Blower Starts, Gas Stops.


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Old 01-07-19, 07:31 PM
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Blower Starts, Gas Stops.

I have an old Trane XL 90 high efficiency natural gas furnace. Produced in 1989, I am unable to find any information. Model # TUC120A960A0, google makes me feel as if this model never even existed. Last week I noticed my furnace was randomly shutting down and immediately restarting before reaching set temperature. This was random, sometimes it ran the whole cycle without stopping. Sometimes it stopped after 2 minutes, sometimes it stopped after 10 minutes but no matter what it always re-lit and got to the
set temp. Now it doesnít want to make heat at all, here is what happens. 1. Inducer motor starts, 2. igniter glows, 3. All 3 burners light and stay lit, 4. limit switch triggers blower to start, within 10 seconds of blower starting I hear a ďclickĒ and all 3 burners shut off. I have no HVAC knowledge, i have spent hours on google attempting to learn something. I have removed and cleaned my flame sensor, but I donít believe thatís the problem because prior to blower coming on the burners are all lit for at least 30-45 seconds until limit switch starts blower. Flame does not flicker or move so that the sensor is not engulfed. I have been suspecting some type of pressure or safety sensor, but here is where I really got stumped. After cleaning the flame sensor i was so excited to try it out, that I didnít reinstall the silver burner plate/door. Furnace fired right up, flames stayed lit when blower started, whole house warmed up with no problem. Iíve come to realize that when the burner door is left off, my furnace runs and heats with no issue. once I reinstall the burner plate/door, the problem starts again; burners look great, blower starts and within 5 seconds gas is turned off. This is happening around 130/140 degrees, high limit set around 180 so I donít believe itís overheating. What else is weird, as soon as the gas is shut off, the ignitior immediately starts glowing again. This process will repeat as long as I let it. Every vent in the home is open, none covered or closed. My pvc in and out pipes are clear of any debris. Someone please help!
 
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Old 01-07-19, 08:15 PM
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Welcome to the forums.

Can you post a pic or two of your furnace. There are several series of *UC models.
How-to-insert-pictures

Once I see them I'll tell you what to check.
That unit has an ignition control board. Does it have a diagnostic LED on it ?
 
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Old 01-08-19, 02:58 AM
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I have attached some pictures. Please let me know if you need a better picture of anything in particular. The silver flame cover in the last photo is the one that when is removed, the furnace runs just fine.

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Last edited by PJmax; 01-08-19 at 04:09 PM. Reason: cropped/enlarged/added labeled pic
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Old 01-08-19, 11:52 AM
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Had technician come out. He made no attempt at diagnosing the problem, looked inside the heat exchanger and says he sees a crack. As per technician ďfurnace is shot.Ē I donít believe this for 2 reasons: 1. Iím able to heat house perfectly fine with the flame door off. 2. I donít see any crack, nor do I have symptoms of cracked heat exchanger.

Although I do not believe the diagnosis, I am going to play along because my home warranty has authorized them to replace the furnace tomorrow morning. At 30 years old, i cannot argue that it is time for a new furnace. I know it wonít be a trane, but hopefully I can get a few worry free years out of it.

should I let the company dispose of my furnace? Iím just wondering if it has any value... either in scrap or even having extra parts.

thanks in advance
 
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Old 01-08-19, 02:04 PM
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Your old furnace isn’t worth anything beyond scrap.
I wouldn’t let a home warranty replace that furnace. They have a horrible reputation, and your new furnace will likely be installed in the same manner as the old one was condemned.
Do not let them scab in the same size. A load calculation is a must before selecting what furnace is right for installation.
 
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Old 01-08-19, 03:46 PM
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my furnace is a 90% 120,000 BTU. The size has always seemed appropriate for my home. No changes have been made to the home since installation. Wouldnít the replacement furnace size be appropriate as long as they are replacing with same 90% 120,000 BTU? Sorry if I sound stupid ( I am ) but why would a new load need to be calculated? Also what do you mean about new install same as old being condemned? Unfortunately I have to do go through with this, I live in northeast and I have a 1 year old son that I need to keep warm. After reading many hours I found many forums explaining my exact problem, but not a single one ever mentioned the fix. I really donít want to give up my trane but itís useless to me since the technician deemed it unsafe.
 
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Old 01-08-19, 04:10 PM
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I fixed your pictures. In the last one that is one of the other ones but cropped and enlarged...... it looks like rust-thru holes in the heat exchanger.
 
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Old 01-08-19, 04:25 PM
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You should not assume that the original furnace was of the correct size. And the new furnace will likely be above 90% efficiency.
120,000 BTU is a huge furnace.
A bigger unit is not better.
Your units reliability and performance will be determined by the quality of the installation. Since warranty companies often do subpar work, there is a chance your new furnace may suffer from problems caused by poor installation practices.
I would suggest cashing out and using a preferred contractor in your area.
 
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Old 01-08-19, 05:32 PM
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Thank you guys for taking the time to try to help me out. I agree with your cash out recommendation but right now I just donít have the money, or the time to get the money. Iím sure Iíll be back soon with questions about my new furnace once if fails
wish me luck!
 
 

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