Hi, I'm having issues with my UTH and rheostat.

The problem is, even when the thermostat is turned all the way down to the lowest setting, the temperature on the surface of the substrate goes up to like 115-120 degrees. The only way I can remedy this is by stacking the substrate higher, up to about 2 and a half inches instead of 1 inch. I don't like doing this because when Molly scoots around in there, the substrate gets moved.

This has been happening ever since i purchased the UTH and rheostat, right out of the box. The UTH is ZooMed and the rheostat is a ZooMed ReptiTemp RT-10. I am using ZooMed thermometers with probes. Could the rheostat be defective? Am I doing something wrong?