# Lennox furnace won't stay lit.



## Montyg360 (Feb 21, 2012)

I have a Lennox furnace that won't stay lite what happens is it lites for a minute or two then cuts out it does this 4 to 5 times. The led lights flash slowly at different times and the chart reads the burner failed to ignite. I have cleaned the flame sensor, checked for blockage in the tubes and the ignitor and bypass the thermostat and nothing is making a difference. Any suggestions.


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## yuri (Nov 29, 2008)

post the complete model #.


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## Legacyairsystem (Jan 6, 2012)

if you have a multimeter that reads dc millivolts, i would touch the flame sensor male plug and its wire female, sould read 12 millivolts dc plus or minus 1-2, while its going through the sequence of operation, anything around 3-9 should be changed hope that helps


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## Marty S. (Oct 31, 2009)

Between 2 and 5 micro amps is what want to see. It's not a milli volt system


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## Legacyairsystem (Jan 6, 2012)

thanks for the correction Marty, i jumped the gun on the micro amp range, was thinking luxaire


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## Marty S. (Oct 31, 2009)

Don't sweat it. Too many furnaces out there to remember them all.


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## lucasrj (Feb 24, 2012)

I'm having the same exact problem. The LED's are flashing slowly alternately on my Lennox Elite G51 furnace which it shows could be one of 4 things. I've cleaned the flame sensor checked the tubes for clogs on the pressure sensors and check the electrical connections.

I've tried to get a reading on the flame sensor as stated above but my sensor only has the one wire going to it so I'm guessing that I should put the other contact on a ground somewhere. I did that and I'm getting around 17 volts with the furnace in the running state.

Another thing that I'm thinking it might be is the previous home owners seem to have taken the burner cover plate off and thrown it away. I just noticed that it had screws that weren't holding anything the other day so I found a drawing and sure enough there should be a cover plate on it. But it has been this way for the past 3 years since we've owned the house so I'm guessing that isn't causing my issue. 

So if it is a bad flame sensor then where can I get one? I've searched all over the web for parts (trying to find the burner cover) and can't find anything for the Lennox Elite G51 furnace.

Any help would be greatly appreciated.


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## yuri (Nov 29, 2008)

you need that cover plate or the furnace may overfire and get damaged as the gas valve won't modulate properly. it needs to sense the pressure in the burner box and w/o the cover won't get a proper reading. cozyparts.com may be able to special order it but you need to phone them and describe it as it will be special order and may take a few weeks to get. what is that code saying. you need to put your meter in series with that wire and where it attaches to the circuit board. you may have an venting obstruction on the intake and they took the cover off to cheat and take air from the house.


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## lucasrj (Feb 24, 2012)

Thanks yuri. The options for the slow flashing alternating LED codes are all watchguard errors and they are:
A. Burners failed to ignite
or
B. Limit open more than 3 minutes
or
C. Lost flame sense more than 5 times in one heating cycle
or 
D. Pressure switch opened 5 times in one heating cycle

Seems to be somewhat erratic. It will run for a couple days and then fall all over itself for a couple of hours and then will run again out of the blue without me doing anything to it other than opening the access panel.


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## yuri (Nov 29, 2008)

Get a cover for it and then we can go from there. Won't have a proper flame w/o it.


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## lucasrj (Feb 24, 2012)

OK so I called and that stupid thing is $152 so is there any way that I can make a makeshift one to test it out before I shell out that kind of money for a little piece of sheet metal?

Or are there any used parts places that you know of that I might be able to try?


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## yuri (Nov 29, 2008)

It is not just a cheap piece of metal, it has a heat resistant silicone gasket and a viewing window. w/o the gasket the burner will pulsate. no used ones that I know off.


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## hvac5646 (May 1, 2011)

I do sheet metal work and u can have it made up, and a bead of silicone should replace gasket. 

But the cost to make it and install it would be more in the $250 range..I would buy the OEM part because it's going to cost you more in grief and money if you don't/


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## lucasrj (Feb 24, 2012)

OK so I received the burner box cover and installed it and now the furnace won't run at all. I think I know why the previous home owner to the other one off. It doesn't seem to be getting enough air and the flame cuts out. We have about 6 feet of 2 1/2" PVC going from the furnace and then it goes to 3" and there is about 20 feet of the 3" PVC to make it outside. There are roughly 4 or 5 90 degree elbows in the mix to make it outside.


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## how (Feb 26, 2011)

When you say the furnace won't run at all with the cover on, how far does it go in the heating attempt and what code do you now get with the door in place when it locks out?


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## hvac5646 (May 1, 2011)

seems like a lot of elbows to me....cutting out on press switch.

disconnect the fresh air intake with cover on to see if i runs then.


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## lucasrj (Feb 24, 2012)

From a fresh start the flame will last for about 30 seconds (just enough time for the blower to kick on), on the subsequent retries the flame will only last about 5 seconds. The flash code is the same: 

A. Burners failed to ignite
or
B. Limit open more than 3 minutes
or
C. Lost flame sense more than 5 times in one heating cycle
or 
D. Pressure switch opened 5 times in one heating cycle

By "disconnect the fresh air intake..." do you mean from the bottom of the burner box? The air supply line is glued to the fitting for the burner box so I can't disconnect that pipe.


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## hvac5646 (May 1, 2011)

I mean the PVC pipe that connects to your burner box.


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## lucasrj (Feb 24, 2012)

OK so I cut the PVC pipe about 2 inches out from the outside of the furnace (so that I can glue a coupler on it after the test) and it still cuts out at the same time with the burner box cover on it.


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## lucasrj (Feb 24, 2012)

Any thoughts of what I can try next?


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## hvac5646 (May 1, 2011)

now dis connect the exhaust by means of the rubber coupler at the inducer motor.


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## lucasrj (Feb 24, 2012)

OK I was finally able to get some time to mess around with this again. I took the rubber tub out and let the exhaust go out into the mechanical room and it seems to run just fine. So that tells me that either my inducer motor is going bad or I have a blockage or too many 90 degree turns in my exhaust pipe run.

Am I correct in this assumption?


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## how (Feb 26, 2011)

A pro also would manometer the pressure switch hose to see what the "WC measurement is case only the pressure switch is faulty.


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## hvac5646 (May 1, 2011)

did you see how many elbows were on the system, how?

yes, check the press switch, but it's not the problem.


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## hvac5646 (May 1, 2011)

lucasrj said:


> OK I was finally able to get some time to mess around with this again. I took the rubber tub out and let the exhaust go out into the mechanical room and it seems to run just fine. So that tells me that either my inducer motor is going bad or I have a blockage or too many 90 degree turns in my exhaust pipe run.
> 
> Am I correct in this assumption?


so far, yes, with the exception of the inducer being bad. 

Just for fun I would use a manometer on the inducer to check the press.

And the press switch too ....but honestly...with you moving in and finding the burner door missing, i am putting my money on too many elbows.


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## how (Feb 26, 2011)

Hey HVAC
did you see how many elbows were on the system, how?

Yeah I hear ya and don't disagree but

Today there is a thread with 16 posts which ends with the op saying he fixed his problem by changing his filter.

Would you actually be suprised if the previous owner had a problem with a pressure switch that went away when he opened up the burner door to investigate? It's unlikely that either op had/has access to a manometer. 

And

If that system was ever tested & approved, it *was* with the burner door closed.
Time will tell.


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## lucasrj (Feb 24, 2012)

That is correct I don't have access to a manometer but am debating on buying one but wondered if you could tell me how much the pressure switches are roughly for my model?

As I see it I either replace the switches (2 x $??)
or
Pay a tech to come out and tell me they are bad ($150 +)
or
Buy a manometer and test them to see if they are bad ($70 +)

Thanks again for all of the help!

I also just noticed that while running it cut out after around 5 + minutes of run time without the burner box cover on. Do you think this points even more so at the pressure switches?


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## how (Feb 26, 2011)

A. Burners failed to ignite
B. Limit open more than 3 minutes
C. Lost flame sense more than 5 times in one heating cycle
D. Pressure switch opened 5 times in one heating cycle

A. Doesn't apply. You have ignition.
B. Temporarily bypassing the entire limit safety circuit will tell you if any of the limits are causing the problem if the furnace runs fine while they are bypassed. 
C. Arises from the flame sensor not receiving a good enough signal. This can be because something is pulling the flame away from the flame sensor, a bad board, gas restriction or the flame sensor circuit output is too low. 
If the gas is not always enveloping the flame rod, you should probably be able to see that. A meter with a micro amp setting is used to measure the flame sensor circuit to prove if it's good enough.
D. A manometer(in the right hands) will prove if the pressure is fine and the manometer is fauty or if the pressure is too low and the pressure switches are fine.

I also just noticed that while running it cut out after around 5 + minutes of run time without the burner box cover on. Do you think this points even more so at the pressure switches? Not nessesarily! 

There are a lot of possible reasons for your furnace problems that a Tech with the proper HVAC tools should be able to identify. Maybe this
is the time to call one.


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## lucasrj (Feb 24, 2012)

OK so I finally got some time to look into this issue some more. Haven't been too worried about it since summer seems to be here now for us.

But here is what I did. I ran a roto rooter type device through our exhaust pipe to see if maybe there was some sort of blockage (when we moved in there weren't any guards or grates on the supply or exhaust pipes so I was thinking that something could have gotten in there or built a nest etc) and we didn't seem to find anything blocking but when we pulled it back we did find that the end was soaking wet.

So I took a closer look and found that when they originally installed the furnace they covered the drain pipe on the right hand side with sheet metal for the cold air return and also put the sealant over it. So over time it has filled with water. 

Also when we had the basement finished 2 summers ago the HVAC guy had to reroute the pipes and ended up leaving the exhaust pipe a little low so the water was backed up in it also. My guess is that there was about 1/2" or maybe more sitting there in the exhaust pipe and thus hindering the exhaust path.

So today I took the black rubber drain hose and cut it and rigged a sort of P trap so that the water could drain but wouldn't let any exhaust fumes out and then I also cut the PVC exhaust pipe and rerouted it so that the pipe just barely angled down enough that the condensation would run into the black drain hose as it should but shouldn't allow it to run down into the exhaust pipe as it had been. This also allowed me to take 3 of the elbows away in the 2" piping that they had.

Well I put the burner box cover on and ran the furnace and it ran for a half hour (that is all that I could bear to run it for since it was hot today) which made me quite happy since it has never run for more than 30 seconds before with the burner box cover on it.

I'll find out for sure this fall/winter I guess.

Thanks again for all of the help/suggestions!


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