How Does A Hot Tub Thermostat Work? (Here’s How…)

By James Brockbank •  Updated: 01/03/22 •  13 min read

How does a hot tub thermostat work?

Modern hot tubs use LCD panels fitted with digital control buttons connected to the heating controller inside the spa. A temperature probe is used to gauge the water temperature to the nearest degree. Older hot tubs (older than 15 years) typically use a temperature dial which often has a variance of 5 degrees from the temperature you set.

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I recommend a digital floating thermometer for your hot tub for more accurate temperature readings.

Nothing is more disappointing than preparing to soak in a hot tub on a chilly day only to realize that the water is not heating. 

A hot tub is designed to heat water from the thermostat. 

Different reasons could be associated with the improper functioning of the hot tub heater. As such, you can test and observe the hot tub thermostat to determine the cause of the poor or failing water heating. 

Never conduct electrical repairs with the power on no matter how qualified you are because electrocution risks can happen. 

The bottom line is finding how a hot tub thermostat works.

Let’s get started!

How Does A Hot Tub Thermostat Work?

In modern hot tubs, there is an LCD panel fitted with buttons and connected to the controller to assist with raising or lowering the temperatures. A temperature probe on the controller reads and displays the water temperature. Older hot tub versions have a dial thermometer that sets the temperature at a 5 degrees variance between the actual temperature and the temperature you’ll set. 

Hot tubs have not been exempted from technological advances, and the heating system is among the components that have been largely modified. 

As a result, changing the hot tub temperature has changed significantly over the years.

Nevertheless, much more happens in the temperature controls of a hot tub, and you might need more extensive skills beyond just troubleshooting. 

Why Is My Hot Tub Not Getting Hot Enough?

Your hot tub is not getting hot enough because of the clogged pipes, dirty filters, broken pumps, or bad temperature sensors. The failure of the heater element could also be the reason that your hot tub water is not attaining the desired temperature. 

The water could be warm but fail to heat to the set temperatures. 

Heating failure could also be associated with temperature sensor malfunctioning, the air in the plumbing components, fried thermostat, and inoperative high-limit sensor. 

A hot tub is supposed to warm up and get nice for you to sit and relax after a busy day. But this will not happen if it no longer heats water to your desired temperature. 

Sometimes it might fail to get warm completely. 

On the other hand, I have also seen hot tubs that heat up and cool down almost immediately. 

Some of these issues are caused by factors you can diagnose and treat on your own, but you can still call an electrician if you’re uncomfortable working with electricity and water. 


Where Is The Temperature Sensor In A Hot Tub?

A temperature sensor is located inside the filter housing for most hot tubs. When you want to verify your water temperature, look at the reading on the control panel. You should then dip a thermometer into the water to check whether there is a discrepancy in the temperature readings. If you find a difference, be sure that your thermostat or the temperature sensor does have a problem. 

How Do You Calibrate A Hot Tub Thermostat?

The steps for calibrating a thermostat are as follows:

  1. Use a separate thermometer to check the set temperature and the actual water temperature discrepancy.
  2. Select the correct type of screwdriver based on the type of screws for the thermostat. 
  3. Turn the screw clockwise by one-quarter. Based on how far off the actual temperature was from the thermometer. Wait for about an hour and retest the temperature. 
  4. Keep turning the water temperature and the screw at the same rate clockwise until you attain a temperature that matches the thermostat. 
  5. After the gap between the two temperatures is bridged, the thermostat is calibrated, and the hot tub will be ready to use. 

If this process does not work because the temperatures fail to match, the best thing would be to consider replacing the thermostat.

One of the common solutions to hot tub water heating issues is the calibration of a thermostat. This works if you use an older model with a dial thermometer. 

A temperature sensor is usually inserted into a thermal well for the newer models. This brings it very close into contact with the water. In addition, the sensor works with the display panel because both are usually linked to the control box. 

The heater tube has a high-limit sensor intended to prevent overheating. However, it is not usually tied to the topside panel directly. 

The older hot tub models with a knob require calibration of the thermometer to set up the temperature. 

But even the newer models with an LCD-type panel and push buttons for setting the temperature offers some good solution for the thermostat calibration since all you need is to test the hot tub thermostat from its various points of failure. 

Replacing the temperature probe with a few recent models with the LCD panel where the probe rubs to thermal well. But replacing the topside panel could be needed if the problem continues. 

If you’re looking for a great tool kit to help you fix your hot tub, this Mixed Tool Kit by Dekopro is my recommendation.

How To Test A Hot Tub Thermostat?

To test the thermostat, check the temperature probe, verify the water temperature, check the back of the thermostat for calibration, and verify that the hot tub’s operation is running normally.

Testing a thermostat is straightforward. You are only required to have an extra thermometer and a screwdriver based on the type of screws you’ll be adjusting. 

Check The Temperature Probe

The temperature probe is located on the underside of the hot tub. Its tip is connected to the thermal well while other components connect from the controller. 

Ensure that it is appropriately pushed into the thermal well because failure to have this position will affect its effective functioning. 

Verify The Water Temperature

Turn water temperature to 104 degrees Fahrenheit and wait for at least an hour if your hot tub was set to at least 98 degrees. Then, use a separate thermometer in testing the water and compare it to what the hot tub is displaying. 

If there is a difference between the two, you may need to recalibrate the hot tub thermostat for proper functioning. 

Check The Back Of The Thermostat

Calibration is possible if your thermostat has a flat or an Allen screw. Most hot tubs have any of the two types of screws, meaning that calibration is quite easy. However, you may need to call in a professional for assistance for those who don’t. 

Check Water, Filter & Overall Operation Of The Hot Tub

Some things to check are the availability of sufficient water in the hot tub and the possibility of a clogged filter. You also need to be sure that your jets are not blocked and running correctly. 

How Much Is A Thermostat For A Hot Tub?

The cost of a thermostat is variable across different hot tub sizes and models. However, they range between $25 and $150 based on your system’s model or size. 

Here is a list of hot tub sensor parts on if you need something specific.

A quick search from the various online sellers across the United States will average the cost to about $100. 

You might incur a higher cost when using a more expensive hot tub

Another point is that rotary dial thermostats might become extinct in the market because they are for the older hot tub versions. That’s because as people increasingly buy the newer hot tub models, the demand for the older components will go down and reduce their production capacity. 

It’s fancy and fulfilling to own a hot tub until one of its components, such as a thermostat, will require replacement. 

I always advise delaying or preventing such troubles through routine maintenance and regular cleaning. 

You will delay any issues and ensure that your hot tub lasts a little longer when you do this. 

How Do You Test A Hot Tub Temperature Sensor?

It is easy to test a hot tub temperature sensor. All you need to do is shut off the power supply to the hot tub, followed by setting the ohmmeter to 20000 and finding the end sensor wire. You should then unplug the end sensor wire from the control box and place the leads of your ohmmeter onto the green and the red wires. 

Compare the thermistor resistance and the temperature chart from the hot tub manufacturer to check whether the hot tub is heating correctly. If this is not the case, you need to test the hot tub temperature for issues. 

An incorrect temperature reading on the thermometer is also a sign of a possible issue with your sensor. 

So, how do you accomplish this test? 

The resistance of the majority temperature sensors is 10000 at 77 degrees Fahrenheit. However, cold water elevates the reading to go for up to 50000. Interestingly, up to 100 degrees of warm water can have even lower resistance readings. 

For higher accuracy, check a book from your manufacturer that contains a resistance vs. temperature chart. If you get a zero reading during the test, know that your sensor or the cable has a problem. 

I recommend the AstroAI Digital Multimeter if you are doing any testing with electrical equipment.

How Do You Change The Thermostat On A Hot Tub?

Here are the steps to change the thermostat:

  1. Ensure that the power to the hot tub is turned off. 
  2. Wear protective clothing such as electrician gloves and rubber shoes to remain grounded.
  3. Remove the cover to the spa motor and remove the cover to the heating element.
  4. Locate the thermostat and disconnect the wiring connected to the motor.
  5. Remove the thermostat and the nuts holding it in place.
  6. Add the new thermostat into position, add the locking nuts, and connect the wires to the heating element.
  7. Place all coverings back inside the hot tub and ensure nothing is loose.
  8. Turn the power back on and check if the system is working normally.

You will need to replace your hot tub’s thermostat over time. Although the actual replacement process is easy, you’ll be required to dismantle the spa heater housing to do the replacement. 

At the same time, you might need to have your manual at hand when doing this, so always keep it safe. 

The first step is preparation. Here, you’ll turn off the power and unplug the hot tub at the control box. Also, make sure to be in rubber shoes to avoid being grounded. 

Carry a bag for putting the screws and your tools for use. Then remove the spa motor cover and locate the heater housing to remove the cover. 

The next step includes activities for removing the bad thermostat. 

First, locate the thermostat and disconnect the wiring linking to the motor.

Next, look for a nut holding the thermostat in place and use a wrench to make it loose to pull out of its position easily. 

Install the new thermostat where the old one was fixed using the old screws and nuts. 

Next, reattach the new wires to where the old ones had been attached and cover back all areas where you had removed the cover as you close and tighten the screws. 

Next, plugin the spa and flip on the breaker. 

Finally, turn the motor on and run for some time to ascertain that it is heating properly. 

Why Won’t My Hot Tub Temperature Go Down?

There may be several reasons why your hot tub temperature won’t go down. These include summer temperatures exceeding 100 degrees Fahrenheit, problems with the temperature probe, and potential issues with the control panel.

Check out this article I wrote on overheating hot tubs for more information.

The most likely reason that your hot tub temperature will not go down is the extreme summer temperatures in the surroundings. In some regions, the summer temperature might exceed 100-degree Fahrenheit.

At the same time, there might be an issue with your control panel. 

If the digital control is not working correctly, you can ensure that the setting temperature readings are inaccurate. 

For example, the system might not detect that you have set a temperature reduction from 98 degrees to 86 degrees. An issue with the digital control of your hot tub will also prevent the decrease in temperatures even when you set it at a lower level. 

Why Does My Hot Tub Get Hotter Than The Setting?

A hot tub could experience thermal creep when the water is constantly being filtered. This is due to ambient energy from the pumps and the moving water. Outside temperatures and direct sunlight can also cause the temperature inside the hot tub to rise.

Sometimes the hot tub can get hotter than the setting and make bathing uncomfortable for you. 

Experts note that hot tub temperatures exceed the setting during the summer months. 

As such, this must never be a cause for panic with your hot tub. Instead, the problem is called thermal creep, which is an unusual rise in the hot tub temperature because there is no escape. 

The electric motor components in your hot tub might be causing your hot tub to be hotter than the setting. Electric motors pump water around the hot tub, heating, filtration, and powering the jet for the bathers. 

The generated heat on the motor is transferred into the hot tub water and makes it rise beyond the desirable levels. It is common for the temperatures to be higher by between 25 and 30 degrees because of the motor emission. 

How Does A Hot Tub Thermostat Work: Conclusion

how does a hot tub thermostat work

A thermostat is responsible for raising or lowering the water temperature in your hot tub. 

Sometimes the thermostat might run into trouble and require recalibration or replacement. 

In either case, the process is simple. However, remember to turn off the power at the breaker box because water and electricity do not mix. 

If you are not confident about changing the thermostat yourself, call for professional assistance. 

As the old saying goes, prevention is better than cure, so don’t wait for the problem to develop. Instead, routine maintenance and regular hot tub cleaning will keep the system in good shape. 

Remember that higher temperatures can happen during the summer because of the extreme outside heat and the ambient energy from the pumps.

I hope this article on how a hot tub thermostat works was helpful.

Thanks for visiting

James Brockbank

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