How to increase your hot tub spa temperature

Research overview

Research suggests that higher spa hot tub temperatures can provide greater benefits for muscle relaxation, pain relief, improved circulation, and potential cardiovascular health, particularly for individuals with conditions like arthritis or muscle tension, although it’s crucial to be cautious as excessively high temperatures can pose health risks and should be used with moderation and consideration of individual health factors.

Advantages for higher hot tub temperatures:

• Muscle relaxation and pain relief: Higher temperatures can deeply relax muscles and alleviate pain associated with conditions like muscle soreness, arthritis, and joint stiffness by increasing blood flow to the affected areas.
• Improved circulation: The heat from a higher temperature hot tub can dilate blood vessels, promoting better circulation throughout the body.
• Potential cardiovascular benefits: Some studies link regular exposure to high hot tub temperatures with a reduced risk of cardiovascular disease, potentially due to improved blood vessel function.
• Stress reduction: The deep relaxation induced by a hot tub can significantly reduce stress and anxiety levels.

Important considerations:

• Safety concerns: Temperatures exceeding 104°F can be dangerous, especially for individuals with certain health conditions like low blood pressure, heart problems, or pregnancy.
• Individual needs: The optimal hot tub temperature varies based on individual tolerance and health status.
• Time limitations: Limit hot tub sessions to 15-20 minutes to avoid overheating.

Risks for spa temperatures above 104 °F

• Health and Safety Risks: Prolonged exposure to water above 104°F can lead to heat stroke, dehydration, dizziness, or fainting—especially for older adults, children, and people with medical conditions.  Fainting in a spa may likely lead to death by drowning.
• Legal and Liability Issues: The 104°F limit is in place due to safety regulations (e.g., CDC and CPSC guidelines). Installing a device that overrides this could expose you to lawsuits.
• Compliance and Certification: Many jurisdictions have strict regulations on hot tub safety. Bypassing temperature limits could violate local laws and void manufacturer warranties.
• Risk to Pregnant Women and Heart Patients: High water temperatures can be dangerous for pregnant women and those with heart conditions, potentially increasing liability.
• Damage to the Hot Tub: Not all hot tubs are designed to handle temperatures above 104°F. Exceeding this limit could cause premature wear, equipment damage, or failure of components like pumps, heaters, and seals.

How to enable your spa to go a few degrees above 104 degrees

All spas have a sensor to measure the temperature of the water. The controller measures the temperature and if lower than the set point it will apply heat until the temperature reaches the desired value or slightly above. Most spas use a thermistor, which is a type of temperature sensor with a negative temperature coefficient (NTC), meaning its resistance drops as the temperature rises. With an NTC sensor you need to increase the resistance to make the spa think it is cooler than it really is so it will then keep heating until it reaches what it thinks is the real set temperature.  Note that your spa controller will lose its calibration and will not report the new higher temperature. Because of this you should have a floating thermometer in the spa and always check it before entering.

First measure the resistance of your temperature sensor

• Perform the following at your own risk!  There are lethal high voltages inside the spa controller, and you have the added risk of breaking something so your spa will not work anymore.
• For this you will need a volt ohm meter (VOM) that has a resistance (ohm) reading feature.  Most home-owner stores stock them for under $20 or buy from Amazon (paid link).  You should also have a way of measuring the spa temperature independently from the controller. I recommend this thermometer from Amazon (paid link).
• Note the temperature of the water in your spa.  Best to record both the spa keypad temperature readout and use a plain thermometer in the spa water.
• Turn off the circuit breaker or cut-off switch so there will be no lethal high voltages in the controller.
• Open up the controller box and find the temperature sensor.  There should be a wire to a connector on the controller printed circuit board which will go to the sensor either on the side of the spa or to a port on the metal heater tube.  The probe will have 2 or 4 wires.  If only 2 wires, there may be a second sensor for the high limit cutoff.
• Unplug the connector from the circuit board.  Pull on the connector, not the wires.  There may be some hooks on the connector which you have to press to release it.
• Set your VOM to 10K ohms or 20K ohms if it is not auto ranging.  Find a place on the connector where you can reach the wires with the probes on the VOM.  If there is no place for this on the connector, stick paper clip wires or pins into the holes on the end of the connector. If the connector has two wires, measure the resistance between them.  If there are 4 wires, measure between the left two and the right two.
• Typical values for the resistance are 6.4 K ohm at 96°F, 5.8 K ohm at 100°F, and 5.3 K ohm at 104°F.   Your readings might be slightly higher or lower depending on the resistance of the leads. 
• If the readings are close to these values, you have determined the type of NTC thermistor and it is one that has a temperature coefficient of about 140 ohms/°F. Now you are ready to modify your spa to reach higher temperatures and you can skip the next section. 

Measure the temperature coefficient of your sensor

A typical sensor with readings close to the values given above has a temperature coefficient of about 140 ohms/°F at typical spa temperatures in the range of 96-106°F.    Other types of sensors may have a much different temperature coefficient.  To determine what resistance change is needed to increase spa temperature you need to make a graph over the range of about 90°F to 104°F.  Measure at least 4 values and determine the coefficient using the formula (R1-R2)/(T1-T2) where R is in ohms and T is in °F (1K ohm is 1000 ohms).  If you do this for several pairs of the 4 readings you should get about the same values.  If not, you should plot the values on graph paper to see if you have a bad reading, then draw a line through the points and find the slope of the line to determine the temperature coefficient. 

Determine the circuit needed

Now that you have the temperature coefficient of the sensor you can determine the resistance needed to increase the spa temperature.  I recommend an increase of 3°F so a spa that limits you to 104°F you will now be able to increase the spa to 107°F.  Above this the spa will be much less safe and at 107°F you will definitely want to limit your time in the spa to under 15 minutes.

For the typical NTC thermistor, to raise the temperature by 3°F you will need to add a resistance in series of 3°F x140 ohms/°F =420 ohms.  For a different NTC thermistor or different increase just multiply your measured temperature coefficient by the number of °F by which you want to increase the temperature.

If your spa has a positive temperature coefficient (PTC) then you need to reduce the sensor resistance.  You will do this by adding a resistor in parallel to the thermistor.  Here are the equations for calculating the resistance to add in parallel:

R = desired temperature increase X temperature coefficient

R is a positive number since resistance increases with increases of temperature

Rs is the sensor resistance at normal spa temperature (104°F)

Rt is the revised total resistance = sensor resistance minus R

Rt = Rs-R

Rp is the new parallel resistance to be added

Rp = Rt / (1-Rt/Rs)

Please contact me if you have trouble with these equations.

If your spa has a second sensor for the upper limit (typically 108°F) then you should make the same modification to that sensor.  If your sensor has 4 wires then probably two are for the temperature and two are for the upper limit.  If you modify both exactly the same way then you don’t have to determine which is which.

How to build your circuit

Now you know the resistance needed to be added to your sensor probe either in series or parallel.  If you don’t have a supply of resistors and you don’t want to try to figure out the color codes markings, I suggest you purchase this labeled collection of 1/4 watt resistors from Amazon (paid link).

These are packaged with 10 identical resistors taped together with the resistance value printed on the tape.  If one if the resistors is not near the value you need, just choose two or more resistors and add their values to combine in series.  For example, for my spa I needed 420 ohms so I used 330 + 100 = 430 ohms.  I could also have used 200 + 220 = 420 ohms which would have been closer.  You should check your final combination with your VOM.

There are two ways to add your resistor.  One is to cut the sensor cable and solder your resistor(s) directly to that wire.  This is the easiest method but will probably void your spa warranty.  If your spa is old the warranty may not matter, but if you ever need to replace the sensor you will have to modify the new one in the same way.

The other way to add the resistor is to purchase a connector pair (both male and female parts) identical or similar to the one used on your sensor.  Then you can wire it up away from the spa and plug it in when done.  This has the advantage that if you need warranty service on your spa, you can remove it before the technician arrives.

My spa controller is Gecko model IN.YE-5 and has a sensor model 9920-401385.  The sensor has 4 wires and I purchased these parts from Amazon (disclaimer: I get a commission from these links):

Female connector with 4 wires

Female connector with 2 wires

Male connector with up to 4 wires

Circuit board (break each one into 4 or 8 pieces)

If your spa has two wires to the sensor, you can cut off 2 wires from the 4-wire male connector.  The advantage of mounting on a circuit board is that you will not have to insulate individual wires and it is easy to solder together. This circuit board breaks into four pieces and I further break those into two more pieces which ends up about the right size for this circuit.  The male connector solders directly onto the circuit board.  Both have a hole pitch of 0.1” or 2.54 mm.  Note that one side of the board has groups of three holes already connected.  Insert the connector and wires and resistors from the other side and pay attention to where they will be connected.  See the photos below showing how I laid out my circuit.

Let me know by email or comment below if you have any questions.  My email is david at redipix dot com.  If this works for you I would be very interested if you could tell me about your project.

-- David Elmore