5 Ton water to water heat pump with buffer tank. Open loop. 12 GPM on source side. Been in operation for 2 years before any issues. Unit will run for about 25 minutes with high side pressure slowly creeping up until it hits 530 psi and trips the high limit switch. Low side pressure stays steady at 100 psi throughout the entire run time. Temp difference on source side of 7 degrees. Temp difference on hydronic side of 10 degrees. All radiant zones come off of buffer tank, buffer tank set at 100 set point, so temp going back into Geo is 90 degrees. Flow meter on source side shows no signs of scaling whatsoever as it is as clear as the day it was put in. Im not a refrigerant tech, but shouldnt that low side pressure climb along with the high side? Anybody have any thoughts?