I have a gas furnace in our home. Recently the furnace gas preheats and the fan comes on but only will run for 30 seconds, when the gas reignites the fan turns off. I changed the filter but that made no difference. I looked at the fuses and while looking discovered the that the hot and common wire marrets had melted. I respliced and reconnected the wires but this has not made a difference either... this morning I tried putting an old style mercury switched thermostat in place but the system still runs the same... has something blown on the circuit board and if so what am I looking for??

Dumbfounded

I had the same problem with my propane furnace.
The problem was that water is a product of combustion, and
It was not draining out of the burner area, properly---due to the
Little drain hose's being frozen.
What brand of furnace do you have?

Quote:

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Originally Posted by eeseely

I had the same problem with my propane furnace.
The problem was that water is a product of combustion, and
it was not draining out of the burner area, properly---due to the
little drain hose's being frozen.
What brand of furnace do you have?

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This is a Bryant furnace, model 394GAD036100... there aren't any drain hoses on this model that I can see?

If the unit only runs for 30 seconds or so and you have a flame sensor try cleaning it.

If you have a 3 wire pilot system check the height of the pilot flame.

Quote:

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Originally Posted by hvac1000

if the unit only runs for 30 seconds or so and you have a flame sensor try cleaning it.

If you have a 3 wire pilot system check the height of the pilot flame.

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I cannot find a flame sensor that looks like a pencil tip. What I do find is a 2 wire sensor type that is just above the burners, it extends inward to the cavity about 3 inches and has a round sensor type on the end. I tried the system again and what it does is this: the system ignites immediately but continues on a long burn(2-3 minutes) before the fan kicks in. At this point the burner shuts off, but as soon as the burner reignites the fan kicks out and a long burn continues?? any ideas?

It is possible the flame is being dropped out by the high limit switch. You may need to adjust the fan control to come on sooner so the heat does not build up in the furnace and kick the high limit into action. See sticky notes at the beginning of this board for location and a picture of a fan/limit control.

Being a Bryant this control might be a fixed disc and in that case you would have to test it or just replace.