Here is my original thread:

http://hvac-talk.com/vbb/showthread....962#post709962

Since I had lots of new data I wanted to make sure the entire board saw this.

Basically I have a Trane XE90 with an inducer motor that never shuts off.

When the thermostat is off, inducer motor runs.

When the thermostat is disconnected, the inducer motor runs.

No vents are blocked/plenty of air blows from the vents.

Here is what happens (starting from the furnace being off at the switch).

1) Turn on power to furnace -> Inducer turns on, control board flashes slowly like everything is ok.
2) Turn thermostat to a temp that will request heat. Control board (fast) flashes to acknowledge this.
3) Igniter turns on, burners light
4) Blower kicks in
5) About 5-10 minutes later (depends on how long the furnace has been turned off) the high limit switch trips and produces 4 flashes on the control board. Although I cannot take a temp inside the area where the switch is, a measurement against the metal surrouding the switch shows ~170F at the time the switch trips. I'm inclined to believe that the switch is doing it's job, or at least I think it is.
6) About 2-3 minutes after the switch blows the blower shuts off. The inducer motor continues to run.

All that being said, I first of all assume that the control board has a bad relay. That appears to be the only excuse for the never shutting down inducer motor. The inducer motor runs even when the control board declares no problem. (meaning during times of fast or slow flashes).

I have a control board, a high limit switch, and a flame sensor on order. The flame sensor I ordered just because I had to "clean" it a few months ago (furnace is just over 10 years old) and I figured now was a good time to order one.

I guess my question is is the constantly running inducer motor possibly the reason why the high limit is tripping? There is no blockage (in or out) and the filter is brand new.

I guess I'm trying to understand when the burners are supposed to turn off. What set of circumstances (aside from poor airflow) leads to the burners tripping the high limit sensor? I even dialed down the gas pressure a quarter turn to see if it made any difference, no luck.

Thanks in advance,

Jason