Which of the following is NOT a permissible fitting type for concealed locations?

In the context of propane gas safety and installation, specific fitting types are deemed permissible or impermissible for concealed locations based on their material properties and potential risks. The reason why plastic fittings are not permissible for concealed locations is primarily due to concerns regarding their durability and ability to withstand the operating conditions associated with propane systems.

Plastic fittings, while lightweight and easily installed, have limitations when it comes to heat resistance and structural integrity under pressure. They can be more susceptible to degradation over time from exposure to UV light or extreme temperatures, potentially leading to leaks or system failures. Concealed locations may also experience fluctuations in temperature and humidity that could further compromise the integrity of plastic fittings.

In contrast, fittings made from materials like metal, such as threaded, brazed, and welded fittings, are more robust and reliable for concealed areas. These materials provide a high level of strength and resistance to the environmental factors that could lead to installation issues. Therefore, when considering gas system installations in concealed areas, it is crucial to use materials that ensure safety and compliance with regulations, making plastic fittings unsuitable for such applications.