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Hello - i use a 1000w light in a space about the same as yours. Rule of thumb is to get a fan that can expel the air out of your grow room about 3 times a minute (once is adequate but about 3 is ideal). so 4.5ftx2ftx6ft = 54 cubic feet so in order to expel air from that space 3 times a minute you need a fan rated at 150 CFM or above (54 x 3)
Definitely use two fans - one for exhaust and the other to cool your light BUT get a reflector that has duct attachments and a glass cover so you can vent all the hot air directly out instead of blowing it around inside your box.
two more things -
don't forget to add some passive intake holes to avoid a vacuum
don't forget to add an oscillating fan or two to increase air movement and help strengthen the stems
PS - i use a 170 CFM fan to exhaust my light and a 440 CFM fan to exhaust my cab - vortex style - not inline
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