Hello. I'm adding some duct work as part of a kitchen remodel and have an interference from a 6" round duct feeding a bedroom above the kitchen. I would like to change it to an oval duct (6" nominal?). Based on my calcs, the cross sectional area of a 6" oval duct is less than 6" round. However, the 6" round is actually coming off of an oval duct in the stack from the basement. My logic tells me that since the stack has a smaller cross sectional area, the round duct does nothing to increase airflow and can be replaced with an oval without compromising the flow. Basically, the entire run would be 6" oval. Does my logic make sense or is there more to it? Thanks!!