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High Limit tripping at 93 degrees

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I have an older Sundance Spa. Recently I replaced the filter, the pressure sensor (precautionary) and the circ pump as it had failed. The spa now works almost perfectly except the HL warning keeps tripping at 93 degrees. If I re-set the GFI I can normally make it run for a few more minutes and get to 94 or 95 degrees and, if I'm lucky, I can keep repeating that and get it to 100 or 101 before it cools back to 93.

Any ideas what could be the cause?

I've attached photos of the unit. I know the HL sensor is the second red connector on the top right but not where the thermostat is or if there is a "heat well" to check?

Is it possible there is a blockage after the heater that is causing to water flow to slow and peak in the heater element (if that's where the temp sensor is)?



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A blockage in the heater could definitely cause the hi limit sensor to trip.  Or the sensor could just be bad.

I would trace the wire for the HL sensor to the sensor bulb.  It's usually tucked into a well on the outside of the heater can.  Here's what it looks like:  https://goo.gl/photos/V9N7HpEdP5js8ziY9

Sometimes, they use a hose clamp to hold it there.

Visually examine it.  It should look like a silver bulb, anywhere from and inch to 3 inches long and about 1/4 to 1/2 inch thick.  They come in many different sizes.  If it looks corroded or like it's falling apart, it's probably bad.

Remove the sensor and put it in a mug of cold water.  Now, turn on the tub and see if it overheats.  Also, if you leave the sensor hanging in the open air, as long as it's under 118 degrees, it shouldn't trip.


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Thank you so much, ScubaDave! I found the HL sensor tucked behind the heater tube and released it. The sensor looks like new with no corrosion of any kind so I guess the unit has just failed and I can replace. The hot tub is currently heating up nicely to 102 degrees so I will be relaxing in it later watching this big Pacific storm blow through the Sierra Nevada mountains. Many thanks again!

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Glad you got your tub working to your satisfaction.  You should jump on replacing that sensor sooner, not later.  If your heater decided to overheat, without that hi limit sensor, you could burn up the heater element..... and maybe the circuit board as well.  Also, if the heater malfunctioned and wouldn't turn off, your tub could get hot enough to scald you.

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