Rheem Criterion II-heat cycle works, then shuts down

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Old 10-16-11, 06:12 AM
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Rheem Criterion II-heat cycle works, then shuts down

I'm having a problem with my gas furnace. When the thermostat calls for heat, the draft inducer kicks on, the ignitor glows, the gas starts and burns for a short time (4 seconds) and then shuts off. It tries this whole cycle again, with the same results. Sometimes the blower runs by itself with no heat.

I've read through some other posts, and I identified the flame sensor as the likely suspect. I cleaned both rods and the ground with steel wool. The furnace started to and stayed lit for about a 1/2 of last night, then this morning the same problem occurred.

I'm going to try to clean again, but I'm thinking there may be a bigger issue.

Does anyone have any ideas?
 
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Old 10-16-11, 07:08 AM
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If you have already cleaned the flame sensor, then you need to clean the burner faces. Also, I have had a few of those furnaces have a connection problem at the board or the Mullex connector that goes through the blower deck. Remove those Harnesses and put back together ensuring a good connection.
 
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Old 10-16-11, 07:48 AM
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Thanks. I'm a novice. How do I go about cleaning the burner faces?
 
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Old 10-16-11, 08:24 AM
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remove the burners buy removing the 1/4" screws holding them in and then brush with a steel or brass brush.
 
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Old 10-16-11, 08:56 AM
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Thank you very much. That did the trick. Have a good weekend. I'm sure you saved me a service call and some $$$.
 
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Old 10-16-11, 08:58 AM
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Originally Posted by hvactechfw View Post
If you have already cleaned the flame sensor, then you need to clean the burner faces. Also, I have had a few of those furnaces have a connection problem at the board or the Mullex connector that goes through the blower deck. Remove those Harnesses and put back together ensuring a good connection.


Since the burners light reliably, I don't see cleaning the burners to be of much help in solving the problem.

Similarly, about the only wiring problem that seems a possible problem might be the connection of the flame sensor to the board.

Everything is operating normally up to the furnace burners lighting. At that point, the circuit board needs to detect that the burners have lit or the circuit board shuts off the gas and starts another trial for ignition.

The flames are detected through "flame rectification." An AC voltage is applied to the flame sensor through the flame sensor wire. When the burners turn on, a small DC current of a few microamps (1 ua= 1/million amps) is rectified to DC current and flows from the flame sensor to the chassis ground.

If the circuit board detects the DC current, it leaves the burners lit. If it doesn't it shuts them off after 3-5 seconds.

The most common cause of failing to detect the burner flames is a dirty flame sensor, so cleaning that is routine, and should be routine annual maintenance.

After that the likely problem is a defective ignition control board that is failing to apply the AC voltage to the flame sensor wire or failing to detect the DC current. Either would require replacement of the board.

No harm in checking for bad wiring though ---- you might get lucky.

The REAL way to check out this problem is 1) to check for AC voltage bheing applied to the flame sensor and 2) measuring the microamps flowing through the flame sensor wire when the burners are lit. But a lot of DIYers might find those tests a bit daunting to do.
 
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Old 10-16-11, 09:05 AM
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dirty burner faces will cause the same problem as a dirty flame sensor due to the dc current flowing from the flame sensor through the flame to the burner face.
 
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Old 10-16-11, 09:09 AM
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Hello HVAC Tech,


Interesting comment. I've never observed that myself. But no harm polishing up the sheet metal surrounding the flame sensor.
 
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