Got my new expansion tank installed yesterday and everything looked good. Pressure was around 15psi at 190 degrees. Went to check it before going to bed and the pressure gauge was over 40 psi! I drained probably 20 gallons of water out of the system to get it back down to a normal range. Right now i have the ball valve going to the auto fill valve shut off and everything is looking good when i got up this morning. Anyone ever have an autofill valve stick open like this? Do i even need to have an autofill valve in the system? This wouldn't have anything to do with me installing an over sized expansion tank would it? I have a sx160v installed with only 165 gallons of water in the system. I went oversized so that i could add storage later. Any opinion would be appreciated. Thanks, Kevin.