Expansion tank size problem

[Austin83]

I am getting a PRV installed due to high pressure coming into my house. I have two 50 gallon hot water heaters that are right next to each other and connected. The plumbing supply store has a 4 gallon, or a 20 gallon expansion tank, and they told me the 4 gallon expansion tank is for an 80 gallon hot water heater, but they think that it will be just fine for my situation. The difference in price is astronomical between those two tanks, and the local Lowes and Home Depot carry a 4.4 gallon tank. My choices are limited, and I'm sure any hired plumber in this area would slap a 4 gallon tank on it and be done with it, but I'd like to know if this is going to be alright for my home. Any information or help is greatly appreciated.

[hj]

as a practical matter, almost any size expansion tank will be all right, as long as the air precharge is correct. Air is a gas, so it is impossible to "squeeze" it into "nothingness". Therefore there will ALWAYS be an air bubble in the tank to absorb the expansion.

[jadnashua]

I think they're way off...go here: http://www.watts.com/pages/support/sizing_DET.asp and calculate what you need for yourself. The smaller one should be fine.