How to read John Wood and GSW water heaters serial numbers

Reading it yourself

• Before 2008: skip any letter in front. The first two digits are the year and the next two are the month.

  Example: U0106413635 was made in June 2001.

• From 2009: skip any letter in front. The first two digits are the year and the next two are the week of the year.

  Example: U0928413635 was made in week 28 of 2009.

This is the date the unit was made. It was usually installed soon after, but it can be later. If a serial doesn’t fit any of these patterns, look on the label for a printed manufacture date.

Sources

Checked in October 2026.

• Frequently asked questions, John Wood Water Heaters
• John Wood water heater age, Building Intelligence Center
• GSW water heater age, InspectApedia