aquatuner Is aquatuner really cooling by 14 deg?

[Beowulfe]

In regards to the issue in the original post - From what I can see, if the water from your overflow (when the AT is turned off) backs up (which it looks like it might be in the screenshot?), water that was supposed to bypass it and has already passed the temperature sensor can re-enter the AT input pipe when it turns back on. Add a bridge immediately after the AT tuner output, and potentially add an emergency overflow output somewhere on the line.