One more expansion tank question, I swear! I just found out my expansion tank is too small. I installed a Watts PLT-12 expansion tank (4.5 gallon tank) a few months ago, and I decided on that one based on what I was told at the plumbing supply store and the Watts sizing chart for the PLT and DETA expansion tanks on their website. The chart says that 40 PSI air pre-charge is standard, and the value 40 PSI is automatically placed on the chart (even though it can be changed). The problem is, all of the PLT tanks have an air pre-charge of 20 psi. Using the 40 PSI value my tank is adequate, but the 20 PSI pre-charge makes my installed tank too small for my system. The only tanks they have with 40 psi pre-charge are the DETA (commercial) series even though the chart says 40 is standard! Instead of my $40 expansion tank, it looks like I need a $150 tank just because of the pre-charge. Why does the pre-charge make such a difference when you just end up adding air to it to equal the incoming PSI from the water supply line? Can anyone help me make sense of this?