Hot tub stopped heating at 96

[Elle]

Hello. A few months ago we bought a very used hot tub. It’s been working decent up until last night. The temp says 96 and it won’t get any higher than that. I turned the power off and then restarted but I don’t think that did anything. I have zero clue what happened. Just the day before yesterday it was 104 and working fine. It does heat slow but once it gets to 104 stays there well.
Please help! I’m regretting buying so used at this point. You get what you pay for I guess.
It’s a 2005 Sundance spa majesta

 

[Jimrahbe]

Elle,

Most likely the thermister that senses the water temperature is bad, but there is no absolute way to know for sure... What I would do is use a voltmeter and measure the voltage going to the heater. When the water is colder, there should be 220 volts across the heater terminals.. Watch the heater voltage and the temperature and see what happens. If the heater voltage goes away when the temp gets to 96, then you know that the heater is not the problem. That leaves the thermister or the controller... The thermister is pretty cheap to replace, but the controller is not..

If this were my spa, I would invest in a thermister, but would have to think twice before spending money on a new controller.

Of course, if the heater voltage never went away, but the temperature never gets above 96, then that would be a heater failure.

Thanks,

Jim R.

 

[DangerBoy]

While it's possible something has gone wrong with the heater, because heaters are just elements made up of electrically resistant materials that heat up when electrical current passes through them they kinda either work or they don't so when they fail, they most often don't work at all.

As Jimrahabe suggested, your most likely culprit is the temperature sensor thermister. Your tub will have two thermistor sensors: One is used to tell the controller what the water temperature is and the other is used only to tell the computer that the water temp has hit a pre-set high limit (probably around 118 F) so the controller will then cut off power to the heater and flash an error code.

There might be a way to check that temperature sensor thermistor in-situ if you can get a temperature vs resistance graph for it and then check the resistance through the thermistor and compare it to the chart. If the thermistor has drifted out of spec then it will be reading a resistance that would make the controller believe the water temp is 104F when it's only 96F. I had temperature regulation problems when I first got my used tub. With a little searching on the Google, I was able to get a response graph for the thermistor used in my tub and found that it was way out of spec so the controller had no clue what the water temperature actually was. The high limit thermistor was also bad so I replaced them both.

If you don't have the means to check the thermistor in place, my advice would be to first address the most likely cause of the problem - the thermistor - and see if that fixes the problem. Lucky for you, the thermistor is also the least expensive and easiest to fix potential cause of your problem. Chances are, the high limit thermistor is the same age as the temp sensor thermistor so it would be a good idea to replace that one as well. That way you'll know neither are likely to fail for quite some time.

 

[Bama Rambler]

Do you get any error codes on the panel?
Does the heater light stay on or does it go off when the spa reaches 95º?