I am replacing my 41" tall, low boy furnace shortly with a Carrier Infinity 80BTU. The Return air duct on ceiling is 10" x 20" fed from the underside by a 12 x 20" X 90 degree turn which offsets sideways from the cold plenum. See attached .This 12 x 20 x 90 degree offset measures a total of 18.5 inches sideways and up 22" to the underside of the duct. The vertical plenum dimension is 19 x 17.25.
The sales rep says he will retain all this cold plenum and offset and simply build a new plenum extension to floor level for the Carrier.
Am I wrong in thinking that a new 2 x 45 degree offset would be a better, more effective transition to the plenum , then down to the floor where it would meet the 4" filter ?