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 Repairing / Servicing Lennox Gas Furnaces
 G51 no heat, alternating slow flash
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Hexar

17 Posts

Posted - 12/28/2009 :  11:34:06 PM  Show Profile  Reply with Quote
Hi Guys,
I have a 4.5 year old G51 furnace that developed this problem last night:

When the thermostat asks for heat, the inducer motor will kick in, and after it runs about 30 seconds, the flames will be light, but it would not run for more than 30 seconds, then the flames shuts off, (inducer motor still runs), then the igniter will glow red, then the flames will come on, but not more than 2-5 seconds then off. This will repeat for 2-3 times, sometimes the blower motor will turn on, then the inducer motor shuts off, then the blower motor shuts off.

After this, the two red/green lights start flashin alternatively.

I have googled and searched this forum and it seems to me that there are 2 main causes for this problem: vent drain and flame sensor. I have drained all the water in the inducer motor assembly (not much) and inducer housing and the black box outside the furnace. I have taken the flame sensor off and tried to clean it with 1000 grid steel wool, and put it back on.

The furnace worked "fine" during the day, but this evening it started the same problem again, it seemed to me this was because during the afternoon hours, because it was warmer outside, and thermostat did not ask for heat for 2-3 hours, and the furnace completely cools down.

When I got home this evening, the two red/green lights are flashing alternatively, so I turn the main power of the furnace off, then turn it back on, it started the "cycles" described above then shut off, then I turn if off then back on, and doing it for 4-5 times, it is "working" normally now.

But I don't think it is fixed. What can cause this problem?

Also, the inducer motor runs really hot - I can barely touch it after it runs more than 5 minutes, is it normal?


Thanks a lot!

Hexar

Cozy.Support

5739 Posts

Posted - 12/29/2009 :  07:01:25 AM  Show Profile  Reply with Quote
If you have an alternating flash on the ignition control, check the furnace grounding. If the igniter is glowing, this is the most likely issue.

See the list of codes on page 49 of this service manual:
http://www.cozyparts.com/Lennox-Parts/forum/Lennox_service_manual/Furnaces/G51MP_09-01-2007.pdf



CozyParts.com, Inc. is an Independent Lennox Dealer providing a national parts distribution resource for Lennox parts since 2002. The Cozy.Support team does not provide a diagnosis for users of this forum. We do, however, try to provide information that will help servicers arrive at their own diagnosis.

Once you know what part you need, you can contact the Cozy parts department by using the link below
http://www.cozyparts.com/Lennox-Parts/locator_form.aspx
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MechAcc

1499 Posts

Posted - 12/29/2009 :  07:51:21 AM  Show Profile  Reply with Quote
As far as the heat of the motor to determine if it is going bad an amperage check must be done and compared to the data plate for the motor.

You have removed the black drain trap once. Remove it again and flush it out. Debris collects in the trap and during an annual furnace servicing this is one of the tasks that should be done. Also flush the drain line that attaches to the trap that goes to the floor drain. These can also collect debris and restrict the flow of condensate.
Remember that the condensate the furnace produces will be a trickle and does not have the force to move debris easily.

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 all of your fossil fuel appliances tested annually for carbon monoxide production by a professional.

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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Hexar

17 Posts

Posted - 12/29/2009 :  08:52:28 AM  Show Profile  Reply with Quote
quote:
Originally posted by Cozy.Support

If you have an alternating flash on the ignition control, check the furnace grounding. If the igniter is glowing, this is the most likely issue.

See the list of codes on page 49 of this service manual:
http://www.cozyparts.com/Lennox-Parts/forum/Lennox_service_manual/Furnaces/G51MP_09-01-2007.pdf


...



Thanks Cozy.Support for the quick reply. I checked the grounding after reading your post, it seems to me it is gounded good, there is a green line coming from the unit and connected to a bare copper line that is sticking outside the unit, and then attached to the gas pipe.

I think some parts in the furnance *needs* warm up now, because if I turn it off and back on several times (thus flames can run for 30-45 seonds each time, warming up the furnance a little), then the furnace can run. Last evening I did this, and it run full night.

I did the same "trick? earlier yesterday morning, it run fine until in the afternoon hours, when it was warmer and the furnace was completely cool down, then it could not run. I had to do the same tricks last evenin.

When flame sensor is bad, will it show this kind of behavior?

Thanks!
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Hexar

17 Posts

Posted - 12/29/2009 :  08:55:43 AM  Show Profile  Reply with Quote
quote:
Originally posted by MechAcc

As far as the heat of the motor to determine if it is going bad an amperage check must be done and compared to the data plate for the motor.

You have removed the black drain trap once. Remove it again and flush it out. Debris collects in the trap and during an annual furnace servicing this is one of the tasks that should be done. Also flush the drain line that attaches to the trap that goes to the floor drain. These can also collect debris and restrict the flow of condensate.


Thanks MechAcc for the quick reply. The main drain pipe to the floor is free, I tried to blow it, and it seems to me that there is nothing in the pipe.

I will try to clean the black box hanging outside of furnace. but When I took the black cap off yesterday, the water inside flew out quick rapidly, and it took about 5-10 seonds to completely drain the black box.

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Cozy.Support

5739 Posts

Posted - 12/29/2009 :  09:10:48 AM  Show Profile  Reply with Quote
Gas lines are not considered to be a proper ground... but that doesn't mean that the gas line is not grounding the unit.

Check the ground using the information at the link below. It is best to check the ground while the furnace while a flame is present.. this can only be a few seconds in some cases, so it can be challenging.

http://www.cozyparts.com/lennox-parts/forum/topic.asp?TOPIC_ID=1370



CozyParts.com, Inc. is an Independent Lennox Dealer providing a national parts distribution resource for Lennox parts since 2002. The Cozy.Support team does not provide a diagnosis for users of this forum. We do, however, try to provide information that will help servicers arrive at their own diagnosis.

Once you know what part you need, you can contact the Cozy parts department by using the link below
http://www.cozyparts.com/Lennox-Parts/locator_form.aspx
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slctech

2085 Posts

Posted - 12/29/2009 :  09:55:07 AM  Show Profile  Reply with Quote
I had a service call yesterday afternoon on a G51 doing the same thing. This one would start-up through the normal sequence of inducer, ignitor, and then burners would ignite. Sometimes within moments or other times it would take a bit, the burners would start to sputter and cough, then extinguish. In my case it was a very audible coughing or woofing of the burners. I removed the sealed burner access panel to expose the burners and everything ran perfectly. Checked the drain, drain trap, tubing, removed inducer and header box and verified no clog or debris build up on the venting side.
This left me with the incoming combustion pipe from outdoors to the unit. Ran system and verified that it was not accidentally sucking in or recirculating flue gases. Then pulled out my trusty 100ft electrical fish-tape, tied a rag on the end and with the tubing cut in side I pushed my fish-tape through to find that the kids were stuffing their Halloween candy wrappers into the pipe and they finally worked their way to a point that was restricting air from feeding the burners.
Not saying you have candy wrappers in your combustion or venting pipes, but would diligently check both pipes for restrictions. Would also make sure your vent pipe has good slope from the outdoor termination back to your furnace with no bowing or sags causing condensate water backing up creating restrictions.
I really expected the differential pressure switch to catch this issue before the flames snuffed out, but it was not.
I hate the alternating slow flashing diagnostic light..it is basically the only code where the furnace is telling you that it has no clue why it is not working. In my experience 90% of alternating slow flash issues are due to clogged/restricted combustion or venting pipe, drain, or secondary issues.

As for your flame sensor, cleaning it is not proof that it is aligned or working properly. Need to measure the microamp/voltage it is generating back to the board. Need a descent multimeter to measure it but here is a good article on troubleshooting
http://www.hvac.amickracing.com/Furnace%20Info/Flame%20rectificatoin.pdf

Good Luck


As a Lennox Technician I feel basic repairs and maintenance can be performed by the homeowner. But I highly suggest that periodic maintenance should be made by a certified licensed technician. You should contact your local Lennox technician for any repairs or issues that are outside your comfort zone. Property damage, personal injury, and death can occur from improperly repaired or maintained systems. Replacing a part doesn't always resolve the issue that caused the part to fail!
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Hexar

17 Posts

Posted - 12/29/2009 :  10:37:45 AM  Show Profile  Reply with Quote
quote:
Originally posted by Cozy.Support

Gas lines are not considered to be a proper ground... but that doesn't mean that the gas line is not grounding the unit.

Check the ground using the information at the link below. It is best to check the ground while the furnace while a flame is present.. this can only be a few seconds in some cases, so it can be challenging.

http://www.cozyparts.com/lennox-parts/forum/topic.asp?TOPIC_ID=1370


Thanks Cozy.Support again. I will try to do this tonight.

I have called the company that installed the furnace to take a look, I could not afford it not running, this Friday the temperature will drop to -30C.

Normally, how long is the parts warranty from Lennox? 5 years? The company says I did not do the yearly service/maintenace, I may loose the warranty from Lennox. Am i required to do yearly service/maintance on a really new furance like mine?

thanks again!
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Hexar

17 Posts

Posted - 12/29/2009 :  10:47:55 AM  Show Profile  Reply with Quote
quote:
Originally posted by slctech

I had a service call yesterday afternoon on a G51 doing the same thing. This one would start-up through the normal sequence of inducer, ignitor, and then burners would ignite. Sometimes within moments or other times it would take a bit, the burners would start to sputter and cough, then extinguish. In my case it was a very audible coughing or woofing of the burners. I removed the sealed burner access panel to expose the burners and everything ran perfectly. Checked the drain, drain trap, tubing, removed inducer and header box and verified no clog or debris build up on the venting side.
This left me with the incoming combustion pipe from outdoors to the unit. Ran system and verified that it was not accidentally sucking in or recirculating flue gases. Then pulled out my trusty 100ft electrical fish-tape, tied a rag on the end and with the tubing cut in side I pushed my fish-tape through to find that the kids were stuffing their Halloween candy wrappers into the pipe and they finally worked their way to a point that was restricting air from feeding the burners.
Not saying you have candy wrappers in your combustion or venting pipes, but would diligently check both pipes for restrictions. Would also make sure your vent pipe has good slope from the outdoor termination back to your furnace with no bowing or sags causing condensate water backing up creating restrictions.
I really expected the differential pressure switch to catch this issue before the flames snuffed out, but it was not.
I hate the alternating slow flashing diagnostic light..it is basically the only code where the furnace is telling you that it has no clue why it is not working. In my experience 90% of alternating slow flash issues are due to clogged/restricted combustion or venting pipe, drain, or secondary issues.

As for your flame sensor, cleaning it is not proof that it is aligned or working properly. Need to measure the microamp/voltage it is generating back to the board. Need a descent multimeter to measure it but here is a good article on troubleshooting
http://www.hvac.amickracing.com/Furnace%20Info/Flame%20rectificatoin.pdf

Good Luck



slctech, Thanks for the detailed reply.

I had similar problems last year: Ice build up clogged the exhaust pipes outside of the house, to a point no exhaust gas can be vented out. My exhaust pipes are those one in another setup, the middle one is the exhaust pipe, and the outside pipe is the fresh air in-take pipe.

About 1 week ago, the ice build up again, but this time it is outside of the inner exhaust pipe, but inside the outter intake pipe, sort of like intween the ring shaped gap between two pipes. I cleared those with a screw driver, and the furnace started working fine.

I hope the kids did not put anything into the pipe, I tried to shine lights to it, but could not see very far. But it seems my furnace will work "normally" after several tries, the vent pipes maybe ok?

Thanks again!
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Hexar

17 Posts

Posted - 12/29/2009 :  10:53:50 AM  Show Profile  Reply with Quote
Another thing I did before the furnace went bad:

There is a fresh air intake vent from outside, about 8" diameter, (this is from the old furance, a 30 year old one before I replaced it with the G51). It is connected directly to the indoor air return duct, although it is insulated, the outside cold air can go directly to the air return duct, and then to the house, so the air return duct is really really cold. And this is not for combustion.

I don't get really hot air from the furnace, so I thought maybe because this cold air from outside is to blame, so I closed the vent.

Will this cause any troubles? Like low pressure of some sort in the furnace?

Thanks!
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MechAcc

1499 Posts

Posted - 12/29/2009 :  12:06:45 PM  Show Profile  Reply with Quote
Having the building in a slight positive pressure add fresh air to the building to remove contaminants. It also helps to make it more comfortable.


There should be a damper installed in the 8 inch pipe and set for required make up air flow. This reduces the amount of fresh air that will be pulled into the structure. Without a damper uncontrolled fresh air is brought in making the structure more positive than needs to be.

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 all of your fossil fuel appliances tested annually for carbon monoxide production by a professional.

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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Hexar

17 Posts

Posted - 12/29/2009 :  11:42:22 PM  Show Profile  Reply with Quote
quote:
Originally posted by Cozy.Support

Gas lines are not considered to be a proper ground... but that doesn't mean that the gas line is not grounding the unit.

Check the ground using the information at the link below. It is best to check the ground while the furnace while a flame is present.. this can only be a few seconds in some cases, so it can be challenging.

http://www.cozyparts.com/lennox-parts/forum/topic.asp?TOPIC_ID=1370




I double checked, the ground bare copper wire was connect to the cold water line at the end.

The company that installed the furnace came and checked with flame sensor and said it is bad, he checked it with a ohm meter with a beeper for continuity, with one lead to the connector end, and one lead to the sensor rod surface, some times there is no beep when he slides through the rod surface with one meter lead.

Is this the right way to check flame sensor? I don't have a multi meter with uA sensitive to check it myslef, based on the service manual you provided.


Thanks again,
Hexar
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Hexar

17 Posts

Posted - 12/29/2009 :  11:45:27 PM  Show Profile  Reply with Quote
quote:
Originally posted by

Having the building in a slight positive pressure add fresh air to the building to remove contaminants. It also helps to make it more comfortable.


There should be a damper installed in the 8 inch pipe and set for required make up air flow. This reduces the amount of fresh air that will be pulled into the structure. Without a damper uncontrolled fresh air is brought in making the structure more positive than needs to be.



Thanks MechAcc. I took the insualation piece I used to plug the vent, just to elinminate one variable.


The service person replaced the flame sensor and cleaned the combustion box, saying it was full of dusts and things. I will see if it works normal or not.

Thanks for all the advices so far!
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VWilson

1 Posts

Posted - 12/29/2009 :  11:55:19 PM  Show Profile  Reply with Quote
I woke up this morning with no heat. My brother did come take a look at it. He says the vent damper isn't working right.
I have a Lennox, isnt there a way to run the furnace without this part? Is there a way to eliminate it all together? After reading about the part it seems that I would not lose enough effienciency in my furnace to lose sleep.... Second night no heat.. Help
VW
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Hexar

17 Posts

Posted - 12/30/2009 :  02:09:53 AM  Show Profile  Reply with Quote
The problem is still there after they changed the flame sensor. :( the alternating slow flashes still light on.

I checked the grounding, there is a H shaped clamp that is used to clamp the bare ground wire to the cold water pipe. I even scraped the cooper water pipe with a screw driver to make sure it has a good contact.

There is a green ground wire that is connected to the combustion box (using one of the 4 screws), I fastern that.

I checked the grounding using the method provided eariler, and I only get 0.2V between Neutral (white) wire and C terminal (Is this one of the 5 terminals that my thermostat wires connect to?), under all 3 running conditions. I guess I may have a ground issue.

What else can cause this problem? I guess if the vent/exhaust pipes are clogged, then they have to cut them to find out and replace? But I kind of dought they are clogged, the furnace could run good after warming up yesterday.

The inducer motor runs hot as usual, will this cause the problems I am having? I am guessing, if it runs hot and therefore slowly, then it will not generate enough pressure/sucktion? This is totally guess though.

Help please! Thanks again!
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Hexar

17 Posts

Posted - 12/30/2009 :  02:26:57 AM  Show Profile  Reply with Quote
On the service manual, it says the flame sensor is 5/16" from the burner surface, on my furnace, I don't see how this distance can be adjusted. The hole where the sensor slide through and the other hole for screw are all fixed.

The fact that the furnance worked before makes me wonder, are there different flame sensors? I mean, same physical dimension, but differnt uA output?

Thanks!
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