The high limit switch is designed to trip the heater when your spa reaches about 120°F. It is a safety feature to prevent your equipment from being damaged by extreme heat. When troubleshooting the high limit switch, it is important to realize that either something is wrong with your hot tub causing it to overheat or other circumstances have caused your high limit to trip without the temperature getting that hot. If you recently drained your hot tub, the high limit tripping may only be a temporary problem and may go away once all of the air is out of your plumbing. Continue through this troubleshooting list only if the problem persists.
1) Reset the high limit switch. This is done by pressing the large red button on your spa pack.
2) Check your spa thermostat. It may be calibrated too high or it may not open the circuit when the set temperature is reached. Some thermostats can be calibrated with a tiny Allen screw or flat screw on the rear of the thermostat. Turn this screw a 1/4 turn counter-clockwise to calibrate down and see if that makes a difference. Replace the thermostat if you can't get it to function properly. Also make sure the thermostat probe is all the way in the thermal well. If the probe sticks out, it may not be reading 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, continue to step 3.
3) Does the contactor open when the thermostat is turned down? If not, replace the contactor. If yes, proceed to the next step.
4) Measure the temperature of the high limit thermal well. If the temperature inside the thermal well is the same as the water temperature, replace the high limit switch. You may choose to test the temperature that the high limit trips at. It should trip between 110 and 120°F.
If the temperature inside the thermal well is higher than the spa water, check for calcium build-up on the thermal well. If there is excessive scaling on the inside of the thermal well, replace the part. Low flow rate can also cause a higher temperature reading in the thermal well. Make sure your filter is clean and the slice valves (sometimes called knife or gate valves) are open completely to ensure adequate flow.