The way they test fans is the same way they test any consumer product.
Under the conditions that will allow it to perform best.

It is a fair basis for comparison. As well flow is reduced when ever you do anything. If you bend the tube at 90 degrees: flow reduces by 60%

There are ways to keep for flow up, big ducts help a lot. It’s an exponential curve. 8inch is Way probably twice as good as 6”
harris7 Reviewed by harris7 on . CFM backpressure i was reading this on another thread fans are tested without ducting which give them the cfm. The thread also mention that if you use ducting, it will cut the cfm by half. No one on that thread asked if it will cut more than half if ducting and carbon filter is use. It got me thinking since i just upgraded and i need a fan that will cool 2000watt in a 64 cubic feet room. Any input on this would be great. thanks in advance Rating: 5