So, my under-house flex ducting system has failed. The previous designer used fiberglass board to make three plenum boxes (one at supply end of AC and the other two as splitters off the trunkline). As I reconstruct this mess (in a very tight and uncomfortable space), it occurs to me that the use of sheet metal T’s and Y’s could easily replace these plenum boxes. But, since I’m no expert, I’m wondering: Does the use of a plenum box and the associated expansion of the pressurized air help the system where a Y or T would not???
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Air has volume and a proper duct design allows flow of the conditioned air to pass through and into the house to pass this air to where you want it to be.
https://www.google.com/search?q=hvac+duct+design
If you hang too many y and t fittings inline, the flow will not match, and risk is that some rooms above will get too much air, and others too little.