Bryant gas furnace flames going on and off during operation


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Old 09-14-13, 10:37 AM
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Bryant gas furnace flames going on and off during operation

Bryant gas furnace installed the mid 1990s
PRODUCT NUMBER: 394 G A Z 036 100 A A EA

Image of Circuit Diagram on back of furnace access door:
Furnace circuit diagram | Flickr - Photo Sharing!

The furnace is only used for heating, there is no central AC in the house. The thermostat is a Carrier Touch N Go Comfort Series it has a C wire connected.

To test the furnace for the new heating season I set the thermostat very high and ran the furnace.

The furnace appears to start up from cold normally.

After about 10 min the burner tube flames cut off and restart every 3 seconds or so.

After about 5 min of the flames cutting off and coming back the blower shuts down. The Thermostat still calls for heat so the pilot lights and the flames come back on but the fan does not restart.

Things I tried to fix the problem:

I inspected the flue and the chimney. They both seem unblocked. I can see light at the top of the chimney used a mirror.

I removed and cleaned the burner tubes and vacuumed the heat exchanger.

The filter seems clean.

The registers through the house are open and blowing air. The baffles in the air ducts are all open.

I removed the humidifier and inspected the heat exchanger, it seems clear down to the blower motor. During this I ran the fan plenty of air blows through the heat exchanger out the humidifier opening. I reinstalled the humidifier.

After the flames and the blower cut off I can start the blower up again with the 'fan on' setting of the thermostat, I did this to see if the blower motor was overheating and shutting everything down, I do not believe the blower motor is the problem.

As far as I can tell the furnace has 3 sensors connected in series. One determines the temperature inside the heat exchanger, one determines the temperature below the flue entrance and also might act as a flame roll out sensor and one that determines the temperature of the pilot light. I jumped the first 2 and it did not change anything, the flames eventually started cutting on and off. I cannot figure out how to jump the sensor on the pilot light. I did this because I thought it might be a sensor tripping for some reason, but this does not seem to be the case with the first 2 sensors. The pilot light assembly includes a sparker and a sensor to determine the temperature. Even though I did not test it is seems to spark, light the pilot and then eventually the sensor trips and the burners come on. So the pilot assembly also seems to be working.

I removed the thermostat and jumped the red and white wires. The furnace came on and the same problem occurred. I did this to see if the thermostat was the problem, as far as I can tell it is not.

I also changed the thermostat batteries.
 

Last edited by geo8rge; 09-14-13 at 01:08 PM.
  #2  
Old 09-14-13, 01:49 PM
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If I turn the power off to the furnace and after 10 seconds restore the power, the burners light but shut off before the fan can come on. The burners start cycling on and off as before.

But If I turn the power off to the furnace and wait 10 minutes to restore the power, the pilot lights, the burners light, and the fan comes on for about 10 minutes and then the burners start cycling on and off as before.

As to the pilot, I do hear it spark, but since it is already hot, the pilot never goes off.
 
  #3  
Old 09-15-13, 11:43 AM
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On closer examination the pilot flame is not touching the heat sensor continuously which is the problem. So I posted a new question.

http://www.doityourself.com/forum/ga...ml#post2161103
 
 

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