types of thermistors in water temperature sensors, and possible failure modes

Hi,

one of my solar controllers started acting up and I am trying to isolate the problem. I measured the resistance of the sensor in the return pipe but it seems a bit too high at 27,500 ohm. If the sensor uses a 10k thermistor, this corresponds to a 5C water temperature (41F), but the water is more like 15C (60F) which should give a reading of about 16 kOhm.

Is anybody familiar with these sensors? Do they all use 10k thermistors? And if the thermistor goes bad, can it go bad in that direction?

More detail:

In case you're wondering why I am using a solar controller in the middle of the winter: my last heating contractor evidently thought it was a good idea to use a solar controller for my hot tub. Without delving into too much detail (I use a single high-efficiency Viessman boiler for home heat, DHW, hot tub, and pool, with a heat exchanger), the hot tub temperature is regulated by the max temperature cutoff in the solar controller.

In addition to not working right now, this setup is pretty impractical, so I am replacing the solar controller with a Raspberry Pi, a relay board, and an A/D converter board. To wire the sensor to the A/D converter, I need to know the range of resistance values in the range of temperatures of interest. Hence the original question.

Thanks!