Product review (1 stars): didn't stick
I had to return these items.  I tried them on ceiling and wall vents.  Unfortunately they did not begin to stick at all.  I have magnetic vents and couldn't figure out why they wouldn't work.  Very disappointed because I certainly need something like this. I may have to resort to putting packing tape over the vents.  I saw so many other reviews telling me they were disappointed.  I guess I was just hoping.