I recently installed a pressure reducing valve, and a 4.5 gallon Watts expansion tank. The tank came preset at 20 psi, and I'm confused as to why or if the 20 psi should be increased (the instructions are rather vague). According to the Watts sizing calculator my system needs a tank larger than 3.62 gallons so I would assume that 20 psi is okay? My static pressure is 55 psi, two 50 gallon hot water heaters @ 135*. Any help is greatly appreciated.