3-Story House Balancing Questions - Duct Traverse done with data

Question

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

Answer 1

Yes you can add, remove, or partially block returns as part of a solution to improve temperature balance. Beware of certain potential problems:

• Air handlers have constraints (both high and low) for total air flow and intake and output pressure. Blocking returns will reduce flow and increase negative intake pressure. Very recent systems might alarm problems but otherwise you need to keep things within bounds.
• Opening or closing interior doors can have a big impact on flow and on your measurements. Take your measurements when doors are in their "standard" position ... the way they are usually left while the system is running. And/or use doors strategically as part or your system: Open them, close them, or shave them to allow better flow when closed.
• Reducing downstairs intake and increasing upstairs may create undesired air flows. It may, for example, promote air flow from the kitchen to the bedrooms.

I think your approach with measurements and spreadsheets is probably too scientific for the purpose of creating comfort. There are too many factors and the overall system is too complex to rely on that. I think trial and error is probably easier. Do use measurements however to ensure things are within acceptable limits for your air handler.