Check the listing for details. Pentair Thermal Regulator #77707-0010 Replacement by Swimables. Condition: New. Listed at 58.99 USD. How to Test: If you suspect the thermal regulator to be faulty simply place the thermal regulator in water heated to at least 130 degrees Fahrenheit. The regulator should open.Compatible With: Pentair Mastertemp 400 parts, SR333NA, Mastertemp 300, SR200NA, Mastertemp 200 and Pentair Mastertemp 125 Low Nox Pool Heater. This includes all HD heaters as well.How to Fix: If the thermal regulator does not open during the testing stage, replace the thermal regulator. If it is found that the thermal regulator IS working then proceed to Internal BypassLocation: The thermal regulator is located between the inlet and outlet water connections on the water manifold. This is true for all Pentair Mastertemp partsIncludes: This kit for the Pentair thermal regulator max valve includes a clip, spring, thermal regulator and plug. Everything you would need to replace the thermal regulator