Rheem Oil Furnace - Blower stops but burner keeps running


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Old 11-13-13, 05:32 AM
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Rheem Oil Furnace - Blower stops but burner keeps running

I'm troubleshooting an odd problem. It isn't consistently reproducible. Occasionally when the thermostat stops calling for heat the blower will continue to run for its 60 second purge but the oil burner will keep running. The burner will run until the temp hits the high limit switch and then the power is cut and the blower comes back on to purge.

I had a tech from the oil company come out and he thought it was the thermostat. I was able to rule out the thermostat by waiting until the problem happened and then removing the thermostat W wire from the furnace - I also removed the thermostat from the wall for good measure.

What I found is there is a wire that leads from W on the burner to a terminal on the fan cycle control. If I remove that lead from the burner then the burner will shut down. My conclusion from this is that the fan cycle control is some how feeding power back to W on the burner and calling for heat. This doesn't seem right. I replaced the fan cycle control, but it still acts the same way. What else could cause this behavior?

Furnace Model #: ROBF-112QBGA
 
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Old 11-15-13, 02:43 PM
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I replaced the heat/cool relay and the safety relay on a whim thinking maybe one of those is feeding power back through the fan cycle control. But that doesn't appear to be the problem. When the problem decides to happen the burner will stay running until one of these events:

1.) Hits the high limit switch and power is cut
2.) The emergency shut off switch
3.) The lead is disconnected between the W terminal on the burner and the G terminal on the fan cycle control

Any thoughts on where I can go from here?
 
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Old 11-21-13, 02:24 PM
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Problem solved!

To help out anyone who might stumble upon this in the future..

The problem was the fan cycle control. I'm not sure if I got two from a bad batch or what. The model that my oil company used as a replacement was not the "drop-in" replacement that they said it was. I found and ordered the correct part according to the parts list and everything has been running fine for 3 days. It is possible that the part the oil company supplied and my initial replacement were from the same batch. Or it is possible that there is some odd incompatibility. Moral of the story: check the parts list.
 
 

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