A method for evaluating a condition of a water purification system (40, 110) comprising a water purification apparatus arranged to output purified water through an outlet port and through at least one sterilizing grade filter (70a, 70b) of a line set fluidly connected to the outlet port, the method comprises: • monitoring a flow rate correlated with the flow rate of the purified water flowing through the at least one sterilizing grade filter (70a, 70b); • monitoring a pressure correlated with the pressure of the purified water flowing to the at least one sterilizing grade filter (70a, 70b); and • evaluating a condition of the water purification system based on whether a relationship between the monitored flow rate and the monitored pressure fulfils one or more system dysfunction criteria including: a) the relationship being a ratio of monitored flow rate to monitored pressure, wherein the one or more system dysfunction criteria comprise that the ratio exceeds an upper ratio limit value indicating the condition that the at least one sterilizing grade filter leaks, or goes below a lower ratio limit value indicating the condition that the at least one sterilizing grade filter (70a, 70b) is clogged and that the bioburden of the water purification apparatus is too large, or b) determining a slope value of a curve of a function of a ratio of monitored flow rate to monitored pressure versus time, wherein the one or more system dysfunction criteria comprise that the slope value goes beyond one or more slope limit values; • activating an alarm upon the relationship fulfils the one or more system dysfunction criteria; and • stopping outputting purified water through the outlet port upon the relationship fulfils the one or more system dysfunction criteria. • monitoring a flow rate correlated with the flow rate of the purified water flowing through the at least one sterilizing grade filter (70a, 70b); • monitoring a pressure correlated with the pressure of the purified water flowing to the at least one sterilizing grade filter (70a, 70b); and • evaluating a condition of the water purification system based on whether a relationship between the monitored flow rate and the monitored pressure fulfils one or more system dysfunction criteria including: a) the relationship being a ratio of monitored flow rate to monitored pressure, wherein the one or more system dysfunction criteria comprise that the ratio exceeds an upper ratio limit value indicating the condition that the at least one sterilizing grade filter leaks, or goes below a lower ratio limit value indicating the condition that the at least one sterilizing grade filter (70a, 70b) is clogged and that the bioburden of the water purification apparatus is too large, or b) determining a slope value of a curve of a function of a ratio of monitored flow rate to monitored pressure versus time, wherein the one or more system dysfunction criteria comprise that the slope value goes beyond one or more slope limit values; a) the relationship being a ratio of monitored flow rate to monitored pressure, wherein the one or more system dysfunction criteria comprise that the ratio exceeds an upper ratio limit value indicating the condition that the at least one sterilizing grade filter leaks, or goes below a lower ratio limit value indicating the condition that the at least one sterilizing grade filter (70a, 70b) is clogged and that the bioburden of the water purification apparatus is too large, or b) determining a slope value of a curve of a function of a ratio of monitored flow rate to monitored pressure versus time, wherein the one or more system dysfunction criteria comprise that the slope value goes beyond one or more slope limit values; • activating an alarm upon the relationship fulfils the one or more system dysfunction criteria; and • stopping outputting purified water through the outlet port upon the relationship fulfils the one or more system dysfunction criteria.