What Would Trip a High Limit Switch in a Hot Tub?


Owning your own hot tub can be a source of endless delight. Home spas also are outfitted with safety features to forestall damage to the tub itself. One of those features is the high-limit switch, which triggers shutdown of the tub's heating system when temperatures exceed a preset limit.

High-Limit Switch Purpose

  • The hot tub high-limit switch is a safety device designed to prevent high temperatures from damaging the tub. The standard switch-triggering temperature is 120 degrees Fahrenheit. Your high-limit switch may be tripped because the temperature is actually too high. But it may also trip if it is not operating properly.

Draining the Hot Tub

  • The process of draining and refilling your hot tub can lead to high-limit switch issues. When you drain the tub of water, air can enter the plumbing in place of the water. When the piping of your tub is filled with air instead of water, the high-limit switch may inaccurately gauge the tub's internal temperature. In such a case, you may need only to wait until the air has been expunged from the tub's plumbing. Once the tub is refilled and the water has displaced the air in the pipes, the high-limit switch should begin operating properly. Reducing the thermostat temperature before you drain the tub may help keep these issues at bay.

Thermostat-Related Issues

  • Problems with the hot tub's thermostat can also trip the high-limit switch. Calibrating the thermostat too high can lead to elevated internal spa temperatures. The thermostat may need adjusting. If the back of your thermostat is equipped with an Allen or flat screw, try to recalibrate it by turning that screw a one-quarter turn counter-clockwise. Check to ensure the probe of the thermostat is fully inserted into its thermal well. Consult the product manual for your particular hot tub model for alternate modes of recalibration, if any.

High-Limit Switch Thermal Well

  • The high-limit switch is equipped with its own thermal well. While the tub is operating, measure the temperature in that well with a thermometer that accurately measures water temperature. Raise the temperature inside the high-limit thermal well to 120 degrees Fahrenheit. If the switch does not trip, the high-limit switch is faulty. In addition, the temperature in the high-limit switch thermal well should not be the same as the temperature in the tub water. If the temperatures match, the high-limit switch needs replacing.

    If the high-limit switch well temperature is higher than that of the tub water, examine the wall of the high-limit switch well for calcium deposits, which can collect over time. Finally, inspect your filter to ensure it is clean and examine the slice valves to be sure they are in a fully open position to maintain water flow and normal temperatures in the high-limit switch thermal well.

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