I have a large 3-story home in the northeast that has all three floors completely finished. We suffer from the classic balancing issues many people post about i.e. not enough cool air upstairs in the summer (major stack effect), in the winter hot and cold rooms, etc.
Background: I've always had a Ecobee thermostat with ~6 room sensors placed in those challenging parts of the house. Using the Ecobee sensors in the past I've been able to balance per say my house just using the supply register adjustments but that really didn't solve any underlying problems. The other detail is I know prior to buying my house it's HVAC system was built for the upper two stories and the finished basement was quickly added to help sell the house so those supplies and returns may not have been part of the original design, or the whole system re-balanced.
I purchased a vane anemometer and followed the steps outlined here: https://www.achrnews.com/articles/137148-duct-dynasty-estimate-room-airflow-in-six-steps
Therefore I derived the expected airflow in CFM per sq/ft, and determined amounts for all my rooms. I mapped all of my supplies and returns and organized them in order of farthest to closest. I collected all of the K-factors for the various grills in my home, then measured every supply and return to get an estimate of performance. My initial finding is my supplies appear to add up to close to the furnace airflow (1350 CFM in stage 1 heat) while my returns are woefully below that at 830 CFM. I went ahead and tried some tweaking of dampers and register adjustments but haven't completely re-measured everything yet.
My question: Can I partially block returns to redirect return pull onto those 2nd floor returns that have very little CFM throughput?
I believe the large returns are all joist/stud based not using ducting but just sheet metal to the main trunk. Because the basement is entirely finished in sheetrock I have very limited access to dampers or returns - in particular the main 2nd floor return is way out of reach. I don't think a solution exists to correct the stud based return that isn't major but that's why I am posting for ideas and optimization thoughts.
Here is a google sheet with all of the measurements - not perfectly formatted. https://docs.google.com/spreadsheets/d/1qoN4PLykoFYhsCJIBfqgW9__MnkvvJ9_9i99p4FjxgY/edit?usp=sharing