It is possible that the shut-off valve was blocked by the cumulated white stuff. As a result, water supply can not be stopped even when water level is high. % E: A% N8 r" U5 h' U% D$ Z3 d+ C5 M) W k8 o
Solution: turn off water supply and take the humidifier apart and clean it will stop the leaking. * F4 m" Z& g* u6 h1 J$ e3 X& C( \/ q) C* R6 J; p2 O
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Otherwise, find a professional or change a new set.