I was having a problem where my fan was drying out my plants. The bottom leaves (the only leaves my fan was still blowing on as the plant got pretty tall) would turn yellow, dry up, and die. I checked my temps (day and night), my pH (and runoff), made sure I wasn't overdoing the ferts, and I was going crazy because nothing I did made a difference. One day I moved the fan to get in the back of the room and forgot to put it back...the problem immediately stopped.