How to size an expansion tank?

Start with

1) Volume of the system in gallons

2) Expected maximum operating pressure in PSI Absolute. (Add 14.7 to gauge pressure to get absolute pressure if at sea level.)

3) Initial cold fill pressure (static pressure) at point of tank connection, more specifically the elevation at which the tank will reside; this too is expressed in PSI Absolute.

4) Maximum design temperature.

There is software available for selecting tanks, whether conventional or bladder/diaphragm type.

You will note that if the tank is higher in the system (say an attic or upper floor) the tank will be smaller. Likewise a cooler maximum operating temperature will get you a smaller tank than one that runs hotter.

Hope this helps.

Brad

Expansion Tank Spreadsheet

Here's a spreadsheet that is taken right from the Amtrol sizing guidlines.

Be sure to use the coldest and hottest water temperatures the system will experience (you likely filled the system with 45 or 50 deg water).