Heater Circulators (TU-1500 and TU3) User manuals are sorted by product serial number and model codes. The first 4 digits of the serial number represent the manufacture date (YYMM) eg, 2203-XXXX = March 2022 build date. Models: TU3, TU3-2000W October 2023 - current (2310 - current) Models: TU-1500 TU-1500B May 2024 - current (2405 - current)