Lennox G40UH-36a furnace intermittent heating issue


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Old 02-12-14, 04:06 PM
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Exclamation Lennox G40UH-36a furnace intermittent heating issue

I have a Lennox G40UH-36a gas furnace located in the attic. It does not appear to be very reliable.

My latest problem has the following behavior:
* Thermostat will turn furnace on
* It will warm up, burners will ignite, air will blow
* After a few minutes (and before the temperature increases much), the blowers will cut off and the furnace will stop

If I manually turn the thermostat off and then on again, it will repeat this cycle.

I have made sure the air vents are clear of obstructions and have clean filters. Is there anything else I can check? We're snowed in here, so I can't get a professional out until it melts.
 
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Old 02-12-14, 08:10 PM
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It sounds like your furnace is overheating. This could be caused by a blower fan issue, dirty air filter(s), clogged A/C evaporator coil, blocked return vent(s), or too many supply vents closed down.

Your furnace will log a trouble code. There should be a sight glass in the blower door to allow you to see the flashing LED code. You can open the door to look for the LED and the control board but once a code is logged, don't open the door as it will be cleared.

In the event that both are flashing either slow or fast, but at the same time, then you may have a thermostat issue.


Below is the link to your furnace manual. Page 31 will explain the trouble codes.

Lennox installation/maintenance-G40UH.pdf
 
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Old 02-12-14, 08:17 PM
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PJmax - Thanks for the info. I'm getting alternating slow flash.

The unit itself has more information than the manual does. Alternating slow flash can mean:
* Pressure switch triggered 5 times during ignition cycle
* Burners failed to ignite
* Lost flame 5 times during ignition cycle
* Limit opened 5 times during ignition cycle

The burners are most definitely igniting, so it's not that one. I also don't appear to be experiencing a lost flame (at least, I haven't observed a flame loss). I guess it would be one of the other two, then.

Any ideas about things I can do to troubleshoot? It's not a dirty air filter, as I replaced them this month.
 
 

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