I am replacing a 50+ year old Natural Gas forced air furnace with a new 90+ efficiency "Hamilton" single stage forced air furnace model # WUN-100-4C. Both old and new furnaces are 100K BTU.
The new furnace outlet is the same size as the old, (19 3/4w x 20d) as is the plenum. The installation manual for the new furnace specifies a minimum return total size of 360 sq. in. and a minimum supply of 280 sq. in.
There are currently 5 ea. 6" round ducts from the plenum and 5 ea. 6" round ducts used for return air.
5 ea. 6" round ducts = 141 sq. in. Will I have any issues with this duct arrangement even though the old furnace used this duct arrangement for many years? If so, what can I do to resolve the issue/s?