When is it permissible to omit flux when joining copper tube to wrought copper fittings?

Omitting flux when joining copper tube to wrought copper fittings is permissible when using BCuP series filler metals because these types of brazing alloys have inherent properties that allow them to flow and adhere well to copper surfaces without the need for additional flux. The BCuP series contains elements that can help in avoiding oxidation during the heating process, making it effective for creating strong joints even in the absence of flux.

In contrast, while some may assume other options could allow for skipping flux, they do not provide the same guarantees as BCuP fillers. For example, silver solder, while high-quality, still generally requires flux for optimal adhesion and to prevent oxidation. Water presence can help with joining techniques but does not eliminate the need for flux in most joining scenarios, especially under high heat. Soft solder typically also requires flux to ensure a good connection, particularly because it has a lower melting point and relies more on clean surfaces for bonding. Thus, BCuP series filler metals stand out as the most suitable choice for omitting flux in this context.