Flame Sensor Probe Error

Problems related to residential installations.

Flame Sensor Probe Error

Postby kenwood850 » Mon Feb 24, 2014 10:36 pm

I have a Thermo Pride propane hot air furnace model CHA-100P with a White-Rodgers 50A50-113 Controller. After it is powered off, if you turn it on and try to fire it with the thermostat set well above ambient, it turns on for a second or two then dies. The light on the controller flashes six times. According to the manual six flashes indicates a “shorted flame probe or water level sensor”. I could not find any water level sensor terminals on the controller so I think it is safe to rule that out. That leaves the flame sensor probe. I checked between the probe and ground and found the resistance to be 468 k-ohms. My understanding is that the flame sensor circuit determines if the flame is present by the ionization of the gas in the flame. If that is the case I would expect the ionized gas to have a high resistance level so 468 Kohms might in fact be considered a short to the sensor circuit. Based on all this I have three questions.

1. Is 468k-ohms considered a short as I suspect and should the probe be replaced?

2. If the flame probe sensor should measure infinite resistance when disconnected from the controller, would it be a fair test to try to run the furnace with it disconnected. That controller would see infinite resistance in that case and I would think it would run at least until the time a flame should be detected. I would also think that would definitely be more than the 1 or 2 second it takes now to shutdown. I tried that and it still shuts down in 1 or 2 seconds.

3. Would there be any other reasons the furnace would shutdown in 1 to 2 seconds. All the other sensors test fine and are in the correct normally closed or open condition with no power.

Thanks.
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- Flame Sensor Probe Error

Postby Freon » Tue Feb 25, 2014 3:17 pm

I do not know the resistance you should be seeing. That is a question for White-Rogers.

The flame sensor is a metal probe with a wire that, all by itself, is a conductor and should have zero resistance. The W_R controller applies a voltage to the probe. In the presence of the flame the voltage difference between the probe and the closest metal part that is grounded to the furnace will cause a very small current to flow. The W_R control measures that current to determine if a flame is present or not.

Poor connections can add resistance to that simple circuit. Make sure the connections at the W-R controller are secure and that the closest metal furnace part to the flame sensor is also well connected to the furnace (ground). The flame sensor probe can become dirty so you may want to use emery cloth and carefully clean the probe. Also make sure it is well enveloped by the flame when the furnace starts.

From your description it seems your furnace works except when you try to relight it immediately after it has finished a heating cycle. If you can watch the furnace when it fails to ignite and pay close attention to the flame sensor and flame, you may see something that's not right.

Lastly, controllers do fail. Some manufacturers place the controller where they get too hot. Heat kills electronics. However since your system seems to work intermittently I think a dirty flame sensor or poor ground/loose connection may be the problem
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