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Old 01-16-2009, 04:42 AM   #1
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Trane XE78


I have a 16 year old model TUD080C936A1 hot air furnace that intermittently fails to fire up but continues to blow unheated/cool air. I have observed the failure mode: the call for heat is issued and the inducer fan turns on; after a few seconds the ignitor comes on and glows very brightly. There is a click but it appears as though no fuel is being presented. When this happens after a few seconds the ignitor goes out and the bloweer continues to blow thus the cool air. All that is needed to correct this at the time is to reset power to the unit via the shut off switch. It fires up as normal when this is done. I have replaced the thermostat/batteries; the induccer motor was very loose and vibrating a lot and this has been corrected. The unit seems to run for a while (10+ hours) okay and then will fail again. Any thoughts?

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Old 01-17-2009, 12:45 PM   #2
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It ran for 24 hours this time before failing to ignite again. This time I was able to get the controller status: 2 flashes = System Lockout. I think I am going to jumper the air pressure switch closed so as to eliminate it (or prove it) as the culprit. Even though the inducer blower motor is turning could it not be creating enough draft and it is the culprit?

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Old 01-17-2009, 12:50 PM   #3
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It is not the pressure switch as the igniter is glowing. You may have a faulty gas valve or circuit board. Gas valves are not DIY projects. I would call a tech and have him check it. He may have to cycle it many times ( I go up to 30) to catch it failing as it is an intermittent problem. Need to SEE it fail. It probably will get worse/more frequent.
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Old 01-17-2009, 01:25 PM   #4
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Thanks Yuri....I went back a re-read the sequencing and you're right. The ignitor comes on AFTER the air pressure switch senses airflow. Damn ....I though I might get this fixed myself. I could do the controller but I would not mess with the gas valve. Is there any way to determine which is the culprit without simply replacing one or the other?
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Old 01-17-2009, 01:54 PM   #5
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You have to catch the culprit in action/inaction. If you have a voltmeter and understand electricity you can check for 24 volts across the 2 gas valve terminals and have your significant other turn the heat off and on until you catch it being energized with 24 volts and not opening at that time. If it does not get 24 volts then it usually is the board. Most parts changing techs won't spend the time to do that process. No other way to find out unless it stops completely.
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Old 01-17-2009, 02:10 PM   #6
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I'll do that ( I am a service mgr for a hi-tech outfit and use meters all the time). Thanks!
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Old 01-31-2009, 01:05 PM   #7
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The system ran flawlessly for about 10 days! Then the problem started to occur again..usually once or twice a day (usually in middle of the night).
I noticed though that the circulation fan and the inducer fan are coming non at the same time. The the ignitor comes on and glows hot and then there is an audible click from the gas valve..and it does not ignite.
I caught it once failing with no 24v to the gas valve....but 99% of the time the voltage is there. Recently I have also seen it ignite but after about 20 seconds the flame goes out and it re-cycles and tries to re-ignite.
I do not think the circulation fan should be coming on until 45 seconds after the burners light. Could this be what is preventing the ignition? (Previously I replaced the thermostat with a LUX TX9100E) (and when this problem started I don't think it was coming on with the inducer fan).

Any thoughts will be appreciated.
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Old 01-31-2009, 01:48 PM   #8
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Sounds like the circuit board is faulty. There should be a 45 second delay unless you changed the thermostat and have the switch at the back set to elec (electric heat). The average life of a circuit board is 10 yrs. If your is original it probably is going bad.
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Old 02-01-2009, 08:50 PM   #9
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Thanks Yuri.....I did not pay close enough attention when I installed the new thermostat. It was set for electric. The circulator now comes on 45 secs after the burners ignite (when they ignite).
Now I just have to decide what to do next for my original problem.

Last edited by bmik2729; 02-01-2009 at 08:51 PM. Reason: misspelled word
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Old 02-05-2009, 08:20 PM   #10
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Well...after getting the thermostat setup correctly (Thanks again Yuri)...the furnace worked great for 48+ hours but failed again during the early morning Wednesday. Typically it will ignite when I reset the unit so in these cases the 24v always is at the gas valve.
One thing I was wondering it seems to be okay if the outside temp is @ 30F or higer (remember it ran okay for a week after it originally started acting up). It got pretty cold again overnight Tuesday and voila it failed again. I checked the exhaust pipe and cannot find any blockages or anything obvious. I also think that a couple of times the flame has gone out after the unit has been on for a while (because I noticed it trying to reignite itself.
I am rapidly approcahing the clinical definition of insanity....doing the same thing over and over but expecting a different result. I've got to start changing parts.
I am torn between going for the gas valve or the controller. Even if I purchase both parts and replace myself it will likley be less expensive than a service call. Both of which I am confident I can do but would like to hear what others say about a DIY gas valve replacement. It looks only marginally more dificult to do than changing the propane tank on a gas grill. I know I must dope up the threads and check for leaks after re-assembling. Any thoughts?
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Old 02-05-2009, 09:22 PM   #11
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The only thing that changes with temp is the density of the outside air and that can affect the draft in the chimney which will in some very rare cases cause a pressure switch to fail to prove. Yours is okay as the igniter glows. If you caught it once with no voltage to the valve then it ain't the valve. The board is safe and relatively easy to change. They fail 95 % of the time, gas valves fail 5% of the time. Those furnaces run with -30 deg F outside air temp where I am.
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Old 02-06-2009, 01:31 PM   #12
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What about low gas pressure? It could be at the point of proving sometimes and not igniting on others ecpecelly if it is a 2 stage unit starting on Low fire.

Missed the XE78 deffinitly not a 2 stage unit. But still could be low gas pressure. Like yuri says below time for a Pro.

Last edited by JohnH1; 02-06-2009 at 03:13 PM.
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Old 02-06-2009, 02:14 PM   #13
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That could be. The Carrier TUA's are very bad for that. Only a experienced tech with a manometer would figure that one out and our poster likely would not do that himself. The burners may need removing and cleaning etc. IMPOSSIBLE to see and do what we do over the internet. He seems determined to DIY. May be best to call a experienced Trane tech.
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Old 02-06-2009, 03:29 PM   #14
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It is not a two stage burner; single stage only and it's a Trane.
I thought of gas pressure as well but my gas hot water heater is okay. I also have a free standing Napoleon Gas Stove and it is okay as is my stove. And, if it was low gas pressure how would I correct it...and as I live in association of cluster homes all built at the same time wouldn't others be having the same issues?
I thought this is a DIY forum? Yes I am somewhat determined to fix this myself. As I indicated earlier I work in a hi tech industry (very electro-mechanical devices) and feel very comfortable replacing any of the components myself. I would be a bit nervous about the gas valve only because it has to do with gas.
I have ordered the new controller and should have it tomorrow. I'll likely replace it on Sunday and hope for the best. If anyone has any suggestions or thoughts on installing the White Rodgers 50A55-843 universal controller I would appreciate them.
Thanks for all the sugestions so far.....
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Old 02-06-2009, 03:59 PM   #15
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It is a DIY forum but some things are not DIY for people with out the skills and since we do not know you or your skills we can not be putting someone in a potentially dangerious spot. We want to help but we want you safe also. Do you have a manometer? The manifold gas pressure should be 3.2 to 3.5in water colume. And each home will or can have different gas pressures. The furnace has a different type of ignition system that your other appliances so it can react different to gas pressure. Also it may not be gas pressure at all. It is tough to fully diognose a problem over the internet. Dont get upset if you dont like the awnsers you are getting. sometimes it is better to error on the side of caution.

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