Pentair Minimax NT reading wrong temp, won't run long

plsbcheap

Well-known member
Jun 24, 2007
75
I've read the posts on other threads about hi-temp sensors and thermistor but wanted advice before I order a part today. Background: I haven't used heater in almost two years. I noticed about 3 weeks ago it said my water temp was 99 or 101 on the HEATER. I have an EasyTouch system which shows correct water temp so I knew it wasn't right. I also read on here about the Trichlor tabs causing problems. While my pool was always salt water... two seasons ago (LIKE AN IDIOT) I tried out the Pentair tab feeder for one season. Hated the upkeep and everything about it. New liner installed last summer and went back to SWG. Based on the info here that Trichlor may have screw up my heater. My question is what the likely first part to replace is?

I fired the heater up but it will only run until the "heater display" said 107 degrees. As it cools back to about 100 it fires again and warms it up until "heater display" say 107 and stops again so I think all the other stuff may be good. My EasyTouch reads water at about 85 degrees while spa is still set to 104. So my EasyTouch says 85d / 104d (which left number is actual and right number is what the setting is on). I did replace the temp probe on the EasyTouch a couple of years ago so I think it is very accurate. Any guidance on how to narrow down which parts to replace first? 1) High-Limit Thermostat - 115F part number 471587,
2) High-Limit Thermostat - 150F part number 471694f or 3) Thermistor Probe part number 471566 ?
 

ps0303

TFP Expert
In The Industry
Jul 6, 2011
4,042
FL
Sounds like a thermistor issue. When you first turn the heater on, what does the temp read? If it's a high temp and the water isn't that warm, thermistor. A limit sensor doesn't read out n the display. So that is no your issue.
 

plsbcheap

Well-known member
Jun 24, 2007
75
Sounds like a thermistor issue. When you first turn the heater on, what does the temp read? If it's a high temp and the water isn't that warm, thermistor. A limit sensor doesn't read out n the display. So that is no your issue.
I'll have to try it tomorrow to see what it does. From memory I think when I stop the pump or flip off the power to the heater and back on it has dashes for a few seconds and then reads 101 or some temp up to 107. I've seen it 99 but I don't think any lower. Water temp is probably only in the low 80s. I have a new probe inserted into a pipe connected to the EasyTouch display and it says approx 83 degrees. I believe that to be accurate.

So bottom line is the thermistor is what actually reads the water temp and sends it ultimately to the display?
 

ps0303

TFP Expert
In The Industry
Jul 6, 2011
4,042
FL
The heaters thermistor is what reads the water temp and displays it on the pool heater.
 

plsbcheap

Well-known member
Jun 24, 2007
75
The heaters thermistor is what reads the water temp and displays it on the pool heater.
Took it apart today. It was just showing a high temp, 99-104 when I started working on it. I read somewhere about how Trichlor tabs negatively affect the Thermistors which made me think about two years ago when I installed the Pentair feeder for tablets. I even installed a Jandy check valve between the heater and the "feeder" but I hated dealing with the chlorine tabs so late last summer I switched back the the SWG. I thought about this feeder and wondered "I'll bet that heavy concentrate of chlorine corroded the Thermistor. So I pulled it out today, wire brushed it, then took some sand paper and polished it and reinstalled it. The temp immediately read 81 which was the same temp as the EasyTouch probe reads. I hit my button to heat the spa to 103 and the heater fired and warmed it up to 103 and it cut off. WORKS LIKE A CHAMP as we say. I just hope it lasts but if not it is an easy fix. Thanks!!!
 

plsbcheap

Well-known member
Jun 24, 2007
75
What an awesome follow up! SWEET!
Kim:kim:
Well, maybe not so fast.... The next day the temp creeped back up to 99 when the pool was really only about 82. So I figured it was just bad and ordered a new probe (themistor). It came in and I decided to unscrew the old one and replace it. As I turned the old one out about 2 turns the temp dropped to 82???? So I stopped and left it there. No leaks and it is holding at the correct temp for two or three days... I will wait and see but I think it may have just been in too deep...