Down-pipe support material must not promote what type of action?

The correct answer is that down-pipe support material must not promote galvanic action. Galvanic action occurs when two dissimilar metals come into contact in the presence of an electrolyte, resulting in the corrosion of one of the metals. This is a significant concern in HVAC systems where various metal components may be used in conjunction. If the support material encourages galvanic action, it can lead to premature failure of the pipes due to corrosion, which can result in leaks and costly repairs. Therefore, choosing support materials that do not foster this type of action is critical for maintaining the integrity and longevity of the HVAC system.

Mechanical action, corrosive action, and thermal action, while important considerations in material selection, do not specifically relate to the issue of galvanic corrosion that arises from metal interactions. Thus, focusing on preventing galvanic action ensures better performance and durability of the HVAC components.