Pentair Mastertemp 400 SFS error light- not heating need help

davedive

Active member
Apr 17, 2017
34
Westport, CT
Hi all!
I know I'm not the first one to have a broken down Mastertemp 400 with an SFS error LED on, but the earlier post didn't solve my particular problem. My propane heater has been working very well for the past few years. Installed in 2013, my first few years had mouse problems but I finally covered all openings with wire mesh and have not seen mice since! Full disclosure; mice lived in the main control box for a few awhile one winter and chewed the heck out of many wires. I got a new wire harness but in the end I simply fixed just the bad wires with fresh splices and have had no problems for the past 4 or 5 years. It baffles me why they can't make the housing mouse proof- but that's another issue. Flash forward to the other day when I found the heater turned of with the service light on. Cranked up the filter pump to top speed and reset the heater- it went for awhile, the exhaust seemed hot, but the water coming out was lukewarm and eventually it stopped and the service light came on. Checked the error LEDs on the control board and the SFS LED was on and temp LCD just said the temp- no error codes. I had a replacement Stack Flue Sensor so I put it in and restarted- heater still stopped. Tested the old stack flue sensor continuity (which looked dirty/sooty on one side) and got a 4.0 reading, which I think is fine, so probably not the problem. Tested the exhaust temp with an instant meat thermometer and got around 350 F, which seems in range. So, far all you heater experts what is my next step??? Going through Pentair troubleshooting steps it says: Check pressure and volume of fuel supply- but how? and all valves are open- would that somehow decrease? It says: Check continuity in wiring harness between Control Board and Probe. Continuity OK? How do I check that? Disconnect the connector from the board and test the connector ends with the meter? Not sure what to do next so hoping someone could help. Maybe I should just replace the whole wire harness with the new one I still have or just go ahead and replace the control board. I don't know. Thanks for your time!
 

JamesW

TFP Expert
Mar 2, 2011
32,445
Turn the heater on. The fan will come on for about 30 seconds, then you will hear a click, and the heater should fire. At that moment, which ever thermostat you are using (POOL or SPA) hold that button down for ~10 seconds. Display should change from water temp to exhaust temp. Normal temps should be in the range of 290-350.


Exhaust Gas Temperature​

While running, press and hold the On button will display the exhaust temperature display.

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.
 

davedive

Active member
Apr 17, 2017
34
Westport, CT
JamesW thanks for the quick reply! It's funny because right after I posted decided to do his test, although I had read you hold down spa on and pool on at the same time. I wasn't sure what to expect but you just need to keep holding down- then you suddenly get a temp readout and it climbs and climbs. Mine kept climbing until it hit 478- then heater shutdown and service light came on. I removed the thermal regulator and put it in water 122 F and the valve didn't really open (pop up) I increased the heat to maybe 128 running it under the tap and it did open- popping up maybe 1/8"-3/16". Cold water made it go down. So...it sort of works? Should I try replacing the thermal regulator? That might be easiest next move. What do you think?
 

JamesW

TFP Expert
Mar 2, 2011
32,445
Do you mean remove he regulator spring etc -just leave in the black cap?
Yes.
And by stack temp you mean the same reading I got before by holding down SPA button?
Yes.

Do you have an external bypass?

Maybe the internal bypass is broken?

If you look into the inlet, you should be able to see the spring part of the bypass.

If you look into the hole where the thermal regulator goes, you should be able to see the plunger disc part of the internal bypass.
 

JamesW

TFP Expert
Mar 2, 2011
32,445
77707-0001_1.jpg

 

JamesW

TFP Expert
Mar 2, 2011
32,445
You have to look into the Inlet to see the spring (blue arrow).

The bypass is about where the other red drawing is.

Look in the thermal regulator hole up on the top to see if you see a disc or just a hole.

1635112626413.png
 
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davedive

Active member
Apr 17, 2017
34
Westport, CT
I think I would have to disassemble more and the sun has set. Wish I had some kind of snake scope camera. I will have to take more apart tomorrow. Right now I've had the heater going for about 30 mins without the regulator and it stays on and heats the water properly- I got my spa from 70 to 90 so does that indicate the bypass valve is OK and only the regulator is bad? Anyway I will shut down and pick it up tomorrow. In the meantime thank you for the help- I feel a solution could be in reach!!!!!!
 

JamesW

TFP Expert
Mar 2, 2011
32,445
The black disc is what you should see if you look into the thermal regulator hole at the top.

Water pressure pushes the disc open to allow water to bypass the heat exchanger.

If the disc broke off, you will just see a hole as shown below.

71Q+TsjkUtL._AC_SL1492_.jpg

1635114001887.png
 

JamesW

TFP Expert
Mar 2, 2011
32,445
OK removed the regulator and got the stack temp- it maxed out at 441 F
That's still too high, which indicates a water flow issue.

Maybe
a broken internal bypass or
an external bypass is open or
a scaled heat exchanger or
the pump is running too slow or
the filter is dirty and blocking too much flow.

What is the pump speed?

What is the filter pressure?
 

davedive

Active member
Apr 17, 2017
34
Westport, CT
he filter may be dirty.
the pump speed is level 3- 2495 RPM with 12 psi at the filter. at this setting BTW the stack temp I was getting was 436F
I could bump up the speed to level 4 - 3270 RPM with 20 psi at the filter. I could see how the stack temp is then.
 

JamesW

TFP Expert
Mar 2, 2011
32,445
That might help, but I suspect that the issue might be a bad internal bypass, an external bypass is open or maybe
a scaled heat exchanger.

If you can look into the thermal regulator hole, you can see if the disc is there.

If you can look into the inlet, you can push down on the top of the spring to push the disc down to make it easier to see in the thermal regulator hole.

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

1635119473191.png


pentair-6290-525dims.jpg
 

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