Pentair Mastertemp 400 SFS error light- not heating need help


TFP Expert
Mar 2, 2011
the pump speed is level 3- 2495 RPM with 12 psi at the filter.

If we estimate the total head loss at 37 feet, the flow should be over the minimum of 40 gpm.

So, I suspect that the pump speed is fine.

Can you check the pump system pressure, flow in GPM, watts and rpm from the keypad display?

You might need to put the automation into Service mode to make the pump work from the keypad.



TFP Expert
Mar 2, 2011
How old is the heater?

Can you show the entire system?

Can you show the heater tag with the barcode and the date?



Active member
Apr 17, 2017
Westport, CT
JamesW: OK I'm back with photos and updates...
I put in the new regulator and the heater ran without stopping. The pump speed was level 3, 2495 RPM and the stack temp maxed at 420F- but it didn't go above that and didn't stop. I consider that "working" but I know that stack temp is too high so I have to figure out why. You asked for some more info/photos so here you are:
1. I can't see GPH or GPM I don't think my pump allows that
Here is what I see after disconnecting the heater and looking in the pipes
inlet pipe.jpg
I see the things you mentioned but still can't tell if that valve is broken and can't really see the little disc inside the regulator tube
Here is an overall view of equipment
pool equip.jpg
Here are the numbers off the equipment
pool equip nums.jpg
so that's what I've got. I don't know if that will help you help me but I want to say thanks again for getting me this far!


TFP Expert
Mar 2, 2011
You can see the bypass disc here.

It looks ok.

If you push down on the top of the spring, the disc should move down and it will be more obvious.

The bypass comes apart, so be careful not to push on the spring in a way that could make it come apart.

stack temp maxed at 420F-

Here is what the normal exhaust temperature should be:
  • Below 250 degrees...very low
  • 250 to 290 ..................low
  • 290 to 350.................acceptable
  • 350 to 480 ..................high
  • Above 480...................error/shutdown.
Note: HD models can be up to 75 degrees higher. HD models use a cupro nickel exchanger and the efficiency is slightly lower resulting in less heat transfer and more waste heat.

Your heater is not a HD model, so the stack temperature is too high.

This part will unscrew if you need to get a better look at the internal bypass.

Last edited:


TFP Expert
Mar 2, 2011
I would take the filter apart and clean the filter really well and also check the salt cell to see if it is clean.

If the cell has scale, that will slow down the flow and it will also indicate that the heater probably also has scale, which might be part of the problem.

You can descale the heater if necessary by running a descaling solution through the heat exchanger.

Make sure that the descaling solution is copper safe.

You have to remove the thermal regulator to allow the mix to flow freely.



TFP Expert
Mar 2, 2011
For descaling, you can unscrew the fitting and then screw in an adaptor for the hose.

Don't descale unless you think that it is necessary because the solution will eat part of the copper and shorten the life of the heat exchanger.


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TFP Expert
Mar 2, 2011
Salt cells scale up due to hydroxide created in the cell.

Heaters scale up due to high temperatures in the heat exchanger due to the thermal regulator blocking flow until the water is over 120 degrees.

If the cell is getting scale, the heater is probably getting scale.

Below, you can see the increase in CSI from 84 degrees to 134 degrees.

Maybe get PoolMath and track the CSI to keep it in the -0.3 to 0.0 range.



TFP Expert
In The Industry
Jul 6, 2011
Before trying the descale process of the heat exchanger, pull the header off and see what the ends of the copper tubes look like. You may or may not see scaling or worse. Also if you had a scope to run thru the heat exchanger you could see what it looks like inside the tubes. The heat exchanger could possibly be sooted up and a cleaning would be needed.
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