The pilot lights easily, the burner fires and burns for about 5 minutes and then everything shuts off including the pilot light. A service man has given up after replacing the pilot assembly and the high limit switch. Any suggestions...
Most common cause of the described symptom overheating the thermocouple! a thermocouple produces millivolts of electricity based upon a differential of temperature between the tip and the base. If it gets too hot it will not work correctly. If the pilot flame is too large or the main burner flame is contacting the thermocouple or the thermocouple is too cheap or old all can cause this issue.
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yes this would appear to me to be the classic symptoms of a bad thermistor , this little device proves that fire has lit (via heat) and keeps gas valve open , otherwise gas valve times out and shuts off (safety)
Does the pilot light stay lit if you continually depress the control valve?
If the pilot light stays lit as long as you keep the valve depressed, then the problem is the thermocouple is bad. The pilot light makes enough electricity via the thermocouple to hold open the gas valve that feeds the burners. If the thermocouple fails to produce that electricity, the gas valve will shut down and switch off the gas flow to the whole unit.
99% of the time it is just dirt and dust in the pilot light. NOT a bad thermocouple! These units generate their own power through the pilot light and it has to burn perfectly. Turn off gas supply, remove the cover and the pilot light assembly, including disconnecting the tiny gas line from the gas valve. Blow compressed air in, around and through it to get dust out. Replace everything, use soap bubbles at gas fittings to leak test and light the pilot. I bet it stays lit.
Thermostat flame sensor. Do you see a little rod next to the pilot? That is what tells the main gas valve that the pilot is lit and it is safe to give gas. Easy swap, and you can usually just pick up a generic one.
The first thing I would do is make sure the flame sensor is clean, this is a metal rod immersed in the burner flame on the opposite side of the igniter. If too much oxidation builds up on it, it will not allow the burner to stay lit. To clean, just turn off furnace and remove flame sensor and clean with steel wool and then reinstall. If this does not solve the problem there may be a problem with the gas valve.
I would first check the gas pressure to your furnace. Then make sure that burners are clean and watch flames when burner first ignites. I have seen burners that were dirty cause delayed ignition that would create a suction when they came on and extingush pilot and burners.
The next thing I would do is change gas valve. Thank you
The thermocoupler is no good. It's that bulb sitting in the pilot flame. when it goes out it shuts everything doen in the main valve. It has to be kept hot by the pilot, that's how it know whether or not it is safe to open the main .