The pump has to be able to generate more pressure than the boiler operates or no water will enter it. Water temperature entering the pump is critical to know. Some pumps do not work well when receiving heated water, others do. At Reimers they use high quality pumps and motors that are oversized compared to the demands. In some applications multiple pumps may be used to transfer water from a single tank. Reimers offers pumps up to 600 volts and pressures to over 150 psi.