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Old 01-17-2013, 04:48 PM   #1
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Lennox G20 problem


I have an older Lennox G20 system that I'm having trouble with. When a call for heat arrives, the pilot light lights, the main burner comes on and after 30 seconds the blower motor comes on. However, within 2 or 3 minutes the system will shutdown and go through a restart. I assumed the problem was similar to other systems like this and it had an older style ignition system, so I replaced both the flame sensor and the ignition control board. The problem is still the same, however now I have an LED to help troubleshoot. The led flashes 8 times which doesn't match up to any of the diagnostic codes in the manual. After further testing, the board is grounded properly, the wire to the flame sensor ohms out good and the connections are tight. 24V line has 24.5 volts and holds steady until shutdown. So, I'm kinda at a loss. Any suggestions?

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Old 01-17-2013, 05:07 PM   #2
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Lennox G20 problem


I just looked at the manual for the G20, and it has a damper that opens through a servo motor. I would say to first make sure your flue is clear, then make sure that damper is working. It sounds like you may have a flame roll out problem due to lack of draft.

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Old 01-17-2013, 05:26 PM   #3
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Lennox G20 problem


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Originally Posted by bclasing View Post
I have an older Lennox G20 system that I'm having trouble with. When a call for heat arrives, the pilot light lights, the main burner comes on and after 30 seconds the blower motor comes on. However, within 2 or 3 minutes the system will shutdown and go through a restart. I assumed the problem was similar to other systems like this and it had an older style ignition system, so I replaced both the flame sensor and the ignition control board. The problem is still the same, however now I have an LED to help troubleshoot. The led flashes 8 times which doesn't match up to any of the diagnostic codes in the manual. After further testing, the board is grounded properly, the wire to the flame sensor ohms out good and the connections are tight. 24V line has 24.5 volts and holds steady until shutdown. So, I'm kinda at a loss. Any suggestions?
Does your blower stay on the entire time as the unit restarts? If so a limit could be going out. Make sure you filters are clean and that the unit is moving plenty of air.

If you could give me your full model # and the part number of the board you installed I may be able to look up that diagnostic code for you
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Old 01-18-2013, 07:30 AM   #4
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Lennox G20 problem


Thanks for the replies.

The motor does not stay on during the restart. I cycles off appropriately. I checked the overlimit circuit and it looks fine.

I also checked the damper circuit. The servo cycles the damper open at the appropriate time. when the damper opens it closes a switch that provides 24v to ignition board which works appropriately.

What appears to be happening is that the ignition board is determining some type of trouble since the LED on it is indicating some type of fault, but the fault is not listed in the manual (8 flashes - btw it is not 6+2 flashes, which is a valid fault code). Then the 24 volt line from the motor control board which supplies the 24 volts to the ignition module and gas valve drops. Perhaps the motor control board is bad, but I just don't think so.

I have a schematic of the ignition control system, but not one of the motor control board or a trouble shooting guide. I haven't been able to find those, but will look some more this morning.

Thanks for any thoughts.

Bruce
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Old 01-18-2013, 11:43 AM   #5
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Lennox G20 problem


Carefully watch that the flame around the flame sensor is not being disturbed just before the gas shuts off.
That flame sensor needs to be fully emersed in flame to keep the furnace going.
If there is a change in the flame, and you don't have a heat outlet blowing directly at it, then the heat exchanger should be checked for cracks.
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Old 01-18-2013, 12:50 PM   #6
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Lennox G20 problem


Found This:

OK hereís the situation: Our furnace decided to not turn on in the mornings a couple weeks ago when the temps dropped into the 20s. I figured it was the thermostat, so I went out and got a nifty Ritetemp one with the remotes, etc. etc. All was good until today. We have had pretty warm conditions for the past couple weeks, but today the temp dropped back into the 20s, not the teens, and the furnace isnít working again. The first time round, the house was cold in the morning, and I would have to turn the heat off, then back on at the thermostat, and all was good again. Now itís doing it every time. Our heat hasnít kicked in automatically all day. Every time I have had to turn the heat off, then back on at the stat. Once we get it going it goes through the cycle with no problems, but when it cools off in the house again it wonít kick back on. It doesnít seem to be trying to kick on. Equipment: Lennox G20 Furnace Model #G20Q3/4 E-100-5 Manufactured 5/8/97 Ignition module is Johnson Control G776RGD-11 The reason I mention the ignition module is because I have heard several stories about these being junk. I have read that there is a Honeywell replacement that lennox is telling people to use. However itís sunday night, and no one to ask. I have not been able to track down a model number for a honeywell replacement for my exact module. Any input would be greatly appretiated. Replies: cable ďRe(1):Lennox G20 problemsĒ , posted Thu 15 Feb 19:44
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Old 01-19-2013, 11:24 AM   #7
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Lennox G20 problem


Quote:
Originally Posted by how View Post
Carefully watch that the flame around the flame sensor is not being disturbed just before the gas shuts off.
That flame sensor needs to be fully emersed in flame to keep the furnace going.
If there is a change in the flame, and you don't have a heat outlet blowing directly at it, then the heat exchanger should be checked for cracks.
Again thanks for the tips. I fixed it yesterday afternoon with a little troubleshooting and a lot of luck. I had heard the problem might be a grounding issue with the ignition module or the burner assembly that holds the flame sensor. So I was running a ground wire between them and got a little tired of running up and down the stairs to fool with the thermostat so I jumpered the contacts closed on the motor control board and found out the system worked fine when it was jumpered for constant "call for heat". After scratching my head for a little while wondering if the grounding was fixing it or was it the thermostat, I removed the grounds and it continued to work. I took the jumper off the thermostat wires and went back to call for heat from the thermostat and the problem returned.

I replaced the thermostat and all is working well. It was an older mercury thermostat and I don't see anything wrong with it, but it is going to the recycle center.

Thanks to everyone that replied.

Bruce

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