Lennox Furnace G40UHX Function issue


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Old 03-27-11, 05:19 AM
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Lennox Furnace G40UHX Function issue

This is my first post here, and while I'm skilled at most construction and repair issues around the home, HVAC problems are my short suit . With that said, I'm hoping to get some direction with the following intermittent problem:

My 8 year old lennox furnace (G40UHX) has been functioning normally until yesterday when it started with the following intermittent symtoms (happened yesterday morning and again this morning):
1. At the point of thermostat demand, it starts as normal, the igniter ignites, the gas valve opens and a normal flame begins.
2. At about 4 to 6 seconds into the heating cycle, it appears that the gas valve closes for some reason and the heat shuts off but the blower continues.
3. It goes through this cycle 2 or 3 more times and then the blower shuts down.

If I turn the main power to the system off and then back on, the system goes back to normal operation.

If it is relevant to the diagnosis, it has had both the combustion air inducer and blower motor replaced within the past 2 years. Any help will be greatly appreciated ... Thanks!
 
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Old 03-27-11, 07:55 AM
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The usual cause of the problem with the symptoms you have is a flame sensor that has become coated with invisible oxides that prevents the burner flames from touching the actual flame sensor.


Your furnace lights the burners with a hot surface ignitor. If the burners don't light because of a burned out HSI or other problem, you don't want the main burner gas to stay on, unlit.

To verify that the burners have lit you will find the flame sensor on the opposite side of the furnace from the HSI. This is a small rod about 1/8" in diameter which sticks up into the burner flame and has a single wire going from the bottom back to the circuit board.

Remove the flame sensor and clean it with a wire brush. Reinstall it and you'll probably have solved the problem.

Cleaning the flame sensor in this way is recommended as annual maintenance to avoid a service problem.
 
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Old 03-27-11, 07:55 AM
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My first inclination would be to remove & clean the flame sensing rod. This rod is usually just a simple rod with a porcelain insulator, normally with just a single wire going to it & located on the opposite end of the burner rack from the ignitor. Remove the rod, clean it with a Scotch Brite pad or very fine sandpaper, re-install, & give the furnace a try. If this fails we can go further.
 
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Old 03-27-11, 10:11 AM
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Originally Posted by Grady
My first inclination would be to remove & clean the flame sensing rod. This rod is usually just a simple rod with a porcelain insulator, normally with just a single wire going to it & located on the opposite end of the burner rack from the ignitor. Remove the rod, clean it with a Scotch Brite pad or very fine sandpaper, re-install, & give the furnace a try. If this fails we can go further.
Thank you Grady and SeattlePioneer for the excellent advice. I'm currently at work but will clean the flame sensing rod when I get home. I'll report back to let you know if it resolved the issue ... Thanks again
 
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Old 03-27-11, 05:12 PM
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Grady & SP ... I cleaned the flame sensing rod per your instructions (there was a whitish green deposit in several places on the rod ). After reassembly, the system started with no problems (I will keep an eye on it for the next few days). Thanks again for your help (I will repay the favor by assisting others on the forum in my areas of expertise)
 
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Old 03-27-11, 05:34 PM
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JeffM54,
Thanks for letting us know the problem seems to be fixed. So often we offer advice & never hear back. Thank you also for helping others in your field of expertise.
 
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Old 03-31-11, 05:07 AM
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4 days with no additional issues ... I think we can pronounce it fixed ... thanks again for the help
 
 

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