cabinet ventalation

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i am gonna be useing a 250 mh conversion and hps kit in a 3 Lx 2 1/2D x 5 H cabinet.im gonna be using a activeair blower for a exaust fan that pulls about 170cfm.Im not really sure if i should leave the exaust on 24hr a day or connect it to my dual timer.If on the timer it will be only on when the light is on which is 18hrs for veg and 12 for flower.My concern is that when the light is off along with the exaust will humidity build up beyond the 70% mark at night.But if im to leave it on 24hrs will the humidity and temps drop too low,kinda a tuff choice to make.

P.s.
I am still thinking about getting a t5 and replaceing the 250w mainly because of the less heat issue and less ventalation.What do yall think?