heater over temperature

[mitman64]

Island spas (2016 model)
Symptoms are similar to others listed in other posts, but also a little different.

Cycles set to 2. Each runs for 2 hours.

Temp set to 101°

from cold start up everything seems fine. But, the temperature will overshoot setpoint and ultimately reach 107° before shutting off.

I measured resistance of both thermometers and got 2.003 and 2.013 ohms.

The water flows seem very strong, but I took out the filters as a good measure.

Then I swapped the two thermometers A and B at the circuit board only.

Once I did that, everything worked for approximately 8 hours before the Heater Getting Too Hot alarm returned.

Also, before doing all of that I ran a descaler/spa cleaner through the tub for 20 minutes. Then I drained, cleaned and refilled the tub

When I pulled the filters out I noticed one port has a protective grate and the other doesn’t.

I beginning to think the plastic grate was sucked into the system and be lodged close to the heater. Maybe the flow is restricted causing the symptoms. But, I have to drain the tub again to check that.

Side note: I found some white plastic looking substance at the bottom of the tub. It looks like it could be part of a seal, but it seems to crumble under pressure. I don’t know what it is, maybe calcium?

I keep the PH, alkalinity and Chlorine at perfect levels.

Any help or guidance would be great!
Thanks

 

[ajw22]

Lets see what @swamprat69 thinks when he passes by.

 

[mitman64]

How Clear is TFP Clear?

Did you see the pictures I posted?
The water is clear and the chemicals are balanced.

 

[RDspaguy]

measured resistance of both thermometers and got 2.003 and 2.013 ohms.

Thermistors are in the thousands and tens of thousands of ohms. Please retest and post results.
"A" sensor should be the closest to the pump output and heater input.

When I pulled the filters out I noticed one port has a protective grate and the other doesn’t.

Typical for multi-filter tubs. The grate will be the circulation/heater pump.

found some white plastic looking substance at the bottom of the tub. It looks like it could be part of a seal, but it seems to crumble under pressure. I don’t know what it is, maybe calcium?

Sounds like it.

keep the PH, alkalinity and Chlorine at perfect levels.

If I had a nickel for every time I've heard that...
Chunks of scale in the footwell would suggest otherwise.

the temperature will overshoot setpoint and ultimately reach 107° before shutting off.

107 actual water temp? Measure with a cooking thermometer.

Typically if it is overheating on the display but actual water temp in the tub is lower than the display it is a flow issue or pump shutting off while heater is still hot.
If overheating actual water temp quickly it is likely a stuck heater relay.
If overheating gradually it could be a pump or control issue.

 

[mitman64]

Thanks for all the feedback. I think it’s looking like bad thermocouples.
When I switched A & B at the board, the symptoms changed.
Now the heater will work for a while with no errors and the water reaches setpoint temperature of 101°. But, the displayed temperature continues to rise to 107°.
As recommended I used an actual thermometer (one I had from work) and checked the water temperature. The thermometer matched the setpoint of 101°, suggesting a bad thermocouple.
Just for the fun of it I used an IR temperature gauge to check everything. It agreed with my thermometer and the hot tub setpoint.
Tomorrow I plan to change out the thermocouples.

I may take the heater out and inspect it for any scale build up or any restrictions or obstructions.

I’ll post my findings.

 

[RDspaguy]

It's not the sensors. Any significant discrepancy between the sensors triggers an error code on the display, (sn1, sn3, dry, etc) and the odds of both failing identically are astronomically low. They're cheap, so when it comes to throwing money at it they are a good place to start, but probably not the answer.

Now the heater will work for a while with no errors and the water reaches setpoint temperature of 101°. But, the displayed temperature continues to rise to 107°.

Is the pump running? Is the heat indicator on? How long does it take to rise from 101 to 107?
If the answer is "no, no, a few seconds", it's a stuck heater relay. They're cheap and most computer/electronics shops can put them in if you're not a solderer. Label wires and take pics before removing circuit board.
Eventually, maybe years from now, you'll get an "sn" error, at which point put in both new sensors.

 

[RDspaguy]

Definitely inspect the heater. Heavy scale on the element would cause a slower overheat with the pump off, and may not trigger an OH depending on how long it was on.
Can you post some pics of the stuff we're thinking is scale in the tub?

 

[mitman64]

Both original thermocouples were removed and new ones were install.
The two new ones measured 35.5K Ohms.
One original measured 35.5K (once cooled to ambient) and the other was 36.8K.
Everything seems to be working correctly now. Time will tell!

 

[mitman64]

Definitely inspect the heater. Heavy scale on the element would cause a slower overheat with the pump off, and may not trigger an OH depending on how long it was on.
Can you post some pics of the stuff we're thinking is scale in the tub?

The picture with the small white object at the bottom of the tub is all I have.
I inspected the heater while working on the thermocouples and found zero scale in it.
I’m not sure where else to check for build up.
But, all the jets and the waterfall have excellent flow.