Furnace Plenum Size

Hello, I am installing a new furnce 96000 BTU. I need to upgrade some of
the ductwork. I ordered a supply plenum that is 19 x 19 x34inches
tall.(which is the size of the furnace outlet) My blower speed is 1570.
Is this plenum too large? when I plug this into the ductulator my FPM
is only about 600 FPM. I am not sure if I need to transition this
plenum to the proper CFM closer too the blower or if the transition is
fine at the take offs?
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I'd worry about the duct velocities in the individual branches you take off the main plenum. The plenum is supposed to be where everything meets, and you'll never get it "optimum" there.
To save money on tooling and production lines they make the same sized furnace outer cases for about three or four sizes of furnace - the same case might have two, three, four or five burners and heat exchanger chambers in it, and they use steel barn doors on the sides to blank off the airflow in the unused areas. Need more heat and airflow, then you bump to the larger size can.
So a plenum that would be way too big for a two-burner furnace would be pushing too small for a four burner version of the same size.
This design makes sense when the loads are unbalanced - you need 5 Tons of AC because it gets real hot, but it rarely gets below freezing. You need the larger air handler with lots of room and airflow for a 5-ton A-coil, but they can blank off part of the furnace because you never need that much heat.
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Bruce L. Bergman (munged human readable)

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