I have a heating duct running from the basement to the 2nd floor through a kitchen wall. We have since knocked down the kitchen wall and we need to reroute the duct. The original duct is rectangular 3" x 10". We now need to run the new duct along the basement ceiling and up the outside of an existing wall to the 2nd floor bedroom where the heat is needed. To run it in this path we will need 4 elbows and are thinking about using a 5-6" circular duct to replace the rectangular one (actually to tap into the rectangular one in the basement) once completed we will box the new duct in. can we use a 5-6" duct instead of the oldschool rectangular one or will we be minimizing the airflow too much

The round duct will only give you about 20 sq. inches of volume and the elbowes each knock down the air flow. Maybe two round duct but not the one you described.