I have a Pentair Minimax CH NG millivolt heater. The pilot lights fine as well as the burners. The issue is that once the water temp gets to the target the heater will not shut off. Turning the thermostat down (POT dial) will not turn it off nor will turning the switch to the off position. The pressure valve will turn it off if I shut off the pump. I pulled the PP and TH/PP off of the back of the thermostat board and it did shut off. Since removing and reconnecting those two wires to the board the POT and on/off switch are now able to turn of the burner.

Any idea what might be the problem? From what I can tell this heater is using a fairly standard and basic Honeywell thermostat circuit board, POTs and valve. From the diagram in the manual it almost looks like the problem may be with thermostat board itself (p/n 471677) as the pressure switch is part of the main circuit but the POTs and on/off switch are attached to the board via different connectors. The thermostat board was replaced last year due to continuous short cycling of the heater.

Does anyone have a link to some documentation on millivolt systems that I can read to understand how these things work, i.e. what TH, TH/PP and PP mean and how to troubleshoot?