Originally Posted by bud16415
I have always just used the ones with the butterfly flapper valve that fit in the line. Do you have one that’s not working or just a straight shot?
Yeah, the valve works on gravity.
Use a candle flame to tell if the cold air is actually moving in under pressure or only sinking due to the stack effect.
If a negative pressure in your house, or winds, prevent yours from working to your satisfaction you could go with a 24vac motorized damper.