How much cold can the heat tape waterer handle?

We enjoyed an abnormally warm December, so our heated chicken waterer didn't get much of a test.  Then came January's arctic blast.

The first night of the cold spell, temperatures plummeted to 16 degrees Fahrenheit.  When I went out to check on the flock, one of the nipples had frozen up, but the other was still flowing.  The thawed nipple was on the side of the bucket facing the roosting area, so it's possible that the chickens' body heat was enough to warm that area very slightly.  Alternatively, the heat tape might have simply been closer to the bottom of the bucket on that side.

That day, it never got anywhere near freezing, with temperatures hovering in the high teens to low twenties.  The second night, we got down to about 10 degrees Fahrenheit, which was far too low for the heat tape waterer.  Not only were both nipples frozen, there was a skim of ice two inches thick on top of the bucket.

We've got plenty more options to making the heated waterer stay warmer --- we were just waiting to see how this simplest option fared in cold weather.  A longer section of heat tape might help since it could wrap all the way around the bucket (as in Lu Ann's protype), and the bucket could definitely use an insulative sleeve.  We could even add a heat lamp facing the nipples, preferably one plugged into a thermocube so it only came on when temperatures approached freezing.

Or we might just leave the waterer the way it is --- there aren't many nights in our neck of the woods that drop below 16!