I’m adding an ERV to an existing 2 story home in zone 2 with separate HVAC systems, one per floor. The home tested 2.1 ACH50, is about 10 years old and is spray-foam with a conditioned attic. I’m planning dedicated supply and return ducting for the ERV with a supply to each bedroom, including 2 upstairs bedrooms. I’m not sure if the upstairs needs a dedicated return to balance the supplies to the bedrooms, or if it’s OK to place all the returns downstairs. There’s a large switchback staircase connecting the floors with unrestricted airflow between them. The upstairs bedrooms have jump ducts to an open upstairs room directly above the staircase and the downstairs returns have unimpeded airflow to the staircase via an open room. Is is safe to assume 10s of CFM of upstairs supply can flow down the staircase to be picked up by a downstairs returns, or will it be fighting some stack effect / draft and I’d be better off with a return upstairs? There aren’t any point sources of bad air quality upstairs – it’s just two bedrooms, a jack & jill bathroom setup with an exhaust fan, and a play room.