Boiler Circulator Pump – Hydronic Components
Boiler 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 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 (more…)
Boiler Components Controls - All hot water boilers must have an expansion tank. Water expands when it is heated. There must be a place for it to expand so that the pressure in the boiler loop remains constant. Without a 
Residential Hot Water Boiler Control - The average residential/light commercial boiler typically uses different controls to sequence the boiler. Boiler control in commercial and industrial applications can be very complex and can have added layers of controls outside of the local boiler controls that come from the manufacturer.