Ok, so today where I live it was in the mid 20's and I had my Tstat set at 65 for the daytime leave temp.
The upper floor of the house receives direct sun while the first floor gets none so It was 65 on the first floor and 73 on the second and third. So I closed the vents in the office on the thrid floor.
When the heat came on in the evening, no air entered the office. I got on a chair and felt the air at the return vent. There was no air movement. I opened the supply vent then checked again and air was rushing through the return grill and into the return duct.
So you are saying that air will not be forced into the return unless some air is entering the room, and air will not enter the room unless some air can escape. Right? Wrong?