This entire thing is getting out of hand and it is too laborious to try to sort through all the "solutions". IF you do not have a pressure regulating valve, OR a check valve in the system then you have an "open" system so you do NOT need an expansion tank because the pressure buildup is absorbed by the city system. However, in that case the pressure should ALWAYS be the same as the city pressure in the street so it should NOT increase, or change, unless the city pressure changes and that seldom occurs to any great extent, assuming the main lines are adequately sized. If your pressure fluctuates, or slowly rises without using hot water, then you need a plumber to check out your system because that is an indication of a failing pressure regulator.