MasterTemp 400 weird heating behavior

Pesh

Gold Supporter
Apr 19, 2020
197
Austin, TX
Surface
Plaster
Chlorine
Liquid Chlorine
I have a MasterTemp 400 (MT400) that is connected to an IntelliCenter automation system. The heater has been working well for years until the last few days.... I decided to troubleshoot this evening and sat for over an hour pushing buttons on the heater... long story short...

I would turn on the heater (thru automation) and it would shut off after heating a few (~5F) degrees. So, I tried manual interaction with the MT400. Start temp was around 60F with a set temp of 100F. Approximately every 5 degrees F in gain, the heater would shutoff. I would manually force the heater to thru back on and it would, then the heater would fire up and gain another ~5 degrees F and the shut off. I would keep manually forcing the heater to cycle (60F -> 65F -> 70F -> 75F -> 80F -> 85F -> 90F)... After over an hour of forcing them temp up the heater finally stablized and from about ~95F to 100F the MT400 started to behave.

I doubt my MT400 its out of it funk. I suspect tomorrow when the water temp returns to 60-ish, I'll experience the same behavior.

Any guidance on what is the root cause (assuming its a failing sensor of some sort) and how to fix?
 
How old is the heater?

Do you have a 5 button or 6 button keypad?

Check the thermal regulator and internal bypass valve.

When did you last clean your filter?

To answer your questions:

How old is the heater? - Old, I think it is from 2012.
Do you have a 5 button or 6 button keypad? - 5 button (Up / Down / Spa On / Pool On / Heater off).
When did you last clean your filter? Long story, but it has been over a year but gets regular backwashes.
 
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