Kept telling myself that cracked door thing made sense until I actually tried it for a week in August and my whole bathroom turned into a swamp. Dave sounds like a guy who knows his trade but forgets half of what he's telling you before it leaves his mouth, and now you're stuck with a peeled strip of paint as proof. Here's what gets me though, if the fan was really pulling too much conditioned air out of the house, wouldn't the fix be a smaller fan or a timer switch instead of just killing the vent? Seems like the real problem is your bathroom fan is sized wrong for the room or hooked up to run way longer than it needs to. And the $18 you saved in August, was that mostly from running the fan again or from the weather cooling off a bit compared to July? I want to know if you ever called Dave back out to make him fix that paint he caused with his half finished advice.