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Old 01-25-2009, 09:20 AM   #1
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Natural gas forced furnace - Burner shuts off prematurely


Hello I am new here. I am struggling with my older model nat gas furnace. 1989 Trane 120k btu (oversized in my smaller home).

A couple of weeks ago it began acting up. The burner would shut off after running for approx 1 minute. I had a service person diagnose the problem as the gas valve. I know how to replace a gas valve so I did it myself. Worked like a champ.

Now two weeks later it is doing something slightly similar but not exactly the same.

The burner comes on and stays on for quite a while. But then as the day goes on the burner may quit coming on and the exhaust fan is blowing all day long waiting for the burner to ignite. If I turn off/on the system it seems to fix it.

It is extremely cold outside and my furnace comes on and off continuously all day and night. Since it is oversized it only has to run for a short while before reaching the thermostat temp. It is almost as if some type of switch is getting stuck or is going out somewhere but I am not too savy with the different components of the electrical within my furnace.

I attached a picture. If you tell me to try something please advise where the item is located in the picture so I can find the correct part. I truly appreciate your help.

Thank you,
Scott
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Old 01-25-2009, 09:22 AM   #2
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Natural gas forced furnace - Burner shuts off prematurely


When it cuts off, before you reset it, look at the control board and see if it has flashing alarm lights. That will help narrowing it down to what to check for.

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Old 01-25-2009, 09:34 AM   #3
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When it cuts off, before you reset it, look at the control board and see if it has flashing alarm lights. That will help narrowing it down to what to check for.

I just tried this and I don't see any lights anywhere. I don't think my has any lights inside it to alert to problems. The problem seems to be getting worse. I wonder if it is the gas valve again. I wonder if something could be burning out my gas valves.

I noticed something else just now. The outside air Intake has a bunch of moisture in it and also there is a bunch of moisture in the exhaust. There is so much in the intake that it is dripping where the fan motor is located.

So long story short, my problem appears to be getting closer and closer to what it was when my gas valve quit working. I am almost wondering if my HVAC technician diagnosed this problem wrong and I didn't need to put in the new gas valve. I can hear the system switch off the gas valve when the burners stop. But the temp has not reached the thermostat level so the intake/exhaust fan keeps running as if it is going to try and relight the pilot soon.

Thanks again for helping me out with this.
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Old 01-25-2009, 10:01 AM   #4
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Natural gas forced furnace - Burner shuts off prematurely


I may have accidentally just now figured out the problem. I was looking for the trouble lights when I could faintly hear water splashing in the exhaust pipe area. Then I noticed the water dripping from the air intake pipe near the electric motor (mid center of pic). I noticed water moisture in all of the clear plastic pipes high near the different switches.

I removed the bottom drain pipe and got flooded with water. The water was sitting in that entire plastic reservoire to the left of my shiny new gas valve all the way up to the exhast motor. After I let all of the water drain out I think the problems I have been facing are all gone now.

Now I am thinking my gas valve was not actually bad but instead it was juts a build up of water. The HVAC service man I guess never did check to see if there was water building up and not draining.

Does all of this make sense? Does it sound like I just had a water drainage problem? At least it is working now (I think it is working).

Scott
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Old 01-25-2009, 10:03 AM   #5
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Natural gas forced furnace - Burner shuts off prematurely


There should be no moisture going into the intake. If so then it is recirculating and that is bad. At 20 yrs old I would want to get another experienced Trane guy to check it in case there may be problems with the heat exchanger cracking. That is about the life expectancy anyway. Going to be a $$ pit now.
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Old 01-25-2009, 10:05 AM   #6
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There should be no moisture going into the intake. If so then it is recirculating and that is bad. At 20 yrs old I would want to get another experienced Trane guy to check it in case there may be problems with the heat exchanger cracking. That is about the life expectancy anyway. Going to be a $$ pit now.

So water is bad and I may have a heat exchanger going out? Is it okay to have water coming in from the intake if it is picking up water from the outside?
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Old 01-25-2009, 10:05 AM   #7
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Natural gas forced furnace - Burner shuts off prematurely


Especially if it was affecting air flow or in the pressure switches.
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Old 01-25-2009, 10:10 AM   #8
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Natural gas forced furnace - Burner shuts off prematurely


Water should NEVER go into the intake. It will corrode the burners etc etc. If the unit is oversized it has 95% chance been overheated which will seriously reduce its lifespan. I would bite the bullet and go for a new one. Lennox G61 and G71s are very good and I hear Trane makes a nice 95% efficient unit with ECM variable speed motor also. The new ones have full diagnostics and a properly sized 2-3 stage unit will be a lot more comfortable.
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Old 01-25-2009, 10:14 AM   #9
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Natural gas forced furnace - Burner shuts off prematurely


Thanks everybody. I am going to keep an eye on it by manually collecting the water into a bucket and have a different repair person check the entire system out again. I am not sure where all of this water is coming from and it sounds like any water is bad during the winter.
Scott
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Old 01-25-2009, 10:46 AM   #10
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Natural gas forced furnace - Burner shuts off prematurely


Dirty/clogging drain line.

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