What is the problem if your circulator pump for your forced hot water heating system is circulating water when the thermostats aren't calling?

You may have a system that runs the pump(s) until the water temperature has dropped to some value in order to retrieve all the heat from the water before shutdown. That would mean, then, most likely the thermostat that controls this operation is probably defective. Although answer one is not wrong, there is another possibility. Usually a thermostats contacts are not "strong." or heavy enough to carry enough electric current to run the pump. So, a relay is placed in the circuit between the thermostat and the pump. Sometimes the relay can become defective and "freeze" in either the always ON position, or permanently OFF. Therefore, in addition to checking the operation of the thermostat[s], check for, and proper operation of, any relay in the pump circuit.j3h.