Humidifier replacement cylinder for HM750A1000/U Humidifier System. The yellow cylinder LED will illuminate when the humidifier has determined that the cylinder is nearing the end of its life, due to scale accumulation on the electrodes. The cylinder LED will flash on and off, during which time the humidifier will continue to operate as normal. After 7 days of flashing, and if the cylinder is not replaced, the humidifier will shut down and the cylinder LED will be on constantly (no flashing). At this point, the cylinder must be replaced. The steam cylinder is disposable and must be replaced at end of cylinder life. Cylinder life is dependent on water supply conditions and humidifier use. Always clean the drain valve before installing a new cylinder. Scale from the spent cylinder may have fallen into the drain valve and could prevent its proper operation. To properly clean the drain valve, it must be removed and disassembled.