A humidifier configured to determine when the humidifier requires cleaning. The humidifier includes a tank for containing water, a heater for heating the water in the tank to generate steam, and one or more water level sensors for detecting the level of water in the tank, including detecting water at first level and a second level, where the first level is lower than the second level. The humidifier further includes a drain valve for draining water from the tank and a controller. The controller is configured to open the drain valve to drain water from the tank, measure a time interval required for the water to drain from the second level to the first level, and compare the time interval against a threshold value. If the time interval exceeds the threshold value, then the controller is configured to provide an indication to clean the humidifier. Methods are also disclosed.