Thank you for the info....now that i've serviced all the rooms with 5" ducts, I'm in the need to know for the "EXTENDED PLENUM" size for the hot air dist and cold air return that are attached to the furnace itself? The house is approx. 1500 sq ft and i'm installing a Carrier MCA40 high eff. furnace. once again thank-you very much for the advice......"MAC"