Watts is a good brand of plumbing supplies. The use of pressure regulators and expansion tanks is a frequent topic here, but this is the first time I've heard of a backflow preventer being installed. They are used when teeing off of a domestic system for irrigation, which is certainly not what you are doing. In fact, a pressure regulator is a backflow preventer and creates a "closed system). Heated water expands, and in a closed system this creates pressure. The pressure regulator prevents this expansion from being absorbed by the water supply. The result is the TP valve on the water heater will trip to relieve this pressure. This is why you need to install an expansion tank when you install a pressure regulator.