Trane XR80 - dirty flame sensor?

mtlockmanNovember 1, 2007

We have a 5-6 year old Trane XR80 furnace. Intermittently it will exhibit the following behavior:

  1. Thermostat calls for heat

  2. Draft motor starts and runs for 15-20 sec.

  3. "Click" is heard (presumably gas valve opening or ignitor)

  4. Nothing happens (normally burners would light now)

  5. Draft motor continues to run, then stops, starts again. On second (or rarely a third) attempt, burners do light and furnace runs normally.

When I had a clean and check done last fall, I relayed this problem to the tech and he confidently said it was likely a dirty flame sensor. He cleaned the sensor with fine sandpaper and polished with steel wool. Reinserted and the furnance ran flawlessly for a couple months before the behavior returned sporadically. Furnce was inspected and flame sensor cleaned again that winter and reduced incident rate of problem, yet problem still occurred not more than 4-5 hours after tech had cleaned flame sensor. Now it's happening again...this morning twice in a row when the thermostat called for heat.

I've read where flame sensor issues usually involve the burners lighting and shutting down after a few seconds. In my situation, the burners don't even light. There is a glow from within the unit and a NG smell, so I know the ignitor is presumably working and the unit is getting gas.

While I've never experienced a situation with this furnace where the burners failed to light enough times to lock it down, it's unnerving to hear the unit cycling on and off trying to light.

Is this really the result of a dirty flame sensor? Other than getting dirty, can flame sensors "go bad?" How long do they usually last? If this isn't a flame sensor issue, what might it be? My logic is pointing to the ignition since the burners don't even light off, but I'm no HVAC professional.

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Flame sensor would only be if the burners lit for about 6 seconds then when out. If they aren't lighting at all, could be no gas to the furnace, bad gas valve or bad board.

    Bookmark   November 1, 2007 at 5:33PM
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Sounds like you have a hot surface ignitor problem. That is the unit that glows white hot and ignites the gas. The only time a flame sensor is activated is when either a pilot flame or flame from the actual burners heats it up. Was an ohm reading ever done on the hsi? Did a tech ever take a microamp reading of the flame sensor output? Is the flame sensor properly positioned in the flame? Is the crossover flame or the burner near the flame sensor clean? A low flame output near the flame sensor causes a lower reading of the flame sensor signal (microamps) which if not enough will cause the gas valve to close. Then a re-ignition sequence will start. 3 failures on a lot of units will cause a 1-3hr shut down. (lock out) How else will the furnace be able to tell you that something is not working properly?

    Bookmark   November 1, 2007 at 11:19PM
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I had a similar problem with my Heil 5000 gas furnace. There was a pressure switch to detect positive pressure (i.e, the exhaust fan is operating) on the exhaust side of the fan. The switch was fed by a rubber hose. I would shut the furnace down and pull off this rubber hose. There was a small nipple and it would be clogged by debris. I just took a small piece of wire and cleaned it out. This worked for me. Good luck.

    Bookmark   November 2, 2007 at 5:08PM
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replaced the ignitor, thermostat, filter. it would light up in the beginning it would run about an 1/2 hour then flame would go off and the blower would keep running. when we put in new thermostat it run a couple of times now it won't even light up. any suggestions? we are on a pension and can't afford to have someone out. please help.

    Bookmark   April 17, 2011 at 3:27PM
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