I'd like to add that I have 3 Zoomed UTHs. One on a 10 gallon, 20 gallon, and a 40 gallon. I have them all on rheostats. I have the 40 gallon and 20 gallon on a fish stand (open bottoms) and the 10 gallon on a shelf (not much air flow). With all the rheostats turned down as far as possible the 40 gallon reads 108 degrees and the 10 gallon 104 degrees. I normally just turn those two off. However the 20 gallon tank on the bottom of the fish stand with the rheostat on the lowest setting reads 85 degrees. I think it is cool only because it is close to the wood floor and our floors are very cold!
I have tried to see how hot the UTH would get without being controlled and it was well over 200 degrees when I finally turned it off (no snake in tank of course). So just because it doesn't feel hot to the touch doesn't mean it isn't. Before I got prob thermometers I had no idea my UTH could get that hot. At that time my snake never burrowed and rarely went to the warm side. I now know why.