What is a Hot Tub High Limit Switch?

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It’s never too late to get acquainted with your hot tub equipment—where it is, what it does, and how to fix it. Here at Hot Tub Warehouse, we do our best to keep our customers informed and satisfied with their hot tub equipment. If you're dealing with some heater issues like the ones we’ve outlined in a previous post, you could have a problem with your high limit switch. But what is that? What does it do?

Your hot tub high limit switch is designed to deactivate the heater when the water in your spa hits extreme numbers, often somewhere around 120° F. It’s a safety feature that will both protect any potential bathers in addition to your sensitive hot tub equipment.

When your hot tub water reaches unsafe temperatures, the high limit switch (which normally remains closed) will open in order to break the circuit, preventing the heater from having a literal meltdown in the cases of equipment failure. The high limit switch must be manually reset after the condition in the tub has returned to normal. A tripped switch cuts the power BEFORE the fuse to stop the function of the heater as well as the hot tub pump. In older systems, this may only cut power to the heater when tripped.

The high limit switch senses the water temperature with a capillary tube that is then attached to a bulb. This is all housed in a THERMOWELL. Another type of high limit switch is a mounted sensor, often referred to as a THERMODISC.

The hot tub high limit switch is sometimes triggered when you’re refilling your hot tub and can be reset by pressing the large red button on your spa pack.

If you need to troubleshoot other issues with your high limit switch, BE CAREFUL. If you don’t have basic electrical knowledge, don’t attempt to fix it.

  • Check the spa thermostat: there’s a chance it’s calibrated too high or the circuit may not open when the high temperature is reached. You can sometimes recalibrate the thermostat using a tool on the back of the thermostat. Turn the screw ¼ turn to the left to try and lower the calibration and see if that helps.

  • If the thermostat is broken, replace it.

  • Make sure the thermostat probe is all the way in the thermowell. If the probe sticks out, it could not be getting a true reading of the water temperature. Insulate the rear of the probe if it sticks out of the thermal well. If the probe is properly inserted and the thermostat is functioning, go to the next step.

  • If the contactor DOES NOT open when the thermostat is turned down, replace the contactor. If it does, proceed to the next step.

  • Measure the temperature of the high limit thermowell. If the temp inside is the same as the water temperature, you’ll need to replace the high limit switch. Test the temperature to see where the high limit trips—typically somewhere between 110°-120° F.

  • If the temperature inside the well is higher than the water, check for calcium deposits on the thermowell. Excess scaling may require a replacement. Low flow rate can also cause a higher temperature reading inside the well. Make sure your filter is clean and that the slice valves open completely.

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