I have Trane XE 90 furnace, works on propane. It is 13.5 years old, has been regularly serviced, no major issues. Failed only once in the past and orifices were changed to fix it. This is a unit in the attic, with 2 burners sitting vertically (another similar unit services downstairs, 3 horizontal burners – no issues with this one). I have been having issues with my attic furnace for the last several months – all this season.

It started with flame showing up inside the burner pipe (i.e right where the gas comes out the orifice). Flame would be there at the ignition, and then disappear. I now learned by reading forums that this is called “flashback”…
I got all orifices and burners replaces by the HVAC company who installed and serviced it. Failed again in 10 days. Same problem developed with the flame. I started observing it and noticed the following. When the flame turns off (thermostat satisfied, call for heat off) a little flame, like a pilot, still burns out of the orifice. It dies down eventually after a few minutes, but if the furnace restarts in the meantime, it does so with a popping sound. Eventually this may lead to a flame appearing inside the burner and staying there for some time.

Not sure that this fact eventually leads to the automatic furnace shut off, but that what happens eventually. Technicians then suspected gas valve, but checking the pressure did not reveal any gas leak through it, pressure was also normal.

Since the problems continued, I had gas valve replaced. After that the furnace worked fine for about 2 months – Dec–Jan – and then the same issue happened again: “pilot” flame burning at the orifice when the unit shuts off. At this point all possible parts have been replaced already - orifices, ignitor, gas valve, burners... Service technician came back to check and was as puzzled as before to figure out what the issue might be. He then just moved the burner back and forth, side to side a bit, cleaned the orifice hole with a paper clip and the unit started just fine.

Since then same symptoms happened several times with a flame and I did the same thing myself – just cleaning the orifice hole with a paper clip. The issue would go away for some time.
In the meantime my HVAC company called me and said they checked with a manufacturer who seemed to think the issue is pointing towards propane pressure being either too high or jumping intermittently. Got it checked by the gas company and pressure was found at 12 inches of water column, i.e. normal.
Since all possible small parts have been replaced already next in line for suspicion was a heat exchanger (that it is cracked). If this indeed can be an issue, I might well be better off replacing the whole furnace since I already spend close to $2k on numerous service calls and parts. However, I am really puzzled by what’s happening and would like to get to the bottom of it, so any expert advice can be greatly appreciated. It is always the same furnace and the same, lower, burner in it. I am very annoyed by total incompetence of my HVAC company to deal with the issue.

 

Could be a control signal issue.

What is the voltage reading at the gas valve when this "pilot" issue occurs?
You want 0 volts AC when no demand is present and 24 volts AC when a heating demand is present.

 

I have not measured this, and I don't believe technicians ever checked voltage on the valve (although replaced it).
I don't have a voltmeter, so i am afraid i won't be able to check now. But looking to find the way to diagnose the issue. What controls voltage on the valve? Can this still be an issue from your point of view (i.e. pump is fine, but may malfunction because of the voltage?)
Also, techs told me to check the gas pressure hinting that it might be too high leading to all these issues. Pressure is 12 inches of water column, normal i was told.

 

You have a fairly uncommon problem there. When the unit no longer calls for heat the valve should close completely. Since you have flames (and gas) when you shouldn't have.... it doesn't leave too many problems. If there is 0 volts on the gas valve and you still get gas..... the valve is defective or there is a pressure issue (could be high or low)..... that's it.... nothing more.

 

Thanks for your suggestions. I do completely agree with you, this remainder gas should not be there. When the problem showed up initially primary suspect was the gas valve. When it was replaced the new one worked fine for 2 coldest months. But then the problem resurfaced again...next thing we checked the pressure, and it is 12 inches wc, probably a notch high (11.5 is considered perfect i was told). So if this is not mechanical problem, then the only other option must be electric.
I will check the voltage as soon as I can. So you say if this is NOT 0 at no demand time, the valve can still be partially open and let some gas in?
One other question - gas valve has an on-off switch on it. If I turn this off at the time when the furnace is off, this should shut the valve off I presume and there should be no flame on the orifice?

 

On propane I always set to 10.5" WC. You may be getting junk under the valve seat. Have them take the gas line apart and check the drip leg.

 

definitely will have them check this again --- granted techs were here already 7 times during the season to look at the issue... Do you think 1 inch difference in pressure can contribute to the problem? Once, 10 years ago or so, i had some issues with the stability of the flame and they advised me to raise from 10.5 to 11.5. I also noted that at least twice the problem appeared when the propane tanks were just refilled to full capacity.
Thinking about junk - the only simple part not yet replaced is the manifold pipe. Could some junk accumulate there and contribute to what's happening?

 

Got evertyhing checked today again by a tech. Pressure is normal, voltage is fine on the valve, pipes are clean... but this pilot flame and flame inside the low burner pipe still shows up intermittently time after time... techs best gues now is heat exchanger - he says most likely some blockage somewhere so that the gas somehow remains in the pipes... does it make sense? I mean after all money and effort spent on these visits and repairs I am ready to go for the replacement, but hate to think that something may still be going wrong elsewhere. Any thoughts are appreciated.

 

Is any of the inlet propane copper? Had one a few years back that had accumulated almost a shrapnel at the valve. Copper and propane is a bad deal.

 

"inlet propane copper" -? I am not sure I understand what the question is... could you please clarify?

 

Some installs use copper pipe from the tank to or near the furnace. That can cause a problem.

 

no, I don't think there are any copper pipes near the furnace. The main one is iron I presume. Pipe coming from the tanks is covered in yellow plastic, so not sure which material it is made of. The only copper I see is a thin pipe that connect my propane tanks outside to each other (i have 3 of them).