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. $ e1 J3 \& N; m4 N# v* B
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Solution: turn off water supply and take the humidifier apart and clean it will stop the leaking. 5 ^; z8 q4 U7 ^- o/ K& ]1 z6 H/ \7 s) ?
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Otherwise, find a professional or change a new set.