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How Does a Hot Tub Thermostat Work? (Fixing Heating Issues)

Changing the temperature on a hot tub has changed a lot over the years. But for those with older-style hot tubs, it’s not uncommon to wonder how does a hot tub thermostat work?

Modern hot tubs have an LCD panel with buttons, connected to the controller, to raise or lower the temperature. The controller has a temperature probe that reads the water temperature. But older models have a dial thermometer to set the temp, which often has a 5° variance from the temp you set and the actual temp.

But there’s a lot more that goes into hot tub temperatures, and especially troubleshooting them when the water temp isn’t what it should be.

So let’s dig in further.

Why is my hot tub not getting hot enough?

If a hot tub is warm, but not at the set temperature, the possible causes include dirty filters, clogged pipes, a bad temperature sensor, or a broken pump. If the water isn’t hot whatsoever, the heater element has likely failed.

Your hot tub is supposed to get nice and warm for you, so you can sit and relax.

What happens when your hot tub stops heating up? Or is it just not getting warm enough for you? There are many elements to getting your hot tub to warm up your water, however, if it does not get hot anymore, these are some of the potential issues:

  • Dirty filter
  • Clogged pipes
  • Some air in your plumbing
  • Bad heater element
  • Broken pump
  • A malfunctioning temperature sensor
  • An inoperative high-limit sensor
  • Fried thermostat

There are many different reasons your hot tub may not get hot enough for your liking, whether it heats up then cools down or just does not heat up at all.

Some of these fixes you can do on your own, however, others require a professional, especially if you are uncomfortable or inexperienced in working with water and electricity.

Want to troubleshoot why your hot tub isn’t getting hot?

Luckily, I have a recent article that looks at all the possibilities and solutions for each one. Many of my solutions are perfect for you to DIY without having to call a pro.

Just click that link to read it on my site.

How do you calibrate a hot tub thermostat?

Having issues with your hot tub water heating up can sometimes be fixed by calibrating the thermostat. That is IF you have an older model hot tub with a dial thermometer.

Newer hot tubs typically have a temperature sensor that gets inserted into what’s called a thermal well, basically coming into almost direct contact with the water. That sensor works in conjunction with your display panel as both get connected to your control box.

There is also a high-limit sensor on the heater tube itself. However, that is designed to prevent overheating and is not directly tied to the topside panel.

For older models with a knob to set the temp, you can usually calibrate the thermometer.

But even for new models with an LCD-type panel with push buttons to set the temp, we still have some good solutions. Here are the steps to follow:

  • Check water, filter & overall operation of the hot tub
    • Make sure there is enough water in the hot tub.
    • Ensure your filter is not clogged.
    • Check to make sure the jets aren’t clogged and are turned open.
  • Check the temperature probe
    • This is located on the underside of your hot tub
    • It connects from the controller and the metal tip gets pushed into a “thermal well”
    • Make sure it is pushed all the way into that thermal well
    • Otherwise, it will not be accurate
  • Verify the water temperature
    • Turn the water temperature up to 104°
    • Wait 1 hour, assuming the hot tub was already set to at least 98°
    • Test the water with a separate thermometer
    • Compare to the hot tub’s displayed temp
    • If they do not match the hot tub’s thermostat may need to be recalibrated to work properly.
  • Check the back of the thermostat
    • If it has an Allen or flat screw, it can be calibrated.
    • Most thermostats can be calibrated.
    • If it does not have this screw, a professional may need to fix it for you.

For newer models with an LCD panel and temp probe, if the probe is all the way into the thermal well, your best bet will be to replace it. If the problem still persists, then I would replace the topside panel.

Luckily temperature probes are fairly inexpensive and can be replaced by the average homeowner.

Here are the steps to actually calibrate a thermostat:

Depending on the type of screw (most likely Allen or flathead), you will need that type of screwdriver to turn it.

You need to turn that screw one-quarter clockwise. Then, depending on how far off the actual temp was from your thermometer, wait up to 1 hour and check the water temp again.

Keep checking the water and turning the screw one-quarter clockwise until the water temperature matches the thermostat.

When they match, your hot tub thermostat will be calibrated, and your hot tub is ready for use.

If they do not match and this process does not fix the issue, you may need to replace the thermostat completely. Read below for the cost of replacing a thermostat and how to do it yourself.

How much is a thermostat for a hot tub?

The cost of a thermostat replacement for your hot tub will vary on the type and model of the hot tub you own.

The range is anywhere from $25 to $150, depending on the hot tub itself. But the bulk of what I see currently on Amazon when I search “hot tub thermostat” is well under $100.

Sometimes, the more expensive the hot tub, the more expensive the thermostat part most likely will be, so keep this in mind if you need to replace it. Also, knowing that rotary dial thermostats are for older hot tubs, just know they will eventually be harder to find and more expensive.

That’s because the supply will start to dry up and the demand goes down.

Owning a hot tub can be expensive, especially when you need to replace parts. And the good news is you can often prevent or delay needing to fix or replace a thermostat or temperature sensor on your hot tub.

The best course of action is to keep up maintenance on your hot tub and make sure you clean it regularly. This can reduce issues and make the hot tub last you longer.

Not sure if you have an actual problem with your thermostat or heating system?

Hot tubs take a while to heat up after refilling with fresh water from the hose. In fact, it can take anywhere from 3 hours to 8 hours or maybe even longer!

If you are wondering how long it should take your hot tub to heat up, AND what to do to speed that up, read this recent article.

Just click that link to read it on my site.

How do you change a thermostat on a hot tub?

Have you tried calibrating and decided your thermostat is bad? Follow the steps below to change it:

  • Turn off the power at the disconnect box near the hot tub. For a plug-in hot tub, simply unplug from the GFCI wall outlet.
  • Remove the main panel from your hot tub
  • Locate the thermostat
  • Disconnect the wires from it to both the control box and any temperature probes.
  • Remove the thermostat. You may need to remove some screws, otherwise, it should just be easily pulled off.
  • Replace with the new thermostat
  • Then screw the thermostat back in, or push it back in.
  • Close up the panel where the thermostat is.
  • Turn the power back on

Congrats, you just changed out your thermostat!

If your hot tub warms up properly and the thermostat now tells the right temperature, you have fixed your problem. However, if it’s still off, then you need to test your hot tub temperature sensor, which is covered in the next section.

And if you still haven’t fixed the problem by now, you’re probably ready to have a cold tub!

But, can you actually use a hot tub without heat, you may wonder? Read this recent article for more information on this topic.

Just click that link to read it on my site.

How do you test a hot tub temperature sensor?

Here is how to test a hot tub’s temperature sensor:

  • Shut off the hot tub’s power at the disconnect panel
  • Set the Ohm meter to 20,000
  • Find the end of the sensor wire
  • Unplug the sensor wire from the control box
  • Next, place the leads of your Ohm meter onto the red & green wires
  • Compare the readings to a Thermistor Resistance vs. Temperature Chart from your hot tub manufacturer

If your hot tub is not heating up correctly or the thermometer is telling you an incorrect temperature, you may need to test the hot tub temperature sensor for issues.

How do you go about doing this?

Most temperature sensors have a resistance of 10,000 at 77-degrees Fahrenheit.

However, the colder the water the higher the readings are up to 50,000, whereas warmer water at 100-degrees Fahrenheit will produce much lower readings.

Your hot tub should have a book by the manufacturer with a resistance vs. temperature chart. Though if you end up getting a zero reading, the issue is with the sensor or the cable is bad.

Final Thoughts

A hot tub thermostat is easy to figure out if you have read everything in this article.

Figuring out what is wrong with it and how to fix it may not be that difficult, but if you are uncomfortable messing with the wiring, you should hire a professional.

However, if you are comfortable with fixing any issues with your hot tub, even with it being electricity and water mixed, make sure you take precautions and stay safe.

Changing the thermostat yourself can be simple if you know what to do, but if you need help ask for it.

Make sure you also test your hot tub temperature sensor for issues as well and refer to the manual that came with your hot tub for help or go to their website. Taking good care of your hot tub can make it last a lifetime for you and your family.

As always, this article is based on my experience and research but should not be considered professional electrical advice. If you need professional electrical advice, you should seek out a qualified professional in your area.

Photos which require attribution:

101/365: Spa repair. by David Mulder and Water Heater Thermostat by Mark Florence are licensed under CC2.0

As an Amazon Associate, I may earn a small commission from qualifying purchases if you click on Amazon from my site and choose to make a purchase. You can read my complete affiliate disclosure for more details.

Jeff Campbell