Bought this from Duncraft a few years ago, and it worked very well for about 2.5 years, and then quit working. My theory is that it simply wore out during a cold spell. When it did work, it had no trouble keeping the water unfrozen. Our bird bath is a low pedestal type, with a typical diameter and depth, made of a stone material (cement?). I am replacing it with the same one, but not sure I will again if it doesn't last 5 years this time. Birds were always there, every day, and all varieties of winter birds. Plus the chipmunks, of course. :-)