Inducer never shuts off + high limit switch trips

[jasonv1]

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

[tlcartman]

that limit switch is a safety device, call a pro before someone gets hurt

[jasonv1]

That seems to be the road I'm going down. I'll try the replacement switch to be sure, but if that doesn't do it I'll have to make the call.

If the switch and control board don't do it, it is time to call in the big guns.

[tinner73]

no D.I.Y. here

[PullaVac]

I have had similar problems and a few others with Trane. Most of the time, the problems where indicitive of polairty problems. Trane control boards a very sensitive to incorrect polarity conections. Make sure that the 120vac connections from the ac supply voltage the the board has the correct polarity. Check that line is connected to line and neutral is connected to neutral and ground to ground. This has solved alot of problems for me.

[simpleman]

I do agree with the polarity issue,all tho in most cases the system will not run at all.

As for the inducer being the cause for limit switching,that not the case,the inducer would cool the hx.

What you have is a system that the safety are working properly,because you have a problem with airflow,be it clogged air filter,collaspe duct or a few other thing that
come to mind.

Please make the call,and put your mind at peace.

[Wild Leg]

I even dialed down the gas pressure a quarter turn to see if it made any difference, no luck.

Why would ANYBODY EVER do that?

Back away from the furnace...
Pick up the phonebook...
Make the call.

[grodan1]

You said plenty blowing from the vents, but didn't say what vents.Did you check the flu vent? Systems are very sophisticated, but sometimes the old basics can still throw the monkey wrench in.By the way, I'm still a student, but even a blind hog finds an acorn now and then.

[jasonv1]

It was the board.

Replaced it earlier today and the unit has been running for 90 minutes already with no problems. (It is bringing the house from 61 to 70 so it will take awhile).

Thanks for the help!

Jason

[tlcartman]

Originally posted by tinner73
no D.I.Y. here

maybe not on this thread, but he got it on this thread.
http://hvac-talk.com/vbb/showthread.php?threadid=71826

wheres dave when you need him?

[docholiday]

So let me toss this out ... If the relay was stuck, the pressure switch would be closed and the board should fault because the pressure switch was not open proir to a call for heat. This feature is built in so you cannot jump the pressure switch and walk away.

Me thinks we are being messed with.