I can feel the tiniest amount of heat, I'd ask "is this thing on?" then I'd press my palm on it and be like "Oh yeah, it's on." Anyway, so far my hotspot, with the probe foil taped to the floor of the tub on pure plastic w/ no substrate is 92.3F. It's 10:26PM where I live, so I'll go to bed, and check what the temperature peaked at in the morning, if it's less than 92.6F then I know it's all good and I will never need to worry about it being too hot. At least I hope so.