What flow is needed for a boiler to maintain operation. The common method is have the pump resupply water to the vessel about 4 times the rate it leaves as steam. When this occurs a signal to start the pump is triggered from water level drop, and a signal is sent to stop the pump when it has been restored to normal level. The pump must be capable of receiving adequate water supply to generate a steady flow outward. It also must be at a psi greater than the vessel psi resisting it. The pumps receive water that is room temperature or from a condensate tank often about 180F. Make sure the pump can tolerate the temperature. The less a pump has to suck is better, a head pressure into the pump helps prevent boiling in the pump head and helps increase the flow capabilities.