Hot tub overheating

My hot tub over heats to the point it has ruined the cover. It blistered it on the side next to the water. This happenns with the temp set to less than 100. I cant find anything wrong with it. The temp sensors have been replaced , the controller has been replaced even though there seemmd to be nothing wrong and it still gets too hot. I thought maybe the heater was coming on when the pump came on even when it wasnt suppose too so I connected a lightbulb in paralell with the heater. After 2 weeks of monitoring I never once found the heater energized when it was not supposed to be
Can the water get hot just through pumping?
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what does the thermometer in the hot tub say the temp of the water is? the water may get a little heat going to it from pumping, but not enough to even measure with standard hot tub thermometers.
i suspect a faulty cover.
randy

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I had this happen to my hot tub. I replaced the High temp limit safety switch.(rebuilt one) It worked fine but then it went out again. I called the parts store and was told that sometimes these switches are "bad" even when they sell them as reconditioned replaced it (for free) and my unit has worked fine.

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This is so weird, The thing went from 102 to 105 with the heater disconnected(wires disconnected from the heating element).Yes temp reading is accurate. Have checked it against a medical and darkroom thermometer. Temp does go down if I completely turn the hot tub off so this elimates external sources such as the sun.
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Jimmie wrote:

Does it only heat up when you want to get in? Maybe some other "domestic" problems need repair!
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This is very strange indeed. How long does it take to go up 3 degrees and what is the ambient temp? The only possibility left would be a short that is somehow heating a metal part of the water circulating system. This seems very unlikely, there should be a ground fault interupter on the supply circuit and a short to ground would trip it. I'd get an amp meter and measure how much current it draws with nothing running, then with the pump, etc. to see if there is a higher than expected current draw. You could also turn off the AC, then feel around all the components with your hand to see if anything is hotter than it should be.
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