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

[Pegasus_RPG]

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]

Did you buy a "generic" 10K thermister or the original part? Quality varies a lot with generics and even some OEM's.

 

[Pegasus_RPG]

I got an original, but off of eBay. It looked like it had been sitting around awhile, but still in the sealed bag.

 

[ajw22]

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....

Temperature Sensors - Further Reading

www.troublefreepool.com

 

[Pegasus_RPG]

I do need to clean the filter. Ordering a new set of Pleatco cartridges today as the bands around my current ones are starting to break.