I'm having a problem with my hot tub heater. It's the exact opposite of what most people have (heater not working)- it is actually working TOO well. Let me explain.

About a month ago, my son was complaining that the hot tub was overly hot. When I checked it, it felt pretty hot to me too. So I put the pool thermometer in it. To my surprise, it read 118 degrees! I got another thermometer to double-check the reading, and got 117 degrees. After doing some research, I determined I should probably call a technician to handle this problem. The technician came out, looked things over, and told me it was either the High Limit Switch, or the Thermistor. He pointed out both switches (I already knew where they were from looking at the parts schematic), and explained how they could be changed. He said I could do it myself if I was handy, or I could pay him $150 to come back and take care of this 10 minute job. I opted to do it myself.

So I ordered the parts and changed them out. The result? I am having the same problem. When I set the heater temp to 100 degrees, the burner turns on and it heats up the water until it reads 100 degrees, as requested on the display. The problem is, what the heater thinks is 100 degrees is actually around 115. I double-checked this by turning the requested temp down to 85 degrees and running the heater. The result is water that is actually 100 degrees, although I am requesting 85 degrees. The system works as advertised, except for the fact that its temperature calibration is off by 15 degrees.

Anyone have any ideas on this one? I've already changed out the thermistor probe, which seems like the most likely culprit in this situation. My next step is probably changing out the control board. I don't want to do that if I don't have to, as it is a $400 part. Thank you in advance for any advice/solutions you can offer.