial numbers encode the date. We tested the bosch thermotechnology rules against 5 serials. None decoded to a wrong year; codes whose year mark repeats show every possible reading. Always confirm with Bosch's warranty page before you spend money on the answer.

Quick notes

• Count four characters in, then read three. Dashes don't count.
• A month code of 53 to 60 means 2014 to 2019; 73 to 80 means 2020 to 2029.
• If the rating tag prints a manufacture date, it should match the decoded code. If it does not, trust the printed date.
• Bosch keeps a warranty library by product, which tells you the coverage terms that apply to your model and install year.