Hello Jack,
I had dual 4" muffin fans on the old canopy vented to outside, while it did work ok... there is a serious drop off in efficiency if you are using the fans to "pull or push ", they are next to usless for that purpose.
For an example, the dual fans after a run of about 8', could barley hold a 5 dollar bill on the inside of my canopy intake.
So, i have arranged for a 4" inline blower fan for the new canopy, they are a bit pricey ($180 for a variable speed and 120 cfm) but are much better suited for that purpose.
They also are made to be just slid in to your ducting and hose clamped.
Most hydroponic outfits will carry them.
Do a google search for centrifugal inline fans.
HTH,Marc.
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