Rheem Criterion II - gas/flame not staying lit

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Old 10-07-12, 07:32 PM
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Rheem Criterion II - gas/flame not staying lit

I have a Rheem Criterion II that seems to be having a fairly common symptoms of a dirty flame sensor. But the normal cleaning doesn't seem to be fixing the problem.

After turning on the system the ignitor glows and the gas turns on and provides flames for 5 seconds. Then the gas shuts off. The system attempts to cycle again with the same results. After the second time the system is simply unresponsive unless I power down and restart the furnace.

I have tried cleaning the flame sensor. It looked clean to begin with and didn't get much cleaner after a good rubbing with some steel wool. I've read on other posts that the flame sensor light usually flashes when there is a faulty flame sensor. Mine neither flashes or even comes on to begin with. The flash sensor light never comes on at all from the time I start the system to the end of the second attempted cycle.

Any help is appreciated. Please advise.

-Jimmy
 
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Old 10-07-12, 07:44 PM
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If your flame sensor is like the one mine has, sanding or cleaning it did not help. After replacing it the heater performed normally.
 
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Old 10-07-12, 10:40 PM
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Unfortunately, you probably have a circuit board which isn't working properly.


Measure the AC voltage being applied to the flame sensor while the inducer motor is running and report the results of that test here. You should get 24 VAC or perhaps 120 VAC. If you get no voltage the circuit board is bad and needs to be replaced.

It is unlikely the flame sensor needs to be replaced. It's merely a hunk of metal attached electrically to the circuit board by the wire. Check to see if you have the voltage needed to operate the flame sensor.
 
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Old 10-07-12, 10:55 PM
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The flame sensor is a single rod of metal that changes impedance or changes the circuit impedance based on the presence of flame. I don't believe you can connect anything to that circuit to measure the sensor and just touching that wire with anything will unbalance the circuit causing flame out.

A Scotchbrite pad should be used in the cleaning of the rod. Don't use sandpaper.
 
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Old 10-08-12, 03:57 AM
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PJ... there is 24 - 120V applied to the flame sensor. The flame rectifies the ac voltage and sends back a signal in DC microamps. With the proper meter you CAN check the flame signal.
 
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Old 10-08-12, 07:42 AM
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Normally a current wont flow through the air. However, a current will flow through a flame if one is present. If it is, it will rectify an AC voltage into a DC voltage and allow a DC current to flow from the AC source to ground.

That's the way a flame sensor on a furnace works. An AC voltage is applied to the flame sensor rod. When the burner flame is present it engulfs the flame sensor and allows a small DC current to flow through the flame sensor wire, through the flame to ground and from the chassis ground back to the circuit board.

So the thing to do is first to check to see if that AC voltage is present. If it isn't, the circuit board is probably bad.
 
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