I have a Key Largo Spa manufactured in British Columbia by York Leisure Products (no longer in business). We bought the unit new in 1992. It worked fine for many years with few problems. It has always been used daily except in the summer. One day late last spring I went to use it and the temp was 106! We had it set at 99 degrees, and it always maintained that temp within +or- 1 degree. It has a manual temp setting (knob) at the spa pac, and we use a floating thermometer. I tried adjusting the temp to a lower point on the dial, but could never get it to maintain a usable temp. It was always way too low or way too high. I called a repair guy we had used before, and he replaced the thermostat on three separate occasions without fixing the problem. After his last visit, he quit returning my phone calls. I finally drained it in June (we never use it in the summer). Now, after I filled it this fall we still have the same problem. The temp is either at 96 or 106 – nothing in between. Note: the thermostat is part of the heat control. It has a long, thin copper tube that fits into the heating chamber. There seems to be a dead spot in the middle of the temp range, where once the control is adjusted past it, the heater just stays on until the high limit control shuts it off. This unit does not have a timer – the pump runs at low speed continuously. Any ideas on what’s wrong?

Sounds like a bad thermostat to me, but another thing to look at is where the thermostat bulb goes into the dry-well. A lot of times of that dry-well is exposed to ambient temperature and you will get fluctuation on the thermostat. Once the bulb is inserted in the dry-well is needs to be sealed off, some come with o-rings for this or some silicone will work. It also helps to have some heat sink on the bulb.

I think it's very unlikely that the thermostat is defective. We are on the 4th thermostat since the problem began, and the symptoms have never changed. Also, there is a rubber grommet at the point of entry where the capillary tube enters the heat chamber. It's tightly sealed. I watched the service man when he removed and inserted the capillary. It seems that once the heater kicks on, it just can't stop until the high limit switches it off. If I turn the dial down far enough, it never goes on. Is there a solonoid that switches the heater on, and then could get stuck? Just two nights ago I had gotten the heat up to 100 (by turning the dial up to the point when the heater light went on, and watched it until it reached temperature, then I turned the dial back just slightly to make the heater switch off). Then I got into the spa. After a while I noticed the temperature was rising. I got out and turned the temp dial slightly down again until the heater light went off. When I checked it the next evening the temp was up to 110. I'm baffled by this.

The relay/contactor that feeds the voltage to element does not sound like a plausible issue. If the contacts were stuck closed the thermostat would be irrelevant. They can stick intermittently but by your description is sounds like you have all or nothing, which leads me back to the thermostat or it getting ambient temperature on the bulb or it needs some heat sink.