I have recently read several threads about expansion tanks being needed if a prv is used . I am confused about this . I own a home I once lived in & now use as a rental . It has public water & the incoming pressure is 110 #'s . I installed a prv at the water meter which is about 200 feet from the house . I adjusted the pressure to 50 # 's . This was several years ago . I have had no known problems . What causes the high pressure in the water heater ? The only thing I can think of is that water can't go backwards through the prv . If that is the case why wouldn't a check valve cause the same problem ?