Furnace stops heating

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  #1  
Old 11-08-08, 12:57 PM
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Furnace stops heating

Over the past week, I have noticed that our furnace stops heating even if the demand for heat is still there. I checked the code being output on this furnace(8 blinks) which indicates low gas pressure or faulty gas valve. If I turn the furnace off and back on, the furnace will start up as expected and the burners will ignite. After a few minutes, the flames will go out and the furnace will restart (ignitor turns on, and flame starts up as expected.). After several restarts, the furnace seems to give up and no more heat is output until the furnace is powered down and back up again.

I donít believe the gas pressure is low (we have a separate furnace on that same gas line) and I don't have any problems with that furnace. What would be the next steps to troubleshoot and get the furnace operating correctly.

PS. In case it is relevant, we recently redid our duct work. Working with a fabric duct supplier, we installed a new heating duct designed to operate at 0.5in head at the highest tap setting on the motor. After we installed the duct, the furnace seemed to work correctly for a couple of weeks before I noticed the problem with the furnace not heating.

When the burners are operating, the temperature rise is about 20deg F. Air flow is good. I haven't checked the pressure to see if it is infact 0.5in water.
 
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  #2  
Old 11-08-08, 02:08 PM
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Here is some additional info. The furnace is a natural gas fired LuxAire with the following model information: ELHM834847 PCUHBLD12N100. It was installed in 2001.

Prior to the new fabric duct, the furnace was operated at the lowest tap setting. With the new fabric duct, the furnace is operated at the highest tap setting to get sufficent air flow to inflate the fabric duct. After the installation of the fabric duct, everythign seem to work correctly for a couple of weeks. So the new duct could be a red herring with respect to this problem.
 
  #3  
Old 11-08-08, 04:10 PM
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I think you should check the static pressure. It sounds like it is low. 20 degrees heat rise is too low. Your heat exchanger may be overheating. .5 static is a little high so make sure its in the range of the manufacturers specs.

After reading this I realized it was confusing. With low static, there isnt enough friction on the exchanger to pick up the heat causing it to overheat. In turn, this causes low heat rise. You still may have a gas problem but I would definitely check static pressure. Also, you mentioned there is another furnace served by the same gas line. You may have a bad regulator at the furnace if it is served by 2 lb. gas pressure or the regulator outside may be bad or the gas line may be undersized.
 

Last edited by thermofridge; 11-08-08 at 04:41 PM.
  #4  
Old 11-09-08, 07:42 AM
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I will check the statis pressure later today. With the old metal duct, the heating contractor indicated that he thought the static pressue was in the 1/8in range. The new fabric duct duct was designed for 0.5in head (which was what I believe was minimum to to properly inflate the duct). I provided the duct designer with the furace manufacture's spec sheet.
 
  #5  
Old 11-09-08, 12:14 PM
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I checked several items today
1. temperature rise is about 40degF. When I was measuring the hot air temp wasn't appropriate. So temperature rise is from 48F to 90F.
2. Static pressue in the duct just beyond the furnace is about 0.4in water.
3. I have high efficiency filters so thinking that the pressure drop across the filter might be too high, I removed one of the two intake filters. It felt like things improved but I still have an issue.
 
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