Pentair MasterTemp 400 temperature reading wiggling around, causing heater to cycle rapidly - bad thermistor?

Pegasus_RPG

Active member
Jun 10, 2019
36
Las Vegas, NV
Hey everyone.

I just replaced my MasterTemp 400's thermistor in March (old one was reading 126F so was definitely bad) and this one has been working fine all season until this week when the heater started cycling on and off within the span of a few minutes, now down to a few seconds! The temp is reading 5-10 degrees above the actual water temperature and it wiggles around that value rapidly (number changes by one degree either way multiple times a second.)

Does this also indicate a bad thermistor?
 

setsailsoon

Gold Supporter
LifeTime Supporter
TFP Guide
Oct 25, 2015
3,873
Stuart/FL
Pool Size
12800
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
CircuPool RJ-30 Plus
Did you buy a "generic" 10K thermister or the original part? Quality varies a lot with generics and even some OEM's.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
24,985
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Sounds more like a low flow problem and the water is boiling inside the heater causing the tenp to vary and trigger the HLS which shuts it down. When it cools off it firs back up and repeats.

What speed are you running your pump? Increase your pump speed by 300 rpm and see if the behavior changes.

What is your clean filter pressure and current filter pressure at a specific RPM?

Test the thermistor using a multimeter....

 
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