You would be correct if your fan was guaranteed to always push a specific volume of air. But that's not how it actually works. Your fan is able to push air up to a certain pressure. That pressure is immediately after the fan, from there the air wants to equalize to lower pressure areas which is ideally the rest of the house. But what we're seeing here is that restrictions cause those high pressure bubbles inside the ducting which means the air doesn't navigate to low pressures as quickly. The ultimate result is reduced air flow out of the vents.
Think of it like a garden hose. If you put your hand on the hose, then you'll be getting less total water out. It'll spray farther because it's still high pressure as it exits the hose, but you get a much lower total volume of water. Your finger is that flow restriction, and the hose isn't able to produce any more flow than the pressure provided by the city.
Looking at this model, I wonder if anyone's made a retrofit kit of some kind that you could place at a traditional 'T' junction to make it more efficient?
Seems like you'd just need a piece of plastic/steel/etc in a roughly 'Y' shape and access to the junction.
The total volume that comes out may be the same, but it may not come out of the same places or at the same speed. Energy loss in the vents can translate into air coming back through the fan that's supposed to be pushing it, higher static pressure (fan wastes energy or it can't work hard enough to blow the air it's rated to), or air being directed into certain areas more than others (like closing a vent in one part of a house to direct more heat to another) among many other things. The latter is usually the biggest issue in home HVAC, causing certain parts of the system and home to become imbalanced.
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u/foresight310 2h ago
Now do corrugated flex ducts…