Intermittent Trane Furnace Problem

[TomB]

I've been battling a intermittent Trane Furnace problem where the furnace occasionally goes into lockout mode. The furnace is a 16 year old Trane XL-90 that looks like new. Over the last few months I have experienced a number of occasions where I wake up in the morning to a cold house. Simply cycling the power to the furnace usually brings it back to life. One time it actually took two tries but it did relight and ran flawlessly with no problem after that.

Initially I thought it was the flame sensor so I checked that. It was very clean but I polished it with steel wool anyway. Knowing that circuit integrity is important for flame sensors I cleaned the connections and added a ground wire from the burner to the furnace controller. Also, I replaced the furnace controller with the latest model that has a little more diagnostic capability but it still hasn't caught the fault condition. The time that I did witness a failure the hot surface ignitor came on, then I heard a click (solenoid for the gas valve?). Immediately after the ignitor went cold and there was no flame; it didn't even try to light. This tells me that it's probably not a flame sensor because it has about a 7 second window to determine if there is ignition. My furnace went into fault immediately, not after any delay. Since that time I've tried to capture the faullt condition by monitoring the voltage applied to the gas valve. I have not witnessed a failure since then and every time I see 24 VAC the gas valve functions. I'm baffled.

The problem seems to be happening before flame ignition since it tripps immediately on attempt to light. This rules out flame sensor issues. Since I have replaced the controller the only thing left is the gas valve. Is this right? The only thing that keeps me from saying this without absolute certainty is that I haven't had my meter on the terminals when there is 24 VAC applied and no gas flowing. I think it's just a question of time and I will eventually see it.

My plan right now is to replace the gas valve since that appears to be the most likely component now that I have peeled away the outer layers of this onion problem. I'd like to hear the ideas of others to see if I am on the right track.

TomB

PS - I'm a electrical / controls engineer and the real issue here is learning about the system, not the fundamentals of combustion control. Thanks in advance for helping me understand the specifics of furnace control and troubleshooting.

[Naydrus]

Honestly....
Call in a gas tech.
Troubleshooting and fixing minor stuff like flame sensor is all good,but when you start getting into the gas valve and burner components and such,call a tech.
To much can go wrong,I know you're trying to save money,but,
how much money will you save if you burn down your house?
how much money will you save if you kill everyone due to carbon monoxide?

A little drastic,I know,but true.

Not sure what field you're in being an electrical engineer but I know some do airline navigation systems.
If something happens to a navigation system on a plane I'm on,I want an electrical engineer to look at it,not the pilot.