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Thread: Lennox furnace shutting off with burner on

  1. #1
    51don is offline New Member
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    Lennox furnace shutting off with burner on

    Heya,

    I have a Lennox natural gas furnace. It cycles the burner on and off during the heating cycle, and will sometimes shut off with the burner on. It's very hot around the furnace when this happens, as well as near the large cold air return (or whatever the proper term is). It smells very hot near the cold air return as well.

    The startup cycle is the same as always. However, when the furnace is in its main cycle (burner on, fan on, blowing heated air through ducts to rooms), the burner will shut off. Okay, sure, that's normal - if the furnace was going to shut off in ~45 seconds. Except the burners will relight, and the furnace doesn't shut off. This will happen once or twice before the furnace shuts off. Normally the burner never relights once it's off. Once the burner is off, normally there is about 45 seconds of the fan blowing air through the house before the whole furnace shuts down, as the air near the thermostat has reached the desired temperature. However, now, the burner is relighting, and sometimes the furnace shuts down without doing the cooling down phase. This leads to the aforementioned heat around the furnace and cold air return, as well as the hot smell.

    To be clear, the furnace is heating as fast as it used to. Normally the burner would stay on for several minutes first thing in the morning (when it's bringing the house up ~10c), but now it's on for a few minutes, off, on again, etc. It's heating fine. It keeps the temperature up quite well. Just...it doesn't seem very safe what's happening. Flames are bright blue as always and I don't smell any exhaust. When it's on, I can see white smoke coming out the chimney. So I assume everything is venting fine and there is enough oxygen for the gas to burn properly.


    It's a Lennox Diplomat, beige color. I can't find any information about that product line (I didn't put it in, the guy who lived here before us did, and apparently did it himself). I forget the model number, but I did a search for it once on Google and found very little. I'd estimate the furnace is ~10-15 years old. I had a service tech tell me the circuit board was bad about 5 years ago, when a very similar problem was happening (except then, the furnace wasn't keeping the temperature up or heating the house properly). Another service guy said it was the air switch (old one was improperly installed or something), and since that was $50 vs a $500 circuit board, guess which got replaced?:P The furnace has been fine since then; this problem only cropped up about 2 weeks ago, but it's been getting progressively worse with regards to the smell and hot air around the furnace.


    Yeah this is a long post, but I hope I provided enough information for someone to help.:) I'll be getting someone to service it come next week; don't feel like paying double for weekend service. Besides, it's still about 7c here in BC.:P

  2. #2
    Hube is offline Journeyman
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    Your description of what is happening is very confusing.
    A furnace that has a dirty air filter or other obstructions in the return or supply ducts will shut the burners off on "high-limit". It will then re-fire again after a short time (maybe 1-2 minutes) and then shut down again on hi-limit. The fan usually will continue to operate when this hi-limit burner shut down occurs.
    Is this what is happening?
    Post back with some definite detail and we will try and give some help.

  3. #3
    51don is offline New Member
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    I posted exactly what's happening. Sometimes it does as you explained (and I explained in the original post), and sometimes it shuts down the furnace without the cool down cycle. When the former occurs, there's no excess heat around the cold air return and furnace body. When the latter occurs, it's very hot near both objects, and there is a "hot smell." My post was long and wordy, but was thorough and explained the issue(s).

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    paul52446m is offline Journeyman
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    sounds to me like you have things going on that could get dangerous. Like Hube said you are saying in does different things at different times. I think if i was you i would start by calling a good service man, clean the unit out and start going through it. If the burner and blower turn
    off hot at the same time then you are losing main power or your electronic board it junk.
    You really need a tech man to go through it before you burn the house down. later paul

  5. #5
    Hube is offline Journeyman
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    51Don; NO, you have not explained it completly. When you say the "furnace shut down" are you saying that this includes the burner AND the fan, or just the burner?(confusion)

    Somtimes, a shorter, more defined explanation makes a problem much more easier to solve.
    As was suggested by Paul52446m, because of your lack of knowledge, you need to get a pro in to have a look,and the quicker the better, before you do burn the house down.

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