Master Temp 125 heats a min then shuts down and shows AFS

AquaChic

In The Industry
Aug 20, 2021
1
Shreveport, La
I need help with a Master Temp 125 natural gas heater. It fires up and runs about a minute then shuts down and AFS lights up on the control board. I checked the gas pressure, it's good. I replaced the Air Flow Switch, Gas shut off sensor, Hi Limit sensor, thermistor, and thermo-regulator. It has a sand filter (TA100) and a 1hp FR whisperflo pump. I've backwashed and the sand isn't old. It has good prime so I don't believe it's a water flow issue. Now what???
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
28,522
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
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1poolman1

In The Industry
Jul 14, 2014
114
Sacramento
The Mater Temp heater has a sealed combustion chamber. Usually, that issue is a failed internal bypass assembly allowing too much water to bypass the heat exchanger. I've done 3 this year alone. The manifold has to be removed and disassembled for that repair. While you're at it, replace the thermal regulator kit as well. When one goes the other is usually bad as well or will fail shortly and cause the same issue. Its worth the added expense.