I'm looking to get another hot tub and having issues with maximum water temp. Seems there is a law in Calif. that prohibits the sale of spas that heat above 104 degrees . I currently have a 1994 Cal -Spa that I can maintain heat in the 108-110 range. I don't want to buy an older refurb and can't get any cooperation on how to manipulate this limit switch .Any suggestions would be great . Thanks . P.S. In Japan , hot tubs are routinely kept at 115 degrees for therapeautic purposes.

I don't believe that law is just for California, but, yes, heaters are only allowed to reach 104 max.

I talked with a spa guy at a dealer about this, he said they used to make a overide board you could get (can't remember which brand it was for) from the factory thru the dealer that was for tharaputic use. It would go to 120 deg. then This is for those poor people with lifelong back problems / injuries and advised by a doctor to help relieve pain. Of course this is for short use like 10-15 mins.

I was told on some boards there was a jumper to simply alow a uncapped high heat. you could look around the board on a store model and see if you get lucky. I would ask your doctor or a cyroprator <bad spelling if he knew anybrands that were "tharaputic friendly".
If all else fails....lets get technical. You can cut the wire to the thermo sensor(heat sensor), its going to be a therocouple or a thermistor. Search how to test them with a digital multimeter on google their in many products. If its a thermistor (heat reacting resistor) things are easier. You can get a Ohm value for water at say 80, then 100deg. for some data we'll use later. There are two types of thermistors
A Negative Temperature Coefficient (NTC) Thermistor will decrease in resistance as temperature increases.
A Positive Temperature Coefficient (PTC) Thermistor will increase in resistance as temperature increases.
IF yours is a NTC thermistor you can add a resistor in series to one of the leads this will trick the board into reading lower and allowing you to go to a higher temp. You could for example choose a resistor that is the same OHM value as the reading differential you get between 80and 100 deg. this would then make your board read 20deg. low so if from the factory 104 was the top well now when you set it to 104 you wil be at 124!

IF you have a Positive Temperature Coefficient (PTC) Thermistor then you need to show a lower resistance to the SPA board by adding a resistor in PARALLEL to the two sensor wires. The math to do a parallel circuit is a little more complicated. Here's the formula

R1 x R2
-------- = Rt
R1 + R2

In this case R1 is going to be Resistior 1 (the thermistor) R2 is going to be the resistor you add, Rt means resistance total and is what the Spa board now sees.
Getting data: So you need some data first to know what resistor to add. Get a resistance at 80(if you want to trick the SPA by 20 deg.) we'll call this A1 or take a reading at 90 deg. (if you want to trick the SPA by 10 deg.) we'll call this A2 then at 100deg we'll call this A3. You can do this by running the SPA then shutting in down and within a min get your meter on the wires of the thermisitor. You will have to unplug it (if possible) or cut both wires for a accurate reading. Now compare these readings. lets says the dif between 80 and 100 deg. is 5000 ohms or 5k ohms then thats how much we want to lie to the SPA board in order to make it over drive from a limit of 104 to 124. Of coarse understand in all this that if you have a digital temp readout it will read low, thats the point of the mod.

So you need to find the NEW Rt for the resistor formula. A3-A1=N1 where N1 is our NEW prefered high reading.
so work this formula
R1 x R2
N1= --------
R1 + R2
r1 is your thermisitor, r2 is the resistor you need to find for then wire it in parallel.

If you have a thermocoulpe sensor then these create a very small voltage, you can do the measurements and if the voltage increases with temp then good, you may add a resistor in series with one wire to drop the reading back down and get your extra heat.