Vent Fan Flapper Leak

Okay, so why was there a faint odor of smoke in the upstairs bathroom last night? Hmmm, Mrs. VF noted the bathroom seemed to be perenially cool. It turned out the little plastic flapper in the vent fan exhaust was out of place and laying in the duct. You could feel the cold air coming into the house.

So the house is under vacuum at that point. If it were pressurized could warm air escape from the vent fan ducts-it doesn't take much to move the flapper.

I can't say I understand this whole vacuum (stack?) effect. If the insert is cold, chances are good there's a downdraft and a cracked open and newspaper needed to get things going the right way up the stack. I have a low power radon fan and have checked its effect on the downdraft when turned off as well as the oil burner and there seems to be none. I looked and found no big leaks (like the attic hatch). It just seems when I open a window the cool air wants to come into the house.

In my old place, I could smell the brbq from out back in the bathroom, due to the same thing happening with the exhaust fan in there. and that flapper worked fine. Never did understand it.