Sundance optima (I think), about 4 years old. Starting a couple nights ago it goes into overheat constantly. After resetting serveral times I have determined that once it turns the heater on, it never turns it back off (although the control panel says the heater is off). It is still heating and will continue to do so until the spa overheats (or until the circ pump is turned off and the heater overheats). Any help is appreciated!

Tripp

Tripp, Based on what you have posted, it sounds like a sticking relay or contactor. Do a voltage check on the coil of the relay or contactor, what you are looking for is when you turn down the heat, the voltage to the coil drops out. You will then check for voltage on the heater element, when the coil voltage is 0, you should not have any voltage on the heater element. Always check voltage with one meter lead on each voltage connection at the same time, i.e. place your meter on both heater lead terminals, one red, one black, and read the voltage.

What jeff said...

One other thing to consider... If the spa is not GFCI protected, or has a faulty GFCI..... read on....

Spa heaters are sometimes wired with a hot "common" going to one pole of the heater (which is always hot), and a switched hot (off the heater relay) going to the other heater pole to complete the circuit.
If the heater has grounded, the common hot will continue to energize the heater, albiet only 110v, but enough to overheat the spa if there's flow, and enough to overheat the heater and trip the heater high limit...

Good luck,

~Swine

Good point Swine, that was something I failed to point out.