I think I know what's going on, but it is just too peculiar:
I have a Honeywell 8300C 4134 gas valve.
The pilot remains lit continuously, and as long as the burner does not start, everything is fine, but once the burner ignites, gas is shut off after a minute or so.
Now, I believe that this is because when the burner lights, the gas delivered to the pilot drops down, allowing the thermocouple to cool and cause shutdown. If I keep the thermocouple hot, the burner runs just fine.
The question is: Why did this suddenly happen? As I said, without the burner lit, pilot has a good flame. I tilted the pilot assembly a little bit to get it more into the burner flame (not too much, since there is only so much availble slop in the assembly without bending parts), and the burner stayed lit longer, but still cut out.
So I think I need to understand why the pilot cuts back allowing the thermocouple to cool. I have adjusted the pilot flame, but it only affects flame size significantly when burner is off; adjusting when burner is lit does nothing.
I am leaning toward replacement of the Honeywell valve. They are not cheap...
thanks!
What type of gas supply line do you have? Is it black pipe or copper? If it is copper, there can be a film like substance that builds up on the inside of the copper. The debris will the get caught in a pre screen where the supply enters the valve thus cutting the supply of gas to the unit. Also, the thermocouple may be bad. You didn't say that you replaced it. Try replacing it as well. Also, you could have a limit switch turning the system off and thus cutting the gas off completely but it is highly unusual for the pilot to be cut off as well unless the pilot is being blown out when the gas is shut off. Also, turn the unit to fan only without calling for heat. Using a flame of some sort, put it into each baffle of the heat exchanger. If your flame is being blown around in one of the baffles, then you have a hole in the heat exchanger and what is happening is that the roll out switch is cutting the unit off and the air still moving through the heat exchanger is blowing the pilot out. I have seen this actually happen. If your furnace is in excess of 15 years old, it does need to be checked for possible holes. If it is in excess of 25 years old, it is a huge possibility that the unit has already developed a hole. If you feel hair coming out of one of the baffles, then shut the unit down immediately and have it replaced!
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Thanks for the reponse.
My lines are black pipe. I tried two different new thermocouples, same thing for either of them.
The furnace is a Burnham series 2 boiler- there is no blower. I did get in and clean the dust off the burners.
The flame roll out switch does not cause it- I bypassed it and the flue sensor too and the problem still happened.
At least I don't need much heat now. This is a real brain teaser!
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