Steve and maybe JayJ, the flapper valve I am thinking of is not the one that goes on the outside of the house. I have an outside the house one and tld said that he had one too. The one that I am thinking of and have tried before goes in the line between the back of the dryer and the outlet with flaps that goes outside the house. The idea of the valve I am suggesting is to divert hot air from the dryer into the basement so you don't lose the heat outside. The problem when used this way is that the air is so moist that you can have humidity problems in the house.
I do have one connected in a hose from outside the house to the cold air plentum of the furnace. In the winter, outside air is usually less humid than the inside air. Opening the valve when heating season starts and closing it in the spring has cut down drastically on condensation on the inside of the windows during the winter.