Pentair tempmaster 400 heating up to 88 than turning off

silvrzuki77

New member
Dec 29, 2019
4
las vegas
Hey everyone, New member here. I am having a issue with my heater. The unit is about 3 years old. It will fire up and get up to 88 degrees and then turn off and will try to cycle on and off. The AFS light will flash on and off but not consistently but no service heater light. I have recently replaced the thermal regulator as well. I do notice that the temp on the heater 104 does not match the easy touch controller as its warming up. Pulled the cover and checked all the wires and connections everything looks good. Checked exhaust temp never went above 270 degrees. Need some help!
 

JamesW

TFP Expert
Mar 2, 2011
17,004
Is 104° the water temperature or the set temperature?

If it's reading the temperature incorrectly, then you probably need a new thermistor.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
11,760
Northern NJ
Welcome to TFP.

What type of filter do you have? What is your filter pressure? When was the last time the filter was cleaned?

Symptoms you describe can be caused by low water flow.

What type of pump do you have?

It would be helpful to create your signature with details of your pool.
 

PoolBrews

Well-known member
Oct 16, 2019
90
Florida
The symptom he describes is that the heater works until it hits 88, then shuts off. I don't see this being caused by low flow. Low flow is not dependent on heat setting. I'm thinking JamesW is on the right track with the thermistor if the panel reading does not match the actual temp of the water.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
11,760
Northern NJ
The symptom he describes is that the heater works until it hits 88, then shuts off. I don't see this being caused by low flow. Low flow is not dependent on heat setting. I'm thinking JamesW is on the right track with the thermistor if the panel reading does not match the actual temp of the water.
Low flow causes flue sensor overheat. Not enough heat is being absorbed by the water flow.

AFS will not come on with a thermistor problem.

I have witnessed the problem.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
11,760
Northern NJ
I'm thinking JamesW is on the right track with the thermistor if the panel reading does not match the actual temp of the water.
I don’t think that is what the OP described. The heater display shows 104 because that is what the heater setpoint should be when controlled by automation.

Start with the simple solution before you begin replacing parts.
 

silvrzuki77

New member
Dec 29, 2019
4
las vegas
Wow first off thanks for all the responses!!!!!! So I have the spa set point at 104..... Sooooo I am thinking its the thermistor... So after I posted, I pulled the thermal regulator to make sure I had it correct or what not. Its the only part I've replaced lately besides cleaning the filters 2 months ago. I put silicone grease on the threads and gasket and was letting it run again to see if anything had changed. I checked the exhaust temp which was a steady 273 degrees while running. All of a sudden the spa water temp raised to 98 degrees! We were getting ready to leave for dinner so I shut the spa off. After getting home I fired the spa back up to see if it would work again. Sure enough the temp got to 98 degrees but stayed there. I checked the actual heater which gave me a temp of 110 and a set point of 104. After about 30 min or so the spa temp drop to 95 and the heater kicked on this time going to up to 104!! The temperatures seemed to match on the heater and spa water temp. So Im assuming the thermistor is acting up? Any other thoughts? Ill post my pool equipment in my signature tomorrow after our mining trip to the desert with my boys. Thanks again for the help.
 

silvrzuki77

New member
Dec 29, 2019
4
las vegas
Well just tried it again sat for an hour or so. Temp got right up to 104 and shut off. I wondering best case scenario I didn't have the thermal regulator seated correctly? I should have the thermistor and high limit switch coming tomorrow. Got a Combo buy.
 

JamesW

TFP Expert
Mar 2, 2011
17,004
A bad thermal regulator would usually cause a high limit error.

You might have a low flow issue.

Check the flow and make sure that it's good.

If the flow going through the heater is good, I would suspect that the thermistor might be going bad.

You can remove the thermistor and check the resistance vs. the temperature to see if it's reading the correct resistance vs. the temperature.
 

Pool Clown

In The Industry
Sep 5, 2008
2,016
Silicon Valley, CA
Typically, when the temp sensor goes out, you will get a normal temp when first turned on, then after a few moments climb to a wildly inaccurate temp, i.e. >126 degrees. I may also jump around. If you remove the sensor, first look to see if it is the old type (brass) those used to crack, usually along the length of the sensor. Your replacement should be Stainless. If it isn't, dont buy it. Someone has some old stock and is unloading it, maybe for cheap. Don't get lured in.

It is normal for the AFS light to come on momentarily until the blower spins up, moves air, and satisfies the switch (AFS).

Low flow will first trip the pressure switch, high limit, or even the AGS before it will open the stack flue switch/sensor (SFS). The SFS is supposed to monitor fire box temps for improper fuel mix, a sooted exchanger or a rats nest. Things like that.

Hope this helps.
 
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silvrzuki77

New member
Dec 29, 2019
4
las vegas
Well I definitely had an inaccurate temperature reading during the heating process! I replaced the thermistor and high limit switch since i had it. Everything seems to be working perfect! Used the electric thermometer and the water temp was accurate to the heater temp now! Luckily i did get a stainless steel one. Ill get a picture of the old thermistor the corrosion was kind of weird pitting in two spots.... I did have a period of high phosphate issues awhile back.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
23,029
Laughlin, NV
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