Mastertemp 250 AFS


New member
Dec 7, 2016
Amarillo, Texas
Hey all, Have a different issue that I have not read about on the forum in regards to the AFS code. BTW, you all have helped me through a lot of other issues over the years. So, here it goes: Mastertemp 250 LP, Since the last repairs, heater ran great for 2 years, I just replaced the Thermal Regulator, while doing that noticed a leak, replaced the Heat Exchanger and while in there had to replace the Flameholder and all gaskets. The unit fired right up and ran for 1.5 hrs. Now, after it runs and heats, it shuts off prior to reaching the selected temp and the AFS led illuminates. If I shut off the unit and start it again, it starts normal and goes through the same process of heating for a while (.5 - 2hours), normal operation, then shuts down and throws the AFS code. Inspected all wiring, blower, AFS and all seems to be okay. The temp of the water right now is about 72F and set point is 80. While the heater is on it reaches about 300F which I understand is in the normal range. Anyway, the heater runs fine until it doesn't ;-) I am thinking it may be shutting down because it thinks it is reaching the set point at some time? If I am thinking right, what component drives that and why the AFS code? I dont understand what the AFS would have to do with it.


Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60


Well-known member
Mar 30, 2019
Las Vegas, NV
Either you are not developing enough differential pressure to close the AFS (there can be numerous problems that can cause this) or the AFS is bad. The easiest way to tell is with a dual port digital manometer. The AFS for your heater needs to see a differential pressure greater than 1.25 inches WC at the pressure switch for it to close. If you measure greater than 1.25"WC and the switch does not close, the switch is bad. If you don't measure greater than 1.25"WC, then you have to isolate the problem that is causing the low differential pressure.


New member
Dec 7, 2016
Amarillo, Texas
Thanks for the responses...With the issue being more complex, it seems, from other issues I have had, I decided to get the pool to the desired temp and because resetting it works, everytime for now. I just check it now and then, if its not running just reset it. I will hook the manometer up and hope it fails while I am checking it, because it runs normal for quite a while. BTW, issue has not presented itself yesterday or today, heater is running normal, for now. Maybe after I got it all put back together some debris got inside the can and was causing a disruption in airflow after it ran for a while, IDK, but this blog keeps making me smarter on these heaters, well at least issues are becoming easier to isolate. Thanks.
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