Most hydronic systems stop water flowing through their distribution system, or portions of multiple zone systems, when the thermostat(s) stop calling for heat. It’s a simple approach but not the only way to limit heat output. It also has an Achilles heel.
Consider this scenario: The thermostat in a light commercial building with hydronic baseboard heating is normally set for 68° F. When everyone leaves at 5 p.m., the boss sets the thermostat to its lowest setting, say 50°. He does this because of a mistaken belief that the lower he sets the thermostat the more money he’s going to save by not burning fuel overnight. Water flowing in the system stops the moment the thermostat is set back.