Exhaust hot on my heater

[whitemok]

I have pentair sta rite heater sr 400hd.
My heater is somehow getting too hot. Knocking off at 350- 450 degrees. O know its suppose to knockoff at 480 degrees. All my sensors are good and it is igniting no problem. Thermal regulator is good.
No red light or any diagnostic light comes on. Im thing of pressure washing the tube sheet.. what else can i do?

 

[JamesW]

If it's shutting down due to high stack flue temperature, then it should be showing E05 on the display and the SFS (Stack Flue Sensor) light on the back of the board should be lit and the Service Heater light should be on.

The 400hd can run hotter than the regular 400, but I don't know why.

It might be a sooted exchanger, a calcified exchanger or low water flow.

The SFS might be going bad. At 77 Fahrenheit, the resistance should be 3.8 megohms.

 

[whitemok]

Sfs was tested 3.8 ohms.
Presuure wash the exchanger.
Water flow is good.
Out of options too.
Thanks for trying to help

 

[JamesW]

If it's shutting down due to high stack flue temperature, then it should be showing E05 on the display and the SFS (Stack Flue Sensor) light on the back of the board should be lit and the Service Heater light should be on.

Are you getting any of those indications?

What's your chemistry?

Have you had any scaling anywhere?

Most likely, you're getting low water flow through the exchanger. It might be scaled or water might be bypassing the exchanger.

 

[PoolguyinCT]

If it's shutting down due to high stack flue temperature, then it should be showing E05 on the display and the SFS (Stack Flue Sensor) light on the back of the board should be lit and the Service Heater light should be on.

The 400hd can run hotter than the regular 400, but I don't know why.

It might be a sooted exchanger, a calcified exchanger or low water flow.

The SFS might be going bad. At 77 Fahrenheit, the resistance should be 3.8 megohms.

I had one drive me nuts last year (2 visits) & for James’s description, eo5 etc.

Ended up replacing top board & was good to go - control pad was set for the “other” sfs style sensor.

Incidentally, I found a small fortune in flue stack sensors in the heater case left behind by former service firm.