Issue with a Mastertemp 250 temperature reading. Have already replaced all sensors, still stumped.

I have already replaced all sensors and the thermistor twice, so its not those. So the problem is when the heater is not running the temperature of the actual spa and the heater unit are the same(working). As soon as the heater begins running, the spa temperature will begin rising and the heaters display temperature will drop. For example the spa will be at 101 and still climbing, while the heater reads it at 80F, and therefore thinks it needs to keep heating to reach a set temp of 99 as an example. I originally found this out when I set it to 100 and the water had actually reached 108, with the heater still running and its temperature showing 88F. It feels like its a valve mixing issue or something along those lines. When the temperature in the unit heats up a bypass opens, but this water is mixing in the temp probe area, cooling it down. I've tried to look at diagrams of the manifold, but this mixing doesn't really make sense. As of now I'm stuck. Does anyone have any ideas or thoughts on the cause of this issue? A flow issue? Broken Manifold? Maybe the board(doesn't seem likely)?

When trying to research it seems most people have the opposite problem. The display temp is hotter than the actual, and therefore the heater won't kick on. Mine is the opposite.
 

ajw22

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Jul 21, 2013
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No automation unit. The heater display board itself. Temp set on the heater unit. Spa Temperature was measured at the spa itself with a laser thermometer and a floating thermometer. I'll create a signature now, but its a pretty basic variable pump, filter, heater spa system. Thanks for the reply, Any ideas?
 
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ajw22

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Is this a spa only, no pool?

What speed are you running your VS pump?

For a test replace the thermistor with a 10K resistor.

If the temperature stays at 76F then it rules out a bad board.

If the temperature drops you probably need to check the wiring from the thermistor to the board, and maybe replace the board.

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Appreciate the response. Yea just an in ground Spa about ~100-150ft from the pump/heater. I did look into the thermistor before this finding that it goes from 10k ohms to 5ohms as the temp rises, but hadn't gone out to test it yet. Figured I'd ask here first. I'll head off to home depot for a 10k and 5k resistor and check if the boards good. Thanks.
 
And I have run the pump at its normal 3100,2600,1850 rpm. Even trying to run it on the overide of 3450 to see if the flow was the issue. It still has the same temp issue. I'll give the board a check and report back.
 

ajw22

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Jul 21, 2013
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Do you think it's worth taking off the manifold and looking for anything strange going on?

If you are going to go through the work of removing the manifold then get a new bypass valve and thermal regulator to install while you have it open. And get the O rings for the manifold and the large rectangle O ring as they don't always seal back up after use.

Here is what I don't understand about your problem. The spa actual water temp is 101, the heater is running putting more heat into the water, where is there cold water that would reduce the water at the thermistor to 80? The input water temp into the heater is 101 and the output is something over 101. There is no colder water anywhere to mix?????

I know you said you replaced the thermistor twice. Are you sure you got a Pentair original part and not a knock off? I continue to suspect the thermistor as I don't see how the actual water temp where the thermistor is located can be 80F.

I would run the heater with the 10K resistor installed and attach the leads to an ohmeter to the thermistor. See what the resistance is as the water heats.
 
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Poolbreh

In The Industry
Jul 31, 2021
272
San Diego
Do you have pictures of the equipment? Is this a new problem or was the problem always there? 150ft is a long long way does the spa have a spillover tank? Can you take pictures of the spa?
 
Thanks Ajw22. That's what brought me here, it doesn't make sense. Both replacement thermistors were official pentair parts, I'm not sure about the thermal regulator I can check again which one I purchased, although I did do a 120F water test to see if it opens(which it did). Like you said there shouldn't be any cool water mixing in order to drop the temp down, but that's all I was left with. That is a smart idea. I'll definitely try running with a set resistance, and just checking the resistance across the thermistor as the systems running.

And Poolbreh. I have checked from the heater drain plug, while the systems working. and the water going to the unit will be the same temp as in the spa(actual temp), while the unit will show ~10-15F lower. The unit will keep heating up, thinking the water's cooler than it actually is.

And 150ft might seem far, but I've had the same system for years, with little to no issues in the past. One day I set the temp to 101, and by the time I got in it was 108. From then on it's been acting this way, and I'm in this situation now.


I will attempt to run through every step again this week(when I'm free), and see if I've missed something throughout this whole process. Maybe there's been an error on my part I've overlooked. Thanks again guys.
 
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Poolbreh

In The Industry
Jul 31, 2021
272
San Diego
The heater drain plug could be different from say the filter air relief or if you have spigot before heater to fill a cup. There is nothing in the manifold that would allow say one side be 80f and the other 108f. With the resistance test it proves the only logical explanation is a bad thermistor. Unless there is something that hasn’t been shown or described. That’s why it’s probability best to show a pic of equipment and spa. Your situation is unique. It may be you had a bad thermistor and replaced it with a lemon. It’s hard to be certain without seeing the big picture. Things you may not seem significant could be.
 

MSchutzer

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Jan 15, 2021
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Redwood City, Ca
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Here’s another idea. Maybe the linearity of the control board is off such that the resistance change to temperature display change is off.

Since the temperature is stable at 76 with a 10k resistor installed the control board is reading correctly at that point. While an unlikely problem, the temperature tracking of the control board could be verified by the temperature reading at another resistance point.

If you clipped two 10k resistors in parallel (5k equivalent) across the temperature sensor contacts the display should read about 107 degrees. This quick test will verify the temperature tracking of the main board at a second point.

Since the temperature reads correctly with the 10k in place the board is probably okay, but it won’t hurt to verify this at a second resistance point that is closer to the spa operating temperature.

Even if the calibration of the main board is off this doesn’t explain the temperature dropping when the water temp is rising. If the temperature is also stable and correct with a 5k resistor then the main board is fine for sure.

If an external voltage potential is introduced into the temperature sensor wiring that could cause strange behavior. When you replaced the temperature sensor does all the wiring between the sensor and the control board get replaced, or does the new sensor get spliced (or connected) to the end of a wiring harness?

If all the wiring isn’t replaced with the sensor then it’s possible that the wiring harness insulation is nicked somewhere allowing one of the sensor leads to make contact with something, most likely the enclosure of the heater that is at a ground potential. An external voltage potential connected to one of the thermistor sensor leads could cause errors like you are seeing. I know this is increasingly unlikely, but if the main board is okay and the sensor is new, then something else is going on. Now if all the sensor wiring is replaced with new sensor then you can rule this out.

I hope this helps.
 
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