Pentair, 400 spa and pool heater High limit switch fault light on.

[n6736v]

Hi, and thank you in advance for your response. Our Pentair 400 spa and pool heater ignites briefly about 3 to 5 minutes and then the heat turns off but the blower continues to run. The service heater light appears. No error code on display panel. When examining the error lights on the back of the printed circuit board, it appears that the high limit switch fault light is on when this problem happens. is there any way to distinguish between the fault being due to a defective sensor versus the valve. The former is easy to replace, the ladder will be quite difficult as the manifold bolts are quite corroded.
Thank you again.

 

[ajw22]

Welcome to TFP.

It is rarely the sensor. The corroded manifold bolts indicate that the thermal regulator and bypass valve are probably corroded.

Read Pentair MasterTemp Heaters - Further Reading

Start by checking the thermal regulator.

 

[n6736v]

Thanks for your quick response. It’s cheap to change out the sensor. I’ll do that and report progress. Happy new year.

 

[JamesW]

You can put the sensor in water and slowly heat the water to see where the sensor opens.

The high limit switch (HLS), which opens if the heat exchanger outlet temperature goes above 135° F (57° C).

Pentair MasterTemp Heaters - Further Reading

www.troublefreepool.com

 

[JamesW]

https://www.pentair.com/content/dam/extranet/nam/pentair-pool/residential/heaters/mastertemp/manual/mastertemp-heater-manual-english.pdf

 

[JamesW]