I was able to test my five-gallon Igloo tanks with the small heating pad over the Bright Tap nipples at ten below zero and zero during the day as a brutal cold front went through my area. They remained unfrozen and did not require thawing.
I doubt if there is such a thing. Many years ago, I went to the nipple system and have loved it. But in winter, I tried every thing I could think of to keep the nipples from freezing.
One day while in the bandaid corner of the Walmart pharmacy, I noticed the heating pads. Right before my eyes...