Interesting instructions. A PRV has a check valve built in to it that prevents the heated water from being absorbed by the water main. That's why you need an expansion tank. More over, if you do not have a PRV, you don't have a check valve so they are implying that something about the PRV makes a check valve necessary. It doesn't make sense to me at all. The only caution I would have for you is to make certain the expansion tank is well supported. I have mine hanging down between 2 ceiling joists and the pipe is clamped to the joists. I'd be interested to hear if you ever get any clarification on why Watts deems a backflow is needed.