Spa Hi Limit Sensor/Switch
All spas will have a Hi Limit switch or sensor of some sort. This device is a safety device which acts as the thermal protection for your spa. These sensors are designed to prevent the heater element from burning out as well as preventing the spas water from getting too hot.
Most spa's will not let bathers set the temperature above the manufactures set point (Usually +/-108°F). Hi Limits usually have the high temperature threshold set between 118-121°F. Therefore an error or unusual event must occur leading to the sensor tripping.
Usually poor water flow. When the water heater's temperature rises too quickly due to insufficient water flow through the heater. Other factors such as damage to the sensor, lose of calibration etc.. can cause issues with Hi Limits.
Usually the sensor/switch will have failed & these switches do fail once in a while. When they do your spa will cease to function and shut down or default to a maintenance program.
IMPORTANT - This is a safety device! If it is found to be out of calibration, do not attempt to perform a field adjustment! Replace bad hi limit with a new one.
Mechanical Hi Limit
Older style Hi Limit Sensors feature a red push to reset button. These models are mostly found on spa controls which use air buttons, dial thermostats or are time clock activated.
If these was an event which wasn't related to equipment failure the hi limit switch can be reset, once reset the spa will function as normal but if it trips again there are other more significant problems with your spa.
Typically these switches have a copper bulb & capillary attached to the main switch body. These Hi Limits are either installed in a dry well or groove tube on/or immediately before the heater.
Digital Hi Limit
Modern digital spas with circuit boards & digital control panels typically have digital/electronic temperature sensors and Hi Limits. Functionally they are the same as the mechanical units, the probes use resistance to measure temperature. Once the maximum/ upper safe temperature limit is reached the sensor will shut the spa down or show an error code on the control panel, 'HL' or 'OH' error codes are the most common but there are others. See our error code post for more information.
These sensors plug directly into the circuit board. Note - there are a variety of plug types and bulb sizes it is important to replace your sensor with the correct one, be sure to match plug type, sensor length, bulb length & diameter.
Other Hi Limit Styles
There are many other types of Hi Limits used in the pool & spa industry. All functionally the same, preventing over heating or injury from water too hot.
Thermodisc (Surface Mount)
Typically found in older Hayward, Hydro-Quip, Raypak, Pentair Purex, Sundance electric heaters among others.
Pentair gas heater Surface mount Hi Limit
Typically found in Gecko S & M Class Spa Packs.