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Sure, you could exhaust a jet engine through a 4" pipe but the problem is the velocity of the air. As the size decreases the velocity increases for any given cubic feet per minute rate of exhaust. So, what you have to do is calculate the velocity of the air through a 4" pipe given the CFM of the fan. And then will that velocity cause an objectionable ...


You want a gravity flap or back-draught/draft flap if it's a horizontal duct. For a vertical flue you'll probably need a right-angle bend, rotating downwind type, and a draft flap. (Letting it swing downwind might be enough on its own.)


Your fan is equipped with a backflow prevention flap. Check that it's installed correctly and not gunked up with so much lint that it doesn't function. It's usually a simple task to remove the cover, unplug the fan and light, and drop the motor panel. This usually doesn't require work on the wiring connections. From there you have open access to the flap.

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