I experienced the same problem with air moving out of the house while the HVAC was not running. It stopped after I sealed every duct joint that I could get to and sealed up the gaps between vent outlets and the walls/floors/ceilings. I used water clean-up mastic on the duct joints and insulating foam-in-a-can around the vent outlets. The foam is messy, sticky, stuff, so wear disposable gloves. Since you have vents in the garage, check to see if air is coming out of them while the HVAC is off. If so, they would need to be blocked when not needed.
- Gil Laster Charlotte, NC [Non-text portions of this message have been removed]
