Boiler Circulator PumpBoiler Circulator Pump - The boiler circulator pump induces the flow of hot water through the boiler and the boiler loop. Usually the boiler circulator pump is controlled by the aquastat however there are various levels of control that can be found that control pumps. One way is by the aquastat as mentioned above. Another way is by an end switch in a zone valve. A thermostat in a particular zone will call for heat and the thermostat calls for the zone valve to open. When the zone valve is fully open an end switch closes the circuit to energize the circulator pump. Typically, the circuit is a control circuit so when the end switch closes it closes the circuit for a relay to energize the circulator pump