Product review (5 stars): Excellent external dryer vent
The HEARTLAND 21000 Dryer Vent replaces my old light, aluminum flap sort of encased by an aluminum triangle/box where the flap was just hanging there and opening and closing in the breeze.

While installing the new vent, I was amazed how much cold air was rushing out of the 4-inch hole, and this new vent takes care of that, and no way rodents or bugs can crawl in, too, and the wind won't open it. There's no cold air rushing out of there now.

When the dryer is on, a small inverted plastic cup is blown upwards, and the hot air is directed downward and out the baffles. When the dryer is off, the cup settles down and seals the opening well enough that there's no more cold air escaping, and I bet in the winter, no hot air escapes either, except when the dryer is running, of course.

This thing promises to pay for itself in short order. The biggest problem is making sure it's level, and on slanting clapboard, I glued a couple of popsicle sticks for shims rather than cut away the clapboard.  Then I screwed it down tight and caulked the edges and all is just fine.  It looks cooler, too.  I'll probably paint it when I get around to it, but I think that's optional.