Which fittings should be used for joints between different metallic piping materials in hydronic systems?

In hydronic systems, it is essential to use approved dielectric fittings when joining different metallic piping materials. The reason for this requirement stems from the concept of galvanic corrosion, which occurs when two dissimilar metals come into contact in the presence of an electrolyte, such as water. This contact can lead to one metal corroding more rapidly than the other, potentially compromising the integrity of the piping system over time.

Dielectric fittings are specifically designed to isolate these two dissimilar metals from each other. They typically have a non-metallic barrier that prevents electrical conductivity, thus stopping the galvanic corrosion process. By using these fittings, you ensure the longevity and reliability of the hydronic system.

In contrast, the other choices would not provide the necessary protection against corrosion between different metal types. For example, plastic fittings would not provide the structural integrity needed for metal piping systems, metal couplings could facilitate corrosion between differing metals, and rubber gaskets are not adequate for preventing electrolysis in this context. Thus, approved dielectric fittings are the appropriate choice for these applications.