I have an old tub of an unknown manufacturer. I recently restarted it after it had been off for 2 years. We got a couple of good days out of it but then the heater seemed to fail. Couldn't get it above 90 no matter how long I ran it. I have read a lot of the advice and have done a lot of testing. What seems to be happening is that the thermodisc trips (hi-limit?) I know this because I have checked for voltage at the element side of the thermodisc. I have also tested ohms across the element. It is right around 9 ohms. One additional piece of info: just before I did this, I added a pH stabilizer. I am wondering: could this have, somehow, coated the element causing it to overheat? I have tested for conductivity from the element to ground and gotten nothing but I have not yet tested resistance. There is still apossibility that there is a pinhole. Any ideas?

If you get a ton of hardness that's touching the element and the heaterbody where the disc is, it will create a little hotspot. That might be happening. You should be able to feel it overheating just by touching the heater body at that spot. If you don't feel it overheating, replace the high-limit.

I did an experiment by bypassing the thermodis (hi-limit). The water from the jets was still only lukewarm and it seemed that the wiring was getting hot. I do not have a meter to read amps but I am assuming that the heater element is drawing too much current.

The element isn't drawing too much current.

How long did you give it to heat with the high limit disabled? Do you have a spa cover?

If enough hardness has accumulated around the element, that it effectively connects the element to the heaterbody, it can create a very hot spot on the heaterbody, which the disc-high limit will interpret as high water temp.


You won't be able to tell any difference in the temp of the water coming out of the jets with the heater on or off.

PH stabilizer takes alot of hardness out of suspension. It accumulates on the element, sometimes enough that it acts as a conduit and will act as though the element is touching the heater-body, and subsequently trips the high limit.

You can feel the heater-body to confirm this. Not the water, but the heaterbody where the high limit is attached. Be careful.