I'm sorry are you talking 31 feet or 31 inches?

It is big...the fan rather....

I use my fan OUTSIDE the grow area and suck the air out as opposed to pushing the air out. It wont fir in my 31 X 18 (inches) box.

You could easily suck out the air in your box and blow it into the exhaust as you planned. Both intake and outtake are equipped for hoses or tubes. Just place the fan between the two and viola !

If this is in the attic, you;ll never hear it unless you're in the attic. Amazinngly quiet yet efficient. No more noise than most indoor fan, in fact probably less...

Place exhaust port on your growbox.....attach say a 4" diameter dryer vent to it and run it to the sucking side of the fan.....on the other side or the exhaust port of the fan, run another flex hose to your pipe Y connection.

Just hang the fan from the ceiling or let it sit on the rafters or gables.

WOrks great and dont build a grow room without it. Not only does it remove heat, provide strong flow of incoming air and will blow the smell out