Thermostat Check Valve 12 male Assy. For those who want to utilize our in-line thermostat and still have ability to use a heater or cool-down circulator, this device is the all-in-one Thermo-Check Valve. When the cold water from the circulator hits the thermostat element, it opens, so if your water heater setting is above your thermostat temperature rating, the valve will become ineffective. Our dual -12 Thermo-Check Valve makes plumbing Ford cylinder head single lines a breeze!