A method is provided of controlling a pump including a electrical motor coupled to a rotor which carries first and second impellers at opposite ends thereof. The method includes: (a) driving the rotor using the motor, so as to circulate fluid from the first impeller through a first fluid circuit, the second impeller, a second fluid circuit, and back to the first impeller; (b) determining a resistance of the first fluid circuit, based on a first motor parameter; (c) determining a flow rate through the first fluid circuit based on a second motor parameter; and (d) varying at least one operational parameter of the pump so as to maintain a predetermined relationship between the flow rate and the resistance of the first fluid circuit.