Last week I had my garage painted and I smelled the paint fumes inside the house from one of the air vents. My furnace is in the garage but I assumed these are sealed to keep this from happening? For example, exhaust fumes from a car could get inside the house?
If it is older, more then likely it's not sealed. If it was sealed, they may have used the wrong product to make the seal last. Or they may not have sealed everything they should have. It's simple for a homeowner to seal themselves, buy some aluminum tape (not duct tape) and go through the whole thing sealing all if the ducts or gaps. More then likely i'm guessing the biggest culprit is where the cold air return comes into the furnace. They also make a special caulk for HVAC purposes, but i've never used it so I can't give you any advice on it. But i'm sure that's worth looking into.
the return panel is negative when the fan runs and pulls air from the nearest area...that garage.where is the return to the unit other side of garage door into the house.could be sucking air under the door from that garage