Taco Flo-Cheks prevent gravity circulation in forced hot water heating systems during periods when the boiler is hot, but the space heating zone is not wanted. This makes it possible to use the heating boiler to maintain domestic hot water year-round. You can also use Flo-Cheks in systems with multiple circulator zones to allow heat to flow only into the zones that are calling for heat. In addition, the thumb screw can be opened manually for gravity feed applications, if the main circulator stops operating. 125 PSI maximum operating pressure (862 kPa) and 277 deg F maximum operating temperature (136 deg C).