Trane XL80 TUD120R954K1 Won't Light

[gregger77]

Hey there!

I have been out of town for about a week but my wife reports that our 7-year old (or so) furnace is heating only intermittently. And all day today, it didn't light at all.

The filter is only a month old, so airflow is good. I removed and cleaned with steel wool the flame sensor. Then I checked the sequence of ops, which for this unit is (according to install manual): call for heat; run some pressure sensor checks; energize draft blower; while draft blower comes up to speed, the pressure switch contacts will engage and the ignitor warms up for approx. 20 seconds; gas valve energizes; flame sensor confirms that ignition has been achieved within the 4 second ignition trial period...and away we go.

Okay, in my case, the board and pressure switches click; the board sends power to the ignitor (which I read with a digital meter at 94.5v) for 17 sec, it DOES NOT GLOW at all; the gas valve opens for 4 seconds, and then closes; the board cuts voltage to the ignitor; and the main blower runs for about 3 mins until a the system tries to restart. After a few tries, the board fault light issues two flashes, the code for "External lockout, no. of retries exceeeded."

Questions:

1) Is the ignitor bad, or the board? (Ignitor is a White-Rodgers 768-A silicon nitride "stick.")

2) If the ignitor is bad, did it go bad over time because voltage sent by board was too high?

I read the specs; this ignitor is rated for 80v. I also read that too much voltage can be caused by a bad system ground. I noticed that the 6" green wire that is attached to the ground screw inside the furnace's junction box is stripped, but not connected to anything. It appears it may have been attached at one time to the cabinet interior, or jammed into the cabinet door hinge. There is nothing else nearby it would have been connected to. So,

3) How is a ground usually achieved with the ground wire from the furnace's junction box?

Thanks for your help. Winter in Chicago with no heat while hubby travels = HOSTILE WIFE.

Edited December 4, 2011 by gregger77

[jumptrout]

Sounds like a bad ignitor to me.

Remove the wires from the ignitor and read the ohms on the ignitor. Look very closely at the ignitor for a hair line crack. Do not touch the ignitor element with your fingers.

[gregger77]

I don't see any cracks on the ignitor, although it about half of the "stick" is a white/ash color (starting from the tip).

I am not experienced reading resistance on my digital meter, but when set to 20K ohms, the reading is 3.32.

Do you think the board is bad, too? I can replace the ignitor, but wonder if it's getting too much voltage.

[Budget Appliance Repair]

Where did you come up with the specs that the voltage to the hot surface ignitor should be 80 Volts?

I've never heard of any hot surface ignitors that operated on anything but the standard 120 volts that is feed in from the line circuit feeding the furnace.

I believe you just have a bad ignitor, replace it and you will most likely be all fixed up and good to go...........

[gregger77]

The spec is actually part of the part description in most places, ie., "80v Silcon Nitride Hot Surface Ignitor." It is also in the manual for the control board (found out mine is White Rogers CNT03077; a web search turned up the manual). The voltage can vary between 70 and 100 volts.

http://www.scribd.co...%80%93-CNT03077

Turns out, the board "learns" the lowest possible voltage that will successfully power the ignitor. It does this by lowering the voltage (via sine-wave "clipping," not via a transformer) at each call for heat, until it finally fails. Then, on retry, it bumps the voltage back up slightly, and, if successful, will retain the "good" voltage for the next 255 calls for heat! Then it will start the learning process over again (or, whenever there is a need for an ignition retry or a total furnace power recycle). As a result, the life of the ignitor is significantly extended!

In any case, the bottom line is...bad igntor and as you suggested, I'm replacing it tomorrow! The old ignitor spec has been changed, to a new part IGN00145 (replaced the old IGN00117), which maybe will last even longer?? My old ignitor lasted about 8 or 10 years...I don't remember when I put the furnace in, exactly...in a very cold weather climate.

Edited December 6, 2011 by gregger77

[Budget Appliance Repair]

If you get 8 to 10 years out of an ignitor you really have no reason to complain.

[gregger77]

Very glad to hear that, B.A.R.!

I just replaced the ignitor today and was very happy when the unit fired right up.

Thanks all for the advice.