Bosch (tankless water heater) water heaters carry the build date inside the serial number. We know 2 serial formats for this brand; the decoder tries each one and shows every reading that fits.
Skip the first four characters. The next three are Bosch's date code: the first is the last digit of the year and the other two are a month code that changed in 2014 and 2020. Bosch publishes the table.
399A-720-000031-8733942691 reads as April 2017.
Two sources agree Bosch Home Comfort serial number guide, Building Intelligence Center
The first three digits (or the three after FD) are a date code: the year's last digit, then the month plus 60 for 1998 and 1999 or plus 80 for 2000 to 2009.
388123456 reads as August 2003.
Two sources agree Building Intelligence Center, InspectAPedia
A gas tankless water heater usually lasts 15 to 20 years, and sometimes longer, because there is no tank to rust through. Scale buildup in the heat exchanger is what shortens its life.
How long does a tankless water heater last? · Should I replace my water heater? · DC Dept. of Energy & Environment, U.S. DOE Building America
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