The following is my opinion:
Human heat pads as under tank heaters are a high, repeat HIGH, potential fire hazard. They are not intended to be used for long periods, nor are they intended to have heavy objects placed on top of them, nor are they meant to be glued to a tank.
You would be best off investing in a thermostat to control the temps; however a decent rheostat is a good alternative.
Your problem and Terry's, with bad rheostats, and problem UTHs have me wondering if you are doing something wrong. The odds of a heater alone malfunctioning and getting too hot are one thing, and that is fair to high against it happening if the set up of the heater is correct. The odds that both a heater and a rheostat are malfunctioning at the same time for one particular person (and that this has happened more than once) are something much higher against both happening at once. While I know that the UTHs can overheat, and mine have done so, I have never invested in a rheostat or thermostat - but I no longer have a problem. All it took was to figure out what was wrong. One heat pad I had simply malfunctioned. Others did not malfunction but often would overheat for one or a combination of three reasons. All of thesde reasons were errors on my part. Either I had a heat lamp above the same area under which the UTH was located or I did not have enough ventilation under the tank for the UTH (or both) and/or there was too much or improper substrate atop the UTH. Could it be any of these alone, or in any combination, with your UTH?
Many people, as I did, put the UTH and a heat lamp at the same end of the tank. Many a UTH (I think Zoo Med uses this temp. range) is designed to heat 5-10 degrees hotter than the ambient temp (around it). If a heat lamp is already heating that area, then the under tank heater gets hotter than it should because it is trying to make it 5-10 degrees hotter than the ehat lamp is already making it. If you combine this with improper ventilation, then it gets even hotter (improper ventilation by itself is enough to cause the problem). I find that raising the tank at least 1/4 to 1/2 an inch at the UTH end helps, as does moving the heat lamp more toward the center of the tank. Another thing that helps is reducing the amount of insulation above the heater, or in other words making sure you have the proper substrate and the proper amount of it. Too much substrate will keep in too much heat causing the unit to over heat. An improper sunstrate that does not properly conduct heat will also keep in too much heat. What type of tiles are you using and how thick are they. Some tiles are made with fire and or heat resistant material and do not conduct heat at all well. These probably should be avoided above a UTH.
If you want to be on the safe side, do not use a UTH at all but use a ceramic heat emitter from above. One of the proper wattage will provide a suitable basking area, and it can remain on all day since it does not emit visible light at least for humans. (I don't think any light possibly emitted would be visible to geckos either). These are more than sufficient to provide enough heat for a Leopard Gecko to avoid the dreaded 'lack of belly heat syndrome' that I often hear herpers worry about. If you are truly worried about a 'belly' heat issue, I assure the ceramic heat emitter will emit enough heat to warm a basking stone or tile placed in the area directly under the unit on the tank substrate that will allow the stored heat to be taken up into the belly of the beast. Heating from above is a fully suitable method to heat a gecko because any heat radiating down will also be conducted throughout the body of a basking lizard so long as you use a sufficient wattage emitter.
Just my opinion, hope it is helpful.
Best of luck
Glenn B
