A method for heating medical fluid with different supply voltages comprising: configuring first, second and third switches in electrical communication with first and second power lines, the switches configured such that (i) in a first switch state first and second heater elements are placed in series, solely via a closing of the first switch, for operation with a first supply voltage applied to the first and second power lines, and (ii) in a second switch state the first and second heater elements are placed in parallel, via the second and third switches, for operation with a second supply voltage applied to the first and second power lines, and configuring a control element to automatically set the switches in the first or the second switch state based upon a signal sent to the control element indicative of the first or the second supply voltage.