Which of the following is NOT required as part of a whirlpool bathtub installation?

In the context of whirlpool bathtub installations, an atmospheric vacuum breaker is not a necessary component. This device is primarily used in plumbing systems to prevent backflow and protect the potable water supply. While backflow prevention is crucial in many plumbing scenarios, it is not specifically required for the installation of a whirlpool bathtub, as this type of fixture typically incorporates a separate system for managing water drainage and circulation.

On the other hand, water supply valves are essential for controlling the water flow to the bathtub. These valves allow for the safe operation of the bathtub, enabling users to turn the water supply on and off easily. Similarly, a drain assembly is critical for the proper disposal of the water used in the bathtub, ensuring that it's properly channeled out of the unit after use. A showerhead, while it can be part of the bathtub experience in some designs, is often optional and not necessary for the basic function of a whirlpool bathtub. Therefore, while all these components serve important roles in related plumbing systems, the atmospheric vacuum breaker stands out as the component that is not required for the installation of a whirlpool bathtub.