Furnace burner keeps cutting out

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Old 01-15-16, 10:37 AM
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Furnace burner keeps cutting out

Hey everyone. Im new to the forum and would like to start out by saying hi and am happy to be a new member. That being said i need some help.

Im having trouble with my furnace. My furnace is Lennox 80MFG2-75A-6
So what happens is the furnace will start it will do its prepurge, than the igniter will start and all the burners will light. The furnace will run for about 5 minutes putting out good heat (i would consider it good i'm new to this house) and than the furnace will trip. The fan will continue to run and eventually the furnace will start up again tripping after about 5 minutes again.

Things i inspected

1) Flame rod - thought maybe the flame rod was dirty and it wasnt getting a good milivolt signal so the gas valve would trip. I cleaned it with some sand paper and its nice and clean

2) The filter is a cleanable filter i took it out and cleaned it really good, also tried running without a filter.. Thought maybe there was some bad airflow.

3) Which brings me to what i think the problem is. I'm pretty sure the limit switch on my heat box is getting too hot and cutting out the furnace. I cleaned the filter, i made sure all of my registers were open, i inspected my A-frame H/E(it was clean) I also looked at the setting on the circuit board and i see the fan is running at speed 2 out of a maximum of speed 3. I did all this to ensure there was enough air flow to keep the limit from getting too hot. I hooked up a multimeter and the highlmit does cut out. Is it possible im still not getting enough air flow and its still getting too hot? or could it be a bad limit? When i touch the heat box it is too hot too touch, as well as the exhaust ducting at the furnace and even on the roof the exhaust ducting is hot.

Thanks in advance

Jesse
 
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Old 01-15-16, 11:42 AM
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So far pretty good troubleshooting in that you found the limit switch is what is tripping.

Next thing: Is temp within spec? Check for rise temperature rating on the plate/sticker inside the furnace. This would be how hot the air is coming out relative to going in.

Can see if it is a restriction in the supply air by running with the blower box cover off and the safety switch taped for a few minutes.

fyi - manual at http://www.hvacpartsshop.com/manuals/80MGF_Series.pdf
 
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Old 01-15-16, 12:55 PM
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ok thanks. the limit switch says L175-30F
Im assuming this means the temperature coming out is 175F and is allowed to raise 30F to a maximum of 205F.

I tried running with the furnace box open.. no air restrictions and i ran the furnace for 10 minutes. It didnt cut out. I than turned the furnace off and put the cover back on, found 2 dampers in the basement that were probably 25% closed so i opened those wide waited 10 minutes, just to let things cool down fired it back on and it tripped after 3.5 minutes.

So if the problem is air flow and all my dampers are open does that mean i have a problem with my fan?
 
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Old 01-15-16, 12:59 PM
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The L175-30F opens at 175F and closes at 145F.
The 175 is the upper limit temperature and the 30 is the differential.

If your furnace runs ok with the blower door off..... but not when it's on..... sounds like a problem in the return side. Undersized ductwork, blocked return register.
 
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Old 01-15-16, 01:10 PM
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Ok makes sense with the temperatures.. the cover i had off was on the discharge side. So all the hot air was just blowing into the basement. The inlet side i left in tact.
So what would be my next step in trouble shooting this.
 
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Old 01-15-16, 03:54 PM
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<<ok thanks. the limit switch says L175-30F
Im assuming this means the temperature coming out is 175F and is allowed to raise 30F to a maximum of 205F.
>>



This means that the limit switch opens and shuts off the burners at a temperature of 175, and that the switch will close and turn on the burners at 145 degrees.


Very likely you have an air flow problem through the ductwork or furnace. You need to do a through inspection of the whole system until you identify the problem or problems.


You can start with the furnace. If you have air conditioning, the AC coil in the furnace may be plugged with dirt. You need to inspect that carefully, which may require that the blower assembly be removed. While you have that out, check to see that the fan wheel turns freely, that the fan blades are clean and not broken or damaged.

Power up the fan to verify that it starts up easily and appears to come up to a normal operating speed.

Often ductwork and plenums are lined with insulation to reduce noise. That insulation can come loose and restrict the air flow. Check for that kind of problem or other defects, such as duct work that is being blown outward or sucked inward.


The furnace rating plate lists the static air pressure for the furnace. A pro would use a manometer to check the static air pressure at various places through the ductwork and furnace to identify where the problem is.
 
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Old 01-16-16, 07:45 AM
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Thanks for the reply.

- I checked the A frame ac H/E it is very clean
- i checked the fan for rotation it spins very easily.
- As for the ducting restricting airflow thats getting to be a big job. If i got my furnace cleaned out would they be able to identity the problems as they were cleaning. ?
 
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