MikeOnBike

RockyMtns, I'm not familiar with your spa model. I'm not a spa tech, I just used my electronics knowledge from building ham radios to solve my problem. The basic premise with many temperature probes is that they are a variable resistor. Most that I'm familiar with decrease their resistance as they get warmer. Given that, higher resistance indicates cooler water. It is fairly easy to add resistance in circuit by putting the resistors in series. In the hot tub case we take the primary resistor which is the temp. probe and add and adjusting resistance in series. If we needed to reduce the resistance it is a bit more problematic to calculate the correct value. http://www.1728.com/resistrs.htm Check the link for the formula. If we assume that you have the same kind of temp. probe or thermistor as I did then you will need to do some experimentation to determine the resistance you need. I cut one of the two wires going to the probe and bared a little copper to clip to. I then used two pieces of wire with alligator clips on each end to clip to the cut ends and clip to the resistor that I was testing. http://www.farnsworthelectronics.com/part%20images/test%20l60.gif I could then easily swap resistors until I found the correct value. In my case I had two temp. probes that I had to adjust concurrently. The VS501 uses two probes and they have to be within 1.5 degrees of each other or the system thinks that something is wrong. The manual for your tub shows two temp sensors. http://www.jacuzzihottubs.com/request-brochure/brochures/2007_LED_Manual-J300.pdf The manual didn't really discuss the interplay between the two but I think you probably need to fool them both. Hopefully they are the same electrically and would need the same resistance adjustment. Good luck...

Well my temperature probe hack was a success. In my case my system was reading 104 when it was only 99. I needed to correct the 5 deg. offset and add some overhead because 104 isn't always warm enough. I was shooting for a 8-9 deg gain overall. A little experimenting revealed that I could get a 1 deg. temp. gain for every 300-325 ohms. As someone else suggested a 1K resistor will get you close to 108 from 104. I added a 2700 ohm resistor in series with each temperature probe. I spliced into the red wire on the probe cable but it doesn't matter if you splice the black or red, the resistor is not polarity sensitive. If I had to do it over I would only use the 2200 ohm resistor because I found that it is plenty hot enough at 105-106 and I won't need the extra range to 108-109 deg. Remember that this modification doesn't let your hot tub go higher like reprogramming it does. It fools the system by making it think that the water is colder than it really is. My system always displays a temperature that is 5 deg. cooler than the water really is. Don't get carried away with this modification. Any time you take the cover off of the control box make sure the power is disconnected. The 220v power to the heater is feed by bare copper straps and it is way too easy to touch these while working with the probe wires as one of them runs right past the connection lugs. If you remove the temp. probes to get better access to them while soldering them you will get water everywhere. If your not comfortable making these modifications get your spa technician or an electrician to make the changes for you. KEEP IT SAFE! DISCONNECT THE POWER! DON'T END UP DEAD! Balboa VS501Z 2.7K resistor (1/8 watt, 5% tolerance) added in series with each temperature probe. 5 deg. correction with 4 deg. range extension. Recommend 1K resistor to just add the 3-4 degree extended range.

I have a Balboa VS501 in my spa. My tub is an accurately measured 99deg. when the display shows 104. I'm going to do the resistor mod this weekend. Balboa confirmed that these are 10k thermistor temperature probes. Yes, both of them most be modified or it will throw the 'Sn' error. Most sensors of this type decrease resistance as they get warmer. Adding resistance makes the CPU think that it is colder than it really is and lowers the observed temperature by the CPU. If you are correcting a system that is not calibrated correctly like I am rather than just trying to make it go hotter than the manufacture allows then you can add resistance until the displayed temperature matches your accurate thermometer. I'll report my findings...