What type of fittings must NOT be used in concealed locations?

Compression fittings must not be used in concealed locations primarily due to safety and reliability concerns. These fittings rely on a mechanical grip to hold the pipe and the fitting together, which can deteriorate over time, especially when exposed to temperature fluctuations, vibrations, or other environmental factors. In concealed areas, any potential failure of these fittings could lead to leaks that are difficult to detect, creating significant safety hazards such as fire or explosion risks due to propane leakage.

In contrast, welded fittings and brazed fittings create a permanent bond by melting the base materials together, which provides a much more secure and durable connection. While these can also be subject to inspection and standards, their nature allows them to be used safely in concealed areas, as they do not have the same risks of loosening or degradation over time.

Using compression fittings in visible locations where they can be easily accessed for inspection and maintenance is generally acceptable, but when it comes to concealed installations, the risks outweigh the benefits, making the use of compression fittings inappropriate in such scenarios.