It has been taking about 3.5 hours to bring my spa (I do not have a pool) up to a temperature of 102 deg. on a couple of occasions. Of the last four times I have used it, it heated up fine in 1.5 hours, the next time took 3.5 hours, the next time 1.5 hours, and most recently 3.5 hours.

I checked out the heater and the Heating and Spa LED lights up when I turn it on.

I had someone come out and look at my spa. His feedback is that my heater needs a new thermal regulator and thermistor. I am not so sure this makes sense. I changed the thermal regulator myself about 2 years ago. Could it go bad again that quickly? Could the actual issue just be the thermistor?

We are in So. Calif. The water temp. is usually in the mid 70's when we start the heater.

To summarize, sometimes I start the heater and it does not consistently heat the water and takes too long to complete the process. Additionally, the service light on the heater does come on when the heater does not work properly, but I do not see any error codes. If I turn off the heater, wait, and turn it on again, the service light does not come on and it works fine.