Gas and Oil Home Heating Furnaces - Trane XE78 keeps running, blows cold

Trane XE78 gas furnace, 10 years old, mounted in a damp crawl space. Temps here in Cleveland, Ohio in the 40's. Digital thermostat.

Two weeks ago the furnace had a startup problem fixed by clearing the pressure sensor tube. At least that is the only thing I did before tesing it and it has functioned fine for two weeks.

Different problem now: It does this almost every heating cycle-set temp on t-stat to 67degrees, unit cycles on and works properly until the mains kick off and then it continues to blow until it is blowing cold air. I have waited up to 30 minutes just to see what will happen and it continues. The only way I can shut it off is to turn the breaker off for the furnace, within seconds I turn the breaker back on and furnace stays off until next demand for heat. It is in a difficult crawl space and I have not yet been able to see if there is any code being flashed by the LED. I work a lot and this just started happening. I am off work tomorrow and wanted some ideas to check before I go down to it, any ideas would be appreciated.

how long do the main burners stay lit? 3 to 5 seconds, look for dirty flame sensor, poor ground. couple of minutes, check filter, blower wheel, and evap coil.....restricted airflow could be tripping hi limit, but i think that is probably an automatic reset....

The mains stay lit throughout the entire heating cycle just as they should. The temp rises until the house is heated to the preset temp (67) then the room t-stat "clicks" and the mains shut down and the fan continues, but instead of the fan cycling off like it should in a few minutes, it just runs and runs. It continues running until I trip the breaker off. I then reset the breaker to the on position almost immmediately and the blower remains off until the next heating cycle. Essentially everything runs exactly as it should except the blower motor does not shut down when it should.

ok, got you. sounds like the fan relay sticking, does this unit have a circuit board in the blower compartment?

Hey, thanks for the help! I actually feel kind of stupid now. Last night on my way home from work I remembered that the batteries in the digital thermostat were 13 or 14 months old. There is supposed to be a low battery warning which has never prompted any action. Anyway, I decided to change batteries to rule them out, so I bought/installed a fresh set and lo and behold, ever since it has worked properly. Apparently marginal batteries in the t-stat don't send a good enough signal to something in the system to shut down the blower even though the signal is strong enough to shut down the mains. I can't say that I understand why, when all else functions right, but I won't argue with success. Just thought I would let you know in case someone else has a problem like this.
Thanks,
Dave

Rarely do I argue with success BUT, in heat mode, the fan is not controlled by the thermostat. Depending on the type of thermostat you are using, it may make a difference if the batteries are dead. If the problem reocurrs, go with hvac's suggestion and look at the time delay on the blower. It seems to be hanging up on the cool down. If it isn't part of a large control circuit board, you probably can just replace that part.

There is a large circuit board available as soon a the covers are removed, the thermostat I installed is sold by a company called "RiteTemp" that I bought at Home Depot around 13-14 months ago. It is similar to expensive units (Honeywell) in features but sells for much less and has performed flawlessly for the last year or so. Trust me, I am a "rule out the cheapest, easiest factors first" kind of D.I.Y. guy, and the fact that the batteries were over a year old made me decide to replace them and test it first before crawling around in the crawl-space. So far it has cycled appropriately around 7-10 times since the batteries were changed and the effect of the batteries was immediate. Also, if the unit starts acting up again I will follow hvac4's instructions. I view my repair as lucky, very lucky at this point.
Thanks,
Dave

I have not written in a while since it started working properly again. But, as luck would have it, it worked for a few days and now the blower is running on and on again so you folks were correct, it was not fixed. Please reread the first 4 posts and disregard my "fix".

Yes, hvac4u, there is a circuit board in the blower compartment. There is also a "flame probe" that projects into the mains when they are lit and it's single wire goes to a plug on the circuit board. I should look for a relay that is seperate from the circuit board that controls power to the motor? That does make sense, I was thinking possibly the Flame Probe was faulty. The circuit board flashes a code indicating that it is "sensing flame with the gas valve closed". If there is no relay seperate from the board then a new board might be required? I will be going back down into the crawl space tomorrow AM to check on the relay situation.

I also have a Trane XE78 with this same problem. I found this post, and the moderator replied that this could be caused by a bad ground. I am replying to state that the moderator knows his stuff. My unit is powered with an attached A/C pigtail, and the ground prong was missing. Upon rewiring, I found also that the hot and neutral were switched. Quick rewire has resulted in a full return to normal operation.