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.

Installed in Oct 2010 - 25HPA536H0031010 Heatpump, 58CVA090 Furnace, CNPHP422 Coil, SYSTXCCUID01-B Thermostat. Have had recurring problems with communication faults. After many return visits by techs from the installing company, no solution has been found. They replaced the controller boards in both furnace and heat pump and the UI. Problem still persists. It is in failure mode now. 40 events outdoor unit, 33 events indoor unit - communication fault. I changed the mode from hybrid to furnace only hoping to eliminate one variable but I think the outdoor unit is still being polled for OAT. Maybe I should disconnect the ABCD bus to the outdoor unit? Any thoughts on how to diagnose this problem? If this follows typical behavior, this will clear in a few hours. Thanks.