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Old 12-04-2011, 01:15 AM   #1
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Trane XL80 2-Stage Furnace Won't Fire


Hey there! I'm posting this here and another forum because it's getting cold and we've got no heat!

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 this ignitor is supposed to get 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.


Last edited by gregger; 12-04-2011 at 01:22 AM.
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Old 12-04-2011, 05:15 AM   #2
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Trane XL80 2-Stage Furnace Won't Fire


Did you check the hoses on the draft inducer motor, if there is a small hole or crack in them your glow plug will not come on. hook that ground up.

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Old 12-04-2011, 06:47 AM   #3
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Trane XL80 2-Stage Furnace Won't Fire


The HSI is not glowing and you have voltage so your ignitor is bad. However, the voltage sounds high for that particular board and furnace. It should be around 80V. You may need to replace the board also. As far as the ground.... Does the incoming power wires to the furnace have metal conduit that is attached to the furnace and a gang box or the breaker panel? If so that is where the ground is coming from. That furnace wont run unless it sees a ground by checking for power between L1 and ground.
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Old 12-04-2011, 11:07 PM   #4
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Trane XL80 2-Stage Furnace Won't Fire


Yes, it's solid metal conduit coming from the service to a gang box with a toggle power switch mounted to the side of the furnace, and then flex conduit takes the power into the furnace's own gang box inside.

I am told the ignitor has changed from a rectangle-ceramic base to a cylindrical ceramic base unit. Maybe the new one is less susceptible to failure?

Anyway, I could change the ignitor and then monitor the board output voltage occasionally to see if it's consistently in the 90s.

Thanks.
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Old 12-05-2011, 05:04 AM   #5
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Trane XL80 2-Stage Furnace Won't Fire


you may want to check the back of the control board for hot or burn marks.
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Old 01-01-2012, 03:13 PM   #6
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Trane XL80 2-Stage Furnace Won't Fire


Sorry, forgot to wrap up this thread. As posted on another forum: The ignitor arrived, and the furnace immediately fired up. I guess 8 or 10 years out of an ignitor is not bad...that's how old the furnace is. The board must be doing a pretty good job of managing the voltage to the ignitor; I read the tech manual, and found it has logic that tapers the voltage down as low as possible over time to preserve the life of the ignitor. Pretty cool feature and I'm glad I kept digging on the 'net until I learned about it, or in my amateurism I could have started by replacing a good board.

Thanks all for your helpful input!
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Old 01-01-2012, 07:34 PM   #7
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Trane XL80 2-Stage Furnace Won't Fire


Quote:
Originally Posted by gregger View Post
Hey there! I'm posting this here and another forum because it's getting cold and we've got no heat!

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 this ignitor is supposed to get 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.
You will want to replace the ignitor as well as the main board, This will correct your current problem.
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Old 01-01-2012, 07:46 PM   #8
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Trane XL80 2-Stage Furnace Won't Fire


Quote:
Originally Posted by harleyrider View Post
You will want to replace the ignitor as well as the main board, This will correct your current problem.
Maybe you should read the whole thread?
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Old 01-01-2012, 08:54 PM   #9
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Trane XL80 2-Stage Furnace Won't Fire


Quote:
Originally Posted by hvactech126 View Post
Maybe you should read the whole thread?
Only time will tell......

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