Which of the following requires a union to be installed?

In plumbing and piping systems, a union is a type of fitting that allows for easy disconnection of two pipes or fittings without having to cut them. This is particularly important for maintenance or replacement purposes.

The connection of a gas regulator to rigid piping often necessitates a union because it allows for straightforward access to the regulator for servicing. Rigid piping does not typically allow for the flexibility of movement, making a union particularly useful. This fitting is designed specifically for ease of installation and removal, while ensuring a secure connection when in use.

The other options either do not require such a fitting or are designed to be permanently installed. For instance, a water supply line typically involves sweat fittings or compression fittings that do not need to be disconnected frequently. Similarly, a gas shut-off valve is mounted in line and generally does not need to be frequently removed. An appliance venting system typically connects through methods like slip fittings, which do not necessitate the use of a union.

Thus, the installation of a union to connect an MP regulator to rigid piping is crucial for maintaining system integrity while ensuring access during maintenance.