I have an 6 year old Waterfurnace Envision 4 ton open loop. When in cooling mode, works fine. When in heating mode, it is faulting out with Low Flow warning after running for just a few minutes. Checked temperature of line where thermistor is attached and found that it faults out just below 50 degrees. If I throw the dip switch to lower the freeze protection sensor from 30 degrees to 15 degrees, it does not fault out. Water flow checks out at the expected 6gpm and 49 degrees exiting temperature, and the line temperature where the thermistor attaches never appears to go below 44 degrees. So I assume the thermistor needs replaced, but I also note that the exposed line where the thermistor clips on is frosting up anywhere the insulation is peeled back. My question is, can frost on the line where the thermistor clips be considered normal? As mentioned above, I am not seeing temperature readings on this line going below 44 degrees or so and the exiting water temperature is around 49 degrees. I used both a non-contact automotive type thermometer and a standard probe thermometer to measure the temps.