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 Repairing / Servicing Lennox Gas Furnaces
 Ignitor problem ? - G24M3/4-100A-2
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computer-guy

8 Posts

Posted - 12/18/2006 :  03:28:25 AM  Show Profile  Reply with Quote

Hello,

Back in October, I began the process of troubleshooting my Lenox G24M3/4-100A-2 furnace. It's now mid December and I'm still troubleshooting the problem. Here's the sequence of events. When I do a call for heat, the combustion blower starts running. A short time afterwards the gas valve opens, the ignitor sparks up and the gas gets lit. All 5 burner tubes light up. Flame current checks as good (single flame sensor). After the gas has been burning for a little while, it shuts off (gas valve turns off)before the thermostat comes up to temperature. Combustion blower and main blower are still running. Here's the problem - when the ignitor sparks up again, it fails to light the gas. I know for sure that the gas valve is open and supplying gas. It could be insufficient gas flow. I tested that possibility by sticking a long necked "butane lighter" to the same spot as the spark ignitor. When the gas valve was open and the ignitor was sparking but not lighting the gas, I flicked on my butane lighter. The butane lighter NEVER failed to light the gas. Am I correct in assuming that the gas pressure is probably O.K. ? Could the control circuit board be NOT supplying a hot enough spark to light the gas ? The ignitor is clean, I polished it. Ceramic on it is not cracked. Ground connection is good and clean. I can see a spark develop between the positive and ground electrodes on the ignitor. Looks normal. The control board in this G24M3/4-100A-2 furnace is the original one.

MechAcc

1499 Posts

Posted - 12/18/2006 :  07:25:36 AM  Show Profile  Reply with Quote
The burners could be dirty. The igniter and flame sensor are positioned, if I recall correctly, in front of the flame carryover or crossover bar. These carry the flame from the first burner that ignites all the way to end for the last burner. Household dust and dryer lint get pulled into the burner during the course of normal operation. The lint will will buildup to the point that it will the crossover bar with enough lint that it will start to interfere with the correct lighting of the burner. Sometimes it will light and others it will not till eventually it will not light at all.First remove the flame sensor and the igniter then the burners and clean them wash them out with water then dry them and blow the insides of the burner out with compressed air and wire brush the face where the flame comes out. Clean the igniter and flame sensor with steel wool or a stainless steel toothbrush. Use an inspection mirror and carefully inspect the face and orifice hole of the burner spud. If you see a buildup of debris inside the orifice the burner manifold bar should be removed from the furnace then the gas valve and the burner spuds. Blow the manifold bar out with compressed air and the orifice holes with a wooden tooth pick. Then reassemble.

The spark that you may have may be a weak spark. The possible cause of this would be a buildup of resistance on the ignition cable or the ignition control board can longer generate a hot enough spark.

I am not an employee of CozyParts. The opinions that I post are my own.

Please have your furnace inspected annually by a qualified service technician. Have them test the undiluted combustion gases for proper combustion and carbon monoxide production.

Get a good Carbon Monoxide Alarm. Replace it according to manufacturers recommendations usually every 3-5 years. CO concerns are not just for the winter but 24-7. UL approved alarms alarm high. For a low level alarm do a search for CO Experts or NSI 3000 a low level CO monitor.
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Cozy.Support

5739 Posts

Posted - 12/18/2006 :  08:44:57 AM  Show Profile  Reply with Quote
The placing of the G24M igniter can be critical on some of these furnaces. The igniter that is included with the 62K50 update kit is slightly different than the original igniter, and solves this issue as far as I know. You might try re-locating it just slightly.

As for your original issue that is causing the furnace to shut down prematurely, I would check the induced draft pressure switch and the limit switches. The limit switches are easy to monitor with a volt meter.

You never said if the main blower is coming on before the burners shut down. If not, the axillary limits on the blower housing should be observed with a volt meter, particularly if the unit is in a horizontal or downflow configuration.

As for the induced draft pressure switch, the easiest way to see if it is the source of the problem is to jump the switch temporarily while the furnace is in operation. The way you do this is to disconnect the wires from the induced draft pressure switch and then start a call for heat. As soon as the induced draft blower comes on, connect the two wires together. The induced draft pressure switch opening is not always apparent to many servicers, and this method of checking them seems to work best.
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computer-guy

8 Posts

Posted - 12/18/2006 :  11:28:35 AM  Show Profile  Reply with Quote
Thanks guys for the replies. The burners (metal with holes cut out - like looking at a cylinder of a police revolver) are definitely clean - no lint or tar or corrosion. The burner "wings" are clean also. The pressure switch that's connected by a small rubber hose to the exhaust blower is working. I verified it by checking with an ohm meter and also by shorting it with a alligator jumper cable after the beginning of a heat cycle. When it's shorted, and if the flame gets ignited, the flame will stay on until the thermostat reaches temperature. If the switch is unjumpered (like normally) the pressure switch seems to cut off after the flame has been burning a while (pressure differance caused by hot exhaust flue gases ?). After the gas supply turns off, the main blower motor still runs. Later on, the gas supply gets turned on again and the ignitor starts to spark. If the gas lights, then it will continue to burn until the controller board cuts it off again, or until the thermostat reaches temperature.

May I pose a question ? Regardless of any pressure switch, or flame sensor or anything, when the gas valve is known to open and supply a combustible mixture of gas, and an ignition source is present, shouldn't the gas burn ? Beacause, that's exatly the conditions that are present now. I know that the gas valve opens - I hear and smell gas. I know that the quantity of gas and air are sufficient for combustion. I held a long necked barbecue lighter right at the same spot as the spark ignitor and lit the gas when the ignitor wouldn't do it at the start of a heating cycle.

Keep in mind, that when a call for heat is done, the furnace will often fail to go through even ONE heating cycle. It will sit there and spark 3 times in a row (with delays in between) and the gas will never light up. When I manually light the gas with a barbecue lighter it always works. If the spark ignitor sparks for more than 1.5 seconds it will usually NEVER light the gas. If it lights the gas, it usually lights the gas in under half a second. When I notice that the spark runs for more than 1.5 seconds, I flick my barbecuse lighter and it always lights the gas.

I cleaned and polished the ignitor electrodes. I changed the distance of the gap between the ground and center electrode - reduced the distance. It made no differance. I slightly bent the center and ground electrodes upward. Upward so that the electrodes were lined up with the large center hole on the burner (first burner on the right). Not such a good idea ! It would still fail perhaps twice during a heating cycle to light the gas. But the bad part was that I got a "flash back". It seems that the spark wouldn't ignite the gas outright. But the gas valve being open for about 4 seconds would cause enough of a gas build up under the metal cover (it covers the burners - has 2 temp. limit switches on it) that when the spark DOES ignite the gas, I get what looks like a flash fire and the gas valve turns off. The controller probably turns off the gas out of safety ! The furnace continues to run, including the main blower motor and combustion blower. The furnace will then attempt to start another ignition cycle.
I readjusted the ignitor electrodes back to the same position. I get no more flash backs.
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computer-guy

8 Posts

Posted - 12/18/2006 :  11:40:44 AM  Show Profile  Reply with Quote

Does anyone know the high voltage reading of a working G24M control board, on the ignitor terminal ? The high voltage transformer on the control board is a flyback transformer much like you'll find on smaller black & white TV's. I seriously doubt that it's putting out more than 20,000 volts. I can borrow a high voltage probe and see what it's putting out.
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MechAcc

1499 Posts

Posted - 12/18/2006 :  6:10:13 PM  Show Profile  Reply with Quote
You can not visually see if the narrow gaps on the crossover (lightover) bars are clean inside. They must be pulled and cleaned out to ensure that they are clean. Those little wings are very narrow on the inside. Please remove and clean them just to eliminate them as a possible cause of the lighting problem.

Is your furnace operating on natural or propane gas?


I am not an employee of CozyParts. The opinions that I post are my own.

Please have your furnace inspected annually by a qualified service technician. Have them test the undiluted combustion gases for proper combustion and carbon monoxide production.

Get a good Carbon Monoxide Alarm. Replace it according to manufacturers recommendations usually every 3-5 years. CO concerns are not just for the winter but 24-7. UL approved alarms alarm high. For a low level alarm do a search for CO Experts or NSI 3000 a low level CO monitor.

Edited by - MechAcc on 12/18/2006 6:12:02 PM
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computer-guy

8 Posts

Posted - 12/18/2006 :  7:18:00 PM  Show Profile  Reply with Quote
Question: I am not trying to refute your suggestion since all suggestions are welcome. But if clogging was an issue with the other burners, how come burner number 1 - with the ignitor in front of it - is failing to light up ? Why am I able to light the gas with a butane lighter ? If it was clogged then NOT ENOUGH gas will flow through the burner holes. Yet I can consistently light all the burner tubes from just lighting the first burner on the right with a long butane lighter. I'm just trying to think logically here. We're talking zero ignition on burner number 1 when the spark goes off. I have had the burners off the furnace and sitting on my work bench under full view. There is no dust or debris inside the body of any of the burner units. Trust me on this one. I've worked as a computer technician for years and paying close attention to detail is my job and hobby. I've even looked at the burners through a jeweler's loupe and I saw no debris.
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computer-guy

8 Posts

Posted - 12/18/2006 :  7:20:11 PM  Show Profile  Reply with Quote
What does Cozy support have to say about all this ? You guys are the experts and I am relying on your opinion.
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Cozy.Support

5739 Posts

Posted - 12/18/2006 :  8:17:45 PM  Show Profile  Reply with Quote
You know what they say about experts... we have a lot of experience, but we see something new every day.

I have resolved the same issue by placement of the ignter.... with that said, I would check the manifold pressure, particularly if you have a propane conversion kit installed (since you have not mentioned turbulaors, I assume this is not the case). There is always the possibility of rare issues such as the manifold not being drilled square or inline. I would think if the unit has worked all of these years, a rare issues such as this would not be likely. Another possibility, while not likely, would be debris in the manifold.

If the igniter placement is not the issue, the most likely issue would be incorrect manifold pressure. Without tools to measure the pressures, it is going to be hard to rule out these kinds of issues.
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Cozy.Support

5739 Posts

Posted - 12/18/2006 :  8:21:47 PM  Show Profile  Reply with Quote
If you still have the original RAM circuit board, you might try cutting about a half inch off of the spark plug wire and plug it back into the barb on the high voltage tower. There has been some reports of the spark plug wire developing a lot of resistance where it connects onto the high voltage tower barb on the circuit board (on some older installations). Usually, these issues will yield a yellow spark, or no spark at all, instead of the normal blue spark.
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MechAcc

1499 Posts

Posted - 12/19/2006 :  04:08:46 AM  Show Profile  Reply with Quote
Because clogging on #1 burner may be more severe than the others if the spark rod is directly in front of the lightover bar gas may not be getting to the spark for ignition before safety time out. Your butane lighter flame provides a much broader ignition source than that skinny spark.

Now as for the jeweler's loupe inspection. Was this inspection done with the burners installed or removed? If it was with the burners installed humor a 59 year old furnace guy. Remove the burners wire brush the face of the burners, clean and then blow the insides of the burners out with compressed air and then across the face of the burner. Then reinstall the igniter and flame sensor. Make sure that the igniter is parallel with the face of the burner. If someone removed the burners in the past without first removing the igniter and flame sensor the tips may have been bent away from the burner face.



Wish I had the specs on the gap between the ground rod and the igniter. On oil burners I know that they are set with a gap of 5/32nds to 1/4 inch. If the gap has narrowed an even smaller ignition target will be presented for the gas to ignite. You have to remember that is not pure gas coming out the burner for ignition but a gas/air combination. Since you stated that you bent the ground rod closer to the spark rod you have reduced the size of the ignition source and the center of the burner is not the optimum location. Has the rod been bent to that location? The locations that I recall is at the edge closer to where the burner starts to flatten for the crossover.




I am not an employee of CozyParts. The opinions that I post are my own.

Please have your furnace inspected annually by a qualified service technician. Have them test the undiluted combustion gases for proper combustion and carbon monoxide production.

Get a good Carbon Monoxide Alarm. Replace it according to manufacturers recommendations usually every 3-5 years. CO concerns are not just for the winter but 24-7. UL approved alarms alarm high. For a low level alarm do a search for CO Experts or NSI 3000 a low level CO monitor.

Edited by - MechAcc on 12/19/2006 04:43:05 AM
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MechAcc

1499 Posts

Posted - 12/19/2006 :  04:19:33 AM  Show Profile  Reply with Quote
Now for premature shutdown. Thermostats are designed to shutdown the burners before it actually reaches the set temperature. Residual heat off the heat exchanger brings the room temperature up to the thermostat setting. If the temperature is not getting up to setting the heat anticipator on the thermostat may be set too low of current flow and may need to be adjusted higher or the thermostat is old the temperature sensing portion may be out of tolerance.

What type of thermostat do you have? Digital or the older mechanical type?

I am not an employee of CozyParts. The opinions that I post are my own.

Please have your furnace inspected annually by a qualified service technician. Have them test the undiluted combustion gases for proper combustion and carbon monoxide production.

Get a good Carbon Monoxide Alarm. Replace it according to manufacturers recommendations usually every 3-5 years. CO concerns are not just for the winter but 24-7. UL approved alarms alarm high. For a low level alarm do a search for CO Experts or NSI 3000 a low level CO monitor.
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computer-guy

8 Posts

Posted - 12/22/2006 :  03:21:32 AM  Show Profile  Reply with Quote
Dear MechAcc: Your suggestion about the placement of the ignitor and the width of the gap on the ignitor helped me fix the problem. I've had heat since the morning of Dec. 19. After bending the center and ground electrodes on the ignitor the gas has consistently lit up. I'll be ordering a new ignitor and dual rod flame sensor kit. The existing one should work O.K. but the center electrode is a little loose. Thank you very much for your help. If you have a Paypal account, I'd like to give you something for your help. If not, maybe you can give me a snail mail address and I can send it to you.

Sean Martin
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MechAcc

1499 Posts

Posted - 12/22/2006 :  06:12:34 AM  Show Profile  Reply with Quote
quote:
Originally posted by computer-guy

Dear MechAcc: Your suggestion about the placement of the ignitor and the width of the gap on the ignitor helped me fix the problem. I've had heat since the morning of Dec. 19. After bending the center and ground electrodes on the ignitor the gas has consistently lit up. I'll be ordering a new ignitor and dual rod flame sensor kit. The existing one should work O.K. but the center electrode is a little loose. Thank you very much for your help. If you have a Paypal account, I'd like to give you something for your help. If not, maybe you can give me a snail mail address and I can send it to you.

Sean Martin



No need. But do me two favors. One get the furnace tested and inspected next year by a professional. Make sure that you want CO and combustion testing done. A service tech can determine if there is any problem with how well the furnace operates by analyzing the undiluted flue gases. Keep calling a company until you find one that does. To the ones that don't do testing don't do testing don't have them come but do ask them " Why don't you test?" Get a good carbon monoxide alarm for the sleeping areas of your house. Kidde and First Alert are alright, but they alarm at around 70 parts per million. Do a search for CO Experts and NSI3000. These are both low level alarms. They use a sensor similar to that used in analyzer meters. Kidde and First Alert models use a different. Since CO Experts and NSI use a better sensor their price will be much higher. They both have digital readouts. They start alarming around 5 parts per million and as such are not UL approved. The Kiddes and First Alerts will save your life the CO Experts and NSI 3000 will save your health.


I am not an employee of CozyParts. The opinions that I post are my own.

Please have your furnace inspected annually by a qualified service technician. Have them test the undiluted combustion gases for proper combustion and carbon monoxide production.

Get a good Carbon Monoxide Alarm. Replace it according to manufacturers recommendations usually every 3-5 years. CO concerns are not just for the winter but 24-7. UL approved alarms alarm high. For a low level alarm do a search for CO Experts or NSI 3000 a low level CO monitor.
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Cozy.Support

5739 Posts

Posted - 12/22/2006 :  06:51:12 AM  Show Profile  Reply with Quote
The spark gap on the G24M3/4-100A-2 should be 1/8" (+/- 1/64th).

The biggest improvement with the 62K50 upgrade kit is flame current. You will get a much stronger flame current with the dual rod flame sensor.
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computer-guy

8 Posts

Posted - 12/22/2006 :  5:59:08 PM  Show Profile  Reply with Quote
MechAcc - thanks for the two tips. I already have new CO detectors and natural gas detectors around the house, but I should get something more sensitive for the CO. I got the Kidde ones.

Cozy Support - thank you for the parts info and for the gap width.

Everyone have a Merry Christmas !

Sean Martin
EDI Computers
Albuquerque, New Mexico
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