After running a quick search did not find answer in this forum as to how much water expands with temperature, which is important in sizing a non-pressurized tank and in sizing an expansion vessel in a presurized system. after *oogling this, these are helpful: http://www.newton.dep.anl.gov/askasci/chem03/chem03335.htm http://www.ucdsb.on.ca/tiss/stretton/chem2/data19.htm A statement made is "from freezing to boiling, [expansion] is 4.3%." Looking at the data table, seems to indicate that expansion in moving water from 50F (10C) to 200F (95C) is 3.8%. So, 1000 gal of storage needs at least 38 gallons of expansion over this temp range. Plus a margin of error -- maybe plan for 5% as a rule of thumb?