The Bosch Aquastar 125B gas tankless water heater was first manufactored back in the late 1980's. The 125B series has since been discontinued and replaced with newer compact models such as the 1600P gas tankless water heater.
When the Bosch Aquastar 125B model first came out, it had a dual port brass watervalve and then years later the Bosch Aquastar 125B series heater had a new single port polymer water valve which was much easier to service and maintain.
The replacement watervalve for the 125B series heater that had a dual port brass watervalve assembly is now a single port polymer watervalve retrofit kit. This watervalve assembly retrofit kit is required for any heater with a serial number beginning with 380 or lower. This kit includes new cold and hot water pipes, washers and the single port polymer watervalve so that it can be easily installed in the place of the old brass water valve. The new watervalve retrofit kit for the 125B part number #8707002499 is available. Please visit the link. There is also a service bulletin available for installation of the new retrofit water valve and the bulletin number is TWH-G1-37 and you can find this on the Bosch website at www.bosch-climate.us
The newer Bosch Aquastar model 125B gas tankless water heaters that had a serial number beginning with 381 or higher had the single port poly watervalve. The new single port Polymer water valve for the 125B part number #8707002649 is available. Please visit the link.