franco b wrote:Keeping more heat in the chimney has got to make a difference and a smaller outlet at the top should do that. You could try that with a piece of sheet metal with a 6 inch hole, or use a chimney pot that will also restrict the opening. Too much cold air can sink into that big opening.
If anything, its worth a try... Just do it when your home to monitor the draft and give the chimney time to adapt to a smaller opening at the top. I know it seems to not make sense, but if cold air is sinking in because the opening is too big, it may help..