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Old 12-04-2006, 10:46 AM   #1
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Furnace power up problems


Hi, and thanks in advance for your help.

Have a Rheem RGDG-07NAVER S/N DF5D302 F4694 8492 which started to sporadically shut down.

Now it is not powering up, it creates enough vacuum to operate the vacuum switch and then shuts down.

I have found the flame sensor to be completely open. By grounding the input from the flame sensor, the power up sequence continues, there is ignition for about 10 seconds or so and then powers down again.

Have found that the current should be more than 1 micro Amp for the microprocessor to continue normal operation. Since the flame sensor was open, and while waiting for a replacement, I built a resistor's network which would generate about 2 microAmps, but the microprocessor is not acepting it as normal. (Or the microprocessor/main board is gone)

When the new sensor arrived, I made a tactical mistake.... I installed it whitout measuring its resistance at room and high temperature....

The new sensor is now open as if the main board fried it.

Have you seen a main board "going bad" and frying flame sensors?

Do you have any suggestions as to how to proceed?

Thanks, Abel

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Old 12-05-2006, 01:34 PM   #2
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Furnace power up problems


Hi Capaz

Just from what I could tell from your post, it sounds like you have a control board malfunction. If your furnace has the two light board in it you would be money ahead to get the conversion kit to make it a three light furnace. I am referring to the number of little light bulbs on the control board itself. If you have any further questions, please let us know.

Good luck
Rusty

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Old 12-05-2006, 03:33 PM   #3
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Furnace power up problems


Thanks Rusty for your response.

I have been reading other threads and have learned quite a bit from you and the other experts. I appreciate your support.

Back to your questions....

Yes, the unit has the three light board. The two green ones are on all the time and the amber which blinks and it is likely to be providing a failure code. However .... I don't have the code to decipher it.

Nevertheless, the code may accurately report "defective flame sensor" because it is now open. However, as mentioned above, the installed replacement is now open too, which led me to ask if anybody had a "board gone bad" which fried the flame sensor.

Is not much left to suspect. Although I'm concerned about ordering the board and a new flame sensor (about $200), and finding out that something else (the transformer for example) is blowing up the board and the sensor.

By the way, I am assuming that the flame sensor is defective because it shows infinite resistance which stops the power on sequence immediately after the pipe vent motor starts. When I shorted the flame sensors, the original and the replacement, then it powers up to igniting and opening the gas valve, for a few seconds, and then shuts down.

Have you ever installed a new flame sensor and the board fried it?

Have you ever measured flame sensor resistance at room temp and/or high temp? Do you remember the readings?

Thanks again for your help
Abel
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Old 12-05-2006, 03:39 PM   #4
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Furnace power up problems


Hi Capaz

If the flame sensor is resting against a piece of metal is will simulate a flame current. As far as the flame sensor being bad, as long as the porcelin is not broken or the stainless rod is not damaged you should be ok. The problem you are more than likely having is the flame rectification circuit is out of the board. Let me know if I can help further.

Good luck
Rusty
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Old 12-05-2006, 04:27 PM   #5
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Furnace power up problems


Rusty, you wrote:

The problem you are more than likely having is the flame rectification circuit is out of the board. Let me know if I can help further."

I don't know what the flame rectification circuit is. Would you please expand on that?

Thanks for your tenacity in helping.

Abel
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Old 12-05-2006, 06:00 PM   #6
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Furnace power up problems


Hi Capaz

The flame rectification circuit is imprinted into the control board. The way it works, it sends a AC signal down the flame sensor wire. The moisture in the air becomes ionized and converts the AC voltage on the rod to a DC circuit. By traveling the ionized air it creates a resistive path to ground. The control board reads this path as a milliamps short. It knows by the amount of milliamps draw on the flame sensor rather or not it has established a good flame. When the rectification circuit goes out of a board it can no longer determine the milliamp draw. In not being able to do so, it knows by default that it is to shut the gas circuit off to keep from a catastrophic problem. Let me know if you don't understand and I will try to simplify it a little more.

Good luck
Rusty
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Old 12-06-2006, 12:26 PM   #7
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Furnace power up problems


Rusty
Thanks for the explanation. Got it.

Let me check the other sensors before I give up and buy a control board.

Be back....

Abel

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