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 boiler aquastat however there are various levels of control that can be found that control circulator pumps. One way is by the boiler 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.

It is important when the circulator pump is installed that the proper size of pump is selected for the amount of flow needed for the loop. Oversized pumps can cause issues with the flow and cause efficiency issues with the system. Too much velocity causes laminar flow issues which makes the loop not heat properly and the boiler to fire unnecessarily. It can be a big difference in fuel consumption of the boiler. The circulator pump should be sized by an HVAC professional familiar with sizing circulator pumps for boiler loops and boiler systems.