Main advantage of the inline exhaust fan is the reduction in noise. Can you opt to run both, the inline and the internal? certainly. Will there be a benefit to running both? I highly doubt it. The limiting factor will be the volume capacity of the exhaust duct; you can only move so much air through the hose. Most (from what I have seen), use the 6” inline fan, which already exceeds the air volume capacity of a 4” duct. In other words, the volume of a 4” duct is already maximized.
If you don’t care about the noise of the internal fan, why bother with the expense and time to install an external inline fan? If your only goal is to maximize the exhaust volume, I can’t see how an external fan will really help. If your goal is noise reduction, then an external inline exhaust fan is the answer.
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