It is both necessary and prudent to use a contractor to install a water heater expansion tank. Only a licensed professional is able to obtain a permit to perform this maintenance upgrade to a water heater. Permit and licensing requirements vary from state to state.
The benefits of adding an expansion tank include compliance with local county and state requirements to install one to lower water costs. The tank makes it possible to retain unused water drawn from the source instead of returning it, translating into less water consumption. Installation by a licensed professional retains the warranty and provides liability assurance.