What is not permissible for gas piping to water heaters?

Concealed bushings are not permissible for gas piping to water heaters because they can create a hidden connection that may not be accessible for inspection or maintenance. This can lead to potential issues such as leaks that are not easily detectable, posing safety hazards. Gas piping systems must be designed and installed in a way that allows for easy access and inspection to ensure the integrity of the system and the safety of the users.

In contrast, threaded joints, flexible connections, and pressure fittings are generally accepted as proper methods for gas piping when installed correctly and meet the relevant code requirements. Threaded joints are commonly used for connecting pipes, while flexible connections allow for movement and can help absorb stresses in the piping system. Pressure fittings are designed to handle the pressures exerted by gas systems, making them suitable for this purpose.