Hot Tub High Limit Switches: What They Do And What To Do If They Fail

There is nothing that can compare to the feeling of slipping into a bubbling hot tub from a company like California Home Spas & Patio. Unfortunately, there is also nothing that compares to the feeling of entering a bubbling spa only to discover the water is ice cold. If your hot tub isn't heating the water, then you're stuck with nothing more than a wading pool. The good news is heating problems can be solved by the do-it-yourself spa owner in many instances. One common cause of a frigid spa is a malfunctioning or failed high limit switch. Below is more information about hot tub high limit switches, how to tell if your switch is broken, and what you can do to fix the trouble:

What a high limit switch does

An overheating hot tub can pose a significant hazard to both the safety of bathers and the spa itself. To prevent hot tub users from being scalded and to protect your tub's internal equipment, the high limit switch exists to monitor water temperature and automatically turn-off the spa's heater. High limit switches consist of a temperature probe that draws water through a small copper tube into the body of the device. If there isn't an adequate flow of water or the water temperature is too high, a mechanically-activated switch will trip and shut-off power to the spa's heater. The switch must be reset by manually pushing a reset button located on the unit.

How to diagnose and resolve problems with a high limit switch

There are several possible issues that can cause the spa to stop heating water, and a limit switch failure is one common option. However, before you spend the money and time to replace a limit switch, you need to be sure that you have ruled-out other sources of trouble. Here are a few things you need to take a look at:


A dirty, clogged filter is a common source of water heating problems in hot tubs. An obstructed filter prevents water from flowing at an adequate rate; slow flow allows the water to remain in close proximity to the heater for a longer period of time, and the water temperature exceeds normal limits. This trips the high limit switch to protect people and equipment, as it should.

Checking to see if your filter is the source of trouble isn't difficult. First, remove the filter from its housing and examine it for obvious accumulations of matter. Next, wash the filter thoroughly in a manner directed by the owner's manual. Reinstall the filter back into its housing, and press the manual reset button for the high limit switch. The reset button is usually colored red, though it may behind a rubber cover. When you press the switch button, you should feel and hear a click that indicates it has been reset.

If the hot tub begins heating and the high limit switch doesn't turn off again after the water has fully heated to its normal operating temperature, then the dirty filter was probably the culprit. That's one reason why it is important for you to regularly clean or replace your spa filter. However, if the high limit switch trips again, then you need to look for other problems:


Another problem that can cause a high limit switch to trip is a buildup of scale. This is caused by hard water that isn't properly treated, and some chlorine-free systems can also be a source of the scale. You can check for scale by looking at the internal components of your spa; scale is a white substance that accumulates on the surfaces of pipes, the high-limit switch sensor and the heater.


Some hot tubs have an adjustable thermostat that can be set by the owner, though not all do. The thermostat consists of a small screw that is turned by a screwdriver to adjust the temperature range. If you have eliminated a dirty filter or scale as the problem, then there is a possibility that your thermostat is set too high. Follow your manufacturer's instructions on how to adjust the thermostat so the water temperature is lowered by a few degrees.

The switch or the heater

Once you rule out all the above possible causes, then you are left with a couple of likely causes: the heater or high limit switch. At this point, replacing the high limit switch is the most cost-effective and easiest repair to make. If you still have a problem after installing a new switch, then you should either replace the heater yourself or contact a qualified spa professional to service your tub.