A method of heat sanitization of a haemodialysis water circuit, the method comprising the steps of: providing a medical device; the medical device defining the haemodialysis water circuit, including: a tank containing a volume of liquid; a sensor arranged to sense the temperature of the volume of liquid; a heater arranged to heat the volume of liquid from an initial temperature to exceed a threshold temperature, and maintain the volume of liquid above the threshold temperature; and a processor, wherein the processor is configured to perform the following steps: sensing the temperature of the volume of liquid with the sensor; heating the volume of liquid from an initial temperature to exceed a threshold temperature; maintaining the volume of liquid above the threshold temperature; determining a time-temperature value for the volume of liquid periodically once the threshold temperature has been exceeded; calculating a cumulative time-temperature value; and characterized by the processor calculating whether there is sufficient thermal energy contained within the water circuit that the temperature of the volume of liquid will remain above the threshold temperature for long enough to ensure a sanitizing dose is reached; and the processor switching off the heater in advance of a sanitizing dose being reached once there is sufficient thermal energy contained within the water circuit to ensure a sanitizing dose is reached; providing an output signal once the cumulative time-temperature value has reached a level indicative of a sanitizing dose. providing a medical device; the medical device defining the haemodialysis water circuit, including: a tank containing a volume of liquid; a sensor arranged to sense the temperature of the volume of liquid; a heater arranged to heat the volume of liquid from an initial temperature to exceed a threshold temperature, and maintain the volume of liquid above the threshold temperature; and a processor, wherein the processor is configured to perform the following steps: sensing the temperature of the volume of liquid with the sensor; heating the volume of liquid from an initial temperature to exceed a threshold temperature; maintaining the volume of liquid above the threshold temperature; determining a time-temperature value for the volume of liquid periodically once the threshold temperature has been exceeded; calculating a cumulative time-temperature value; and characterized by the processor calculating whether there is sufficient thermal energy contained within the water circuit that the temperature of the volume of liquid will remain above the threshold temperature for long enough to ensure a sanitizing dose is reached; and the processor switching off the heater in advance of a sanitizing dose being reached once there is sufficient thermal energy contained within the water circuit to ensure a sanitizing dose is reached; providing an output signal once the cumulative time-temperature value has reached a level indicative of a sanitizing dose. a tank containing a volume of liquid; a sensor arranged to sense the temperature of the volume of liquid; a heater arranged to heat the volume of liquid from an initial temperature to exceed a threshold temperature, and maintain the volume of liquid above the threshold temperature; and a processor, wherein the processor is configured to perform the following steps: sensing the temperature of the volume of liquid with the sensor; heating the volume of liquid from an initial temperature to exceed a threshold temperature; maintaining the volume of liquid above the threshold temperature; determining a time-temperature value for the volume of liquid periodically once the threshold temperature has been exceeded; calculating a cumulative time-temperature value; and characterized by the processor calculating whether there is sufficient thermal energy contained within the water circuit that the temperature of the volume of liquid will remain above the threshold temperature for long enough to ensure a sanitizing dose is reached; and the processor switching off the heater in advance of a sanitizing dose being reached once there is sufficient thermal energy contained within the water circuit to ensure a sanitizing dose is reached; providing an output signal once the cumulative time-temperature value has reached a level indicative of a sanitizing dose. sensing the temperature of the volume of liquid with the sensor; heating the volume of liquid from an initial temperature to exceed a threshold temperature; maintaining the volume of liquid above the threshold temperature; determining a time-temperature value for the volume of liquid periodically once the threshold temperature has been exceeded; calculating a cumulative time-temperature value; and characterized by the processor calculating whether there is sufficient thermal energy contained within the water circuit that the temperature of the volume of liquid will remain above the threshold temperature for long enough to ensure a sanitizing dose is reached; and the processor switching off the heater in advance of a sanitizing dose being reached once there is sufficient thermal energy contained within the water circuit to ensure a sanitizing dose is reached; providing an output signal once the cumulative time-temperature value has reached a level indicative of a sanitizing dose.