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Thread: Lennox Furnace won't light up

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    Default Lennox Furnace won't light up

    Hi guys,

    I have a Lennox furnace and it is going into the Watchguard mode often recently.

    A few years back I had an issue where it wouldn't heat, so I cleaned up the flames sensor and everything has been fine since, until about two weeks ago, when I woke up to a cold morning. Went downstairs, took out the sensor again and cleaned it...then it was fine for a week, and now again this morning, cold.

    I am getting a glow plug to light up, but there is no ignition. It tries this a few times and then craps out. If I power cucle the furnace eventually it lights up and heats, and usually works for a while, until it doesn't again.

    This morning my house dropped to 16 degrees (should be 21), so when I got the furnace going again, I sat there and watched it heat as I knew it would take some time to climb up to 21, and noticed that twice the flame went out...and re-ignited.

    Anyways, I am not sure wtf is going on, perhaps a bad gas valve? I am thinking of calling a furnace guy in, but I want to make sure they fix the problem. Do furnace companies usually guarantee their work? My fear is that they will come in, and it will ignite like it does for me after they do something basic, and they will call it a day, only to have to be brought in again..and charged again, to look at the next part.

    Any recommendations for a furnace place?

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    If its a Lennox furnace, there should be some diagnostic LEDs through the plastic/glass circular window. They should be flashing in a sequence. Find the sequence layout on the front panel and see what the error is
    Originally posted by GTS Jeff
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    Have you got a multimeter? Check for 24 volts at the gas valve when it goes through its ignition sequence when the hot surface ignitor is glowing brightly and see if you get voltage to the gas valve.
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    I had the same thing with mine, it goes into a fail safe mode as it tries to light 5 times and failed. The first 2 times I power cycled it and it was fine but last time, even though the flame sensor that sits in front of one of the torch thingy looked clean I still took it out and gave it a thorough cleaning anyways and the issue hasn't come back yet

    What do your diagnostic lights say when this happens?

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    Originally posted by dj_rice
    If its a Lennox furnace, there should be some diagnostic LEDs through the plastic/glass circular window. They should be flashing in a sequence. Find the sequence layout on the front panel and see what the error is
    The LEDs indicate that it's in the Watchguard mode, which means it failed to ignite 5 tries in a row.


    Originally posted by Maxt
    Have you got a multimeter? Check for 24 volts at the gas valve when it goes through its ignition sequence when the hot surface ignitor is glowing brightly and see if you get voltage to the gas valve.
    I did some testing previously and it was getting 24 volts, but I have to catch it in a phase where it is not igniting, and it's been igniting all morning since it was dead, so it's hard to catch. I almost have to wait for it to fail before I can test, and with the cold weather coming, a little worried.

    I did some reading and people mentioned that the hot surface igniter may not be getting hot enough. The model number is 75M2101 on the igniter. I tested the resistance and it came back at 200ohms, which seemed high, until I read this thread http://www.cozyparts.com/lennox-part...?TOPIC_ID=4373 and someone said that a brand new one of those is 250ohms...so it isn't too far off, using a cheap voltmeter. I might try to find a new one and test it in store with same voltmeter and see if it comes back any differently.

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    Clean your flame sensor and order some backup hot surface igniters.

    Edit: change/check your filters also.
    Last edited by phreezee; 12-04-2016 at 01:08 PM.

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    Is it possible I have low gas pressure? Reason I ask is that there have been a few instances where my hot water heater also shut off. I thought it was the thermocouple and bought a new one, but never replaced it...and haven't had any issues since. It happened three times in the time I have lived here (about 7 years). Is there a way I can get gas pressure tested? Is this something Atco would do free of charge as they do with gas leaks...anyone know?

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    I haven't seen too many gas service pressure regulators go bad, but I have seen gas meters go bad and stop passing gas for lack of a better description.
    Also, old ignitors sometimes don't get hot enough to ignite the gas, even though they still glow orange and seem to work. Over the years I have seen that problem more on Trane equipment but a few lennox here and there as well.
    Atco will show up for free, sometimes they just won't carry out the repair.
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    Originally posted by phreezee
    Clean your flame sensor and order some backup hot surface igniters.

    Edit: change/check your filters also.
    Yup. That fixed it for me. Easy to check and fix.

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    I bet it's sensor or ignitor I had both go in my furnace over the last few years. I have a spare sensor now. If it's that just clean your old one. Looks like a long metal stick about 3-4 inches.

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    Clean the sensor, vacuum up any debris.

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    had the same issue, ended up being my blower motor.. no movement of air, flame kept going out.

    $900.00 to fix/replace
    Originally posted by BlueSTI
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    WTF $900?!? You know a new furnace is like $3500.

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    If its a newer high efficiency furnace, with modulating motors, it can go way higher than that ^^^ for some of the failures.
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    Originally posted by Xtrema


    Yup. That fixed it for me. Easy to check and fix.
    Was it the sensor or the igniter?

    I am leaning more towards the gas valve or the igniter not being hot enough, as I don't think the flame sensor comes into play, as there is no flame yet at that point.

    Got a laptop setup right now on constant record to record what is happening, and a voltmeter hooked up and in view, to capture what the voltmeter says when the furnace doesn't ignite. It's happening more and more often now, 3 times today, until today was fine for the last week, and until last week was fine for over two years. I wish something was just dead, instead of this sporadic behavior.

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    One more thing to watch for, on most lennox furnaces, the ignitor is on one side and the sensor at the other, so also watch that the flame is carrying over from one burner to the next on ignition. It might be lighting but not lighting the last burner where the sensor is...if thats the case, the carryover passages in the burner are probably plugged with dirt.
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    If hot surface ignitor is glowing red hot, but no flame after that, it is a gas related issue. If you get fire and then it goes out promptly, it is likely the flame sensor. And the previous post by Maxt brings up a good point as well

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    Originally posted by jeffh
    If hot surface ignitor is glowing red hot, but no flame after that, it is a gas related issue. If you get fire and then it goes out promptly, it is likely the flame sensor. And the previous post by Maxt brings up a good point as well
    All my burners light up. I think the most likely culprit is the gas....reason being is this morning then it was warming up from 16 to 22....once i got it ignited...it kept going for a good 5 minutes or so....and then abruptly cut off the flame...the igniter came on again..and then it relight and continued heating.

    There is some sort of a pressure adjustment thing on the gas valve....is this something that I should play with...?

    Really not sure where to go from here.

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    Not without a manometer and that adjusts the firing rate. Call atco they'll send someone out.
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    Originally posted by eblend
    All my burners light up. I think the most likely culprit is the gas....reason being is this morning then it was warming up from 16 to 22....once i got it ignited...it kept going for a good 5 minutes or so....and then abruptly cut off the flame...the igniter came on again..and then it relight and continued heating.

    There is some sort of a pressure adjustment thing on the gas valve....is this something that I should play with...?

    Really not sure where to go from here.
    Unless you have a manometer(edit* day late dollar short)I wouldn't. You may end up just complicating any future troubleshooting.

    Maxt mentioned earlier you have to find out if the gas valve is getting 24v and remaining closed, or does a loss of supply voltage cause the gas valve to close? This will help eliminate/confirm the gas valve as the issue. The fact you described the loss of flame as "abrupt" suggests there's a fault somewhere else that causes the gas valve to close.
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