Hot Tub high limit switch

If you’re having issues with your hot tub heater, it may be related to the high limit switch (also known as the high limit thermometer). Certain conditions can cause the switch to trip more than it should.

Often, you can troubleshoot issues with the high limit switch and address them on your own. However, because this is a situation that involves electricity so close to water, never attempt repairs unless you understand your hot tub and its electrical system very well.

If any of the following issues are recurring, and your fixes are temporary at best, contact a professional for assistance. The longer you let the issues go on, the greater the potential for damage to your hot tub and its components.

What is the High Limit Switch?

The high limit switch is a safety feature that keeps hot tubs from overheating. If it senses that the spa water has reached 110 or 120 degrees, the switch trips and your heater (and often the pump, too) turns off.

Your high limit switch measures the water temperature by either a capillary tube attached to a bulb or a mounted sensor. If the switch detects unsafe temperatures, it opens to break the circuit and keep the heater from literally melting down.

Once the water temperature in the hot tub has decreased, the high limit switch must be reset manually.

The high limit switch will trip if:

  • The hot tub water is too hot and may harm bathers or the spa.
  • Something is causing the switch to trip even when the water is not too hot.

Next we’ll discuss some of the common reasons behind high limit switch tripping.

What Causes High Limit Switch Tripping?

The first thing to consider is the water in your hot tub. What might cause it to reach temps of 120 degrees? Generally, new tubs will not allow you to set the programmed temperature that high, so something else is likely going on.

If the air outside the tub is hot enough, it can be difficult for the spa water to cool down. Try adding some cool water (according to filing and treatment instructions) or shifting the spa cover to let hot air out.

Adding very hot or boiling water is not recommended. Even if you want to increase the temperature faster, never add water to the spa in ways that different from the manufacturer’s filling instructions.

If the water isn’t actually reaching dangerous temperatures, something else is wrong and causing the high limit switch to trip. Here are some of the most common causes:

The High Limit Switch Needs to Be Reset or Replaced

Occasionally, the high limit switch will trip when you’re filling your spa. Or it might need a reset from time to time for no known or serious reason. Simply reset by pressing the big red button on your spa pack.

The high limit switch may be housed in a thermowell. If the water temperature in the thermowell is the same as the water temperature in the rest of the spa, the switch is damaged and needs to be replaced.

Thermostat Problems

Hot tub calcium buildupThe thermostat may be calibrated too high, or it may be simply broken. If calibration is the problem, you may be able to recalibrate by turning the screw on the back of the thermostat a quarter-inch to the left.

It’s also possible that the thermostat probe isn’t all the way in the thermowell and isn’t getting a true reading of the water temperature.

If all else fails, it may be time to replace the thermostat.

Calcium Buildup

If the water temperature inside the high limit switch thermowell is higher than the water temperature in the rest of the spa, you may have calcium buildup on the thermowell. The resulting low water flow can impact the high limit switch functioning.

Check to be sure your filter is clean and free of obstructions, and ensure slice and gate valves are open.

Air Trapped in the System

If air remains in the plumbing system after you fill the hot tub it could trip the high limit switch. If the issue occurs each time you drain and refill, that is the probable cause. This issue will likely resolve itself when the air is out of the lines.

Again, if you have any hesitation about working with the high limit switch yourself, don’t hesitate to call a professional or your local Aqua Living dealer for assistance, and for the safety of yourself and others around the spa.