The weather was finally warm enough that I could pull a garden hose over from the house to re-pressurize my system after replacing my undersized (Amtrol #15) expansion tank with an appropriately sized #30.

The difference was clear on the first sunny day.  Instead of steadily climbing pressure as the tank heats up - the expansion tank absorbs the pressure by compressing the air inside.  Typically I would have 140-150F solar heated water in the 80 gallon tank by the end of the day, and the pressure would be at 100psi.  Now the pressure just inches up from normal 45psi to maybe 55psi.  So this is how it was supposed to be!

Here I am installing the larger Amtrol #30 expansion tank:

NOTE!  I have since learned that the tank should be installed BELOW the pipe on the supply side of the heating system.  Trapped air in combination with very hot water can corrode the inside of the tank.  My setup preclues re-orienting the tank, but if you are designing a system, be sure to orient the tank correctly.  The Amtrol Extrol web site has instruction details for installation available.

To learn more about this issue of over pressure with my system click here to jump back to last February 20.

December 2010 update:  The new tank failed - probably due to the orientation issue, so I'm trying a different style pressure tank made by Heliodyne.