Dr. Del said it. That's the reason you don't want the whole floor of the cage covered by heat sources. If one, or heavens forbid BOTH, goes haywire your snake is toast. When you have an area in the center unaffected by heat, the snake can retreat there to avoid the worst of it.

If you do use that large one in winter, make sure you have a good quality reliable t-stat on it to keep it at the cool side levels. Even though I use ZooMed stats regularly, I would NOT trust one, or any of the cheap stats, to control something like that.