Fluctuating flame sense voltage - bad controller board?


  #1  
Old 05-19-19, 03:13 PM
A
Member
Thread Starter
Join Date: Jul 2018
Location: usa
Posts: 2
Received 0 Votes on 0 Posts
Fluctuating flame sense voltage - bad controller board?

I have a Ruud RGGE furnace with a 62-102783-81 controller (IFC) board that is occasionally generating a code-11 error “Failed Ignition” error that soon progresses to a code-10 error “call for service” with a 1-hour lockout on ignition retries. Sometimes it fixes itself and sometimes it continues for a while. I have observed it while it is misbehaving, and all burners fire and the flame sensor rod is well within the periphery of the last burner’s flame. Soon after all are firing, they shut down. I have cleaned the flame sensor rod with a Scotch-Brite pad. I have cleaned the face of the burner that is associated with the flame sensor. With AC power off to the furnace, I have checked the resistance from the flame sensor rod to ground and an infinite resistance is measured (as expected). I have checked the resistance from the IFC board ground to the metal of the burner associated with the flame sensor and found it to be less than 1 ohm. With power on and things working as they should (but no heating demand) I can measure ~120VAC from the flame sensor to ground. As the furnace starts firing, this voltage drops momentarily to ~80VAC and then recovers to ~110VAC as the burner continues to run. On the other hand, when things are not working, the voltage at the flame sensor rod can be as low as 22VAC and it can stay that way for an extended period. Whenever it tries to fire with the AC voltage at level, it fails to sense the flame and shuts down. Given time (hours) the voltage will gradually creep back up and when it finally exceeds about 70VAC, the furnace starts working again. A continuity check from the IFC board flame-sense pin to the flame sensor rod shows less than an ohm of resistance. The too-low AC voltages measured at the flame sensor rod can also be measured at the IFC pin that connects to it. My Fluke meter cannot measure microamperes but, without the needed AC voltage to drive the flame rectification process, the microamp level seems to be rather moot anyway. I have a second RGEF furnace for the upstairs of the house that uses the same IFC board and its flame sensor voltages match the “good” voltages of the RGGE. Since there is very little involved with the flame sense process other than the parts that I’ve described, I’m beginning to think that the only piece that’s complex enough to cause voltage problems that vary over time like this is the IFC. The furnace is about 9 years old and this problem has been occurring for about 2 months now and there were no changes to the system prior to that.


 
  #2  
Old 05-19-19, 03:52 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 62,059
Received 1,505 Votes on 1,393 Posts
Welcome to the forums.

It doesn't fix itself. The problem may be coming or going or a value is on the edge of satisfactory.

Soon after all are firing, they shut down.
How soon is soon ?
5-10 seconds could be a flame sensor issue.

The only true way to read a flame sensor is with a ua/micro ammeter. The AC voltage that is applied to the flame sensor rod is not a published value and is always changing.
 
  #3  
Old 05-19-19, 05:24 PM
A
Member
Thread Starter
Join Date: Jul 2018
Location: usa
Posts: 2
Received 0 Votes on 0 Posts
It is no more than 5 seconds before the shutdown. The furnace uses spark ignition and you can listen to the spark to determine when the flame sense is working or not. When it is working, the spark is shut off about 1-to-2 seconds after the burners light. When the sense is not working, the spark continues for the 5 or so seconds it takes the controller to decide that it is not going to light and then both the spark and gas are shut off.

As for the voltage vs. current, without sufficient voltage, there will not be sufficient current and, from everything that I have read, 22 volts is too little for any known flame sensor of this type. But ignoring the voltage for a moment and given everything else that I said when I had posed the question originally, if I had also said that I had measured a maximum current of 0.1uA during these periods of failure and at other times, 4uA when the flame was successfully sensed, would this be enough to condemn the controller board as the likely cause of the problem?
 

Last edited by apicky-engineer; 05-19-19 at 05:26 PM. Reason: correct typo
  #4  
Old 05-19-19, 09:02 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 62,059
Received 1,505 Votes on 1,393 Posts
A dirty flame rod or it not positioned correctly in the flame so as to be completely engulfed are typical flame sense problems. However.... those problems don't cause the AC voltage to fluctuate with the board in standby.

I tend to agree you have a board problem.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: