Kair Ducting Reducer 150mm to 100mm / 6 to 4 inch Duct Pipe Reduction Connector
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Gurji, that’s what what I got also. I did calculate it after making the statement. In basic duct sizing 4 to 6 is double and 6 to 8 is double, but pi r squared is right. But I only do HVAC part time.
Cutting the area in half may speed the air up in the choke point but fans work by moving air--once the resistance increases the blades start “slipping”. I don’t remember the tech term but it’s like cavitation on a boat prop; you can put all kinds of power in but the boat goes slower until the blades bite and push the water air does similar. You can put a tachometer on the fan and watch it speed up but it is moving less air. This why duct sizing/flow is critical to set up a system.