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prolem with lennox pulse furnace

zepfan
zepfan Member Posts: 315
happy new year to all. I had a call today on a lennox pulse furnace m# g140 4/5-100-6. the ignition model was off on lockout with a code of no flame sensed. I reset the furnace at the thermostat, and it fired off on the first try. upon checking the manifold pressure it was dead on (2'' of w.c as stated on the nameplate to be the maximum) and the flame signal was right where it has been in the past (4.5 ua) I have worked on this furnace for several years, and recorded the readings of the manifold, flame signal, temperature rise and combustion test readings. all seemed to be normal, and was right where it was when I did the maintenance on the furnace back in October. upon cycling the furnace several times, about once every three times, it did not light although you can hear the sparking, and read the correct manifold pressure at the outlet of the gas valve. I confirmed that the combustion blower is operating, and turns off after flame is established, which is normal for pulse ignition. the combustion blower, spark plug, flame sensor, and cables were all replaced on this furnace back in 2009, and appear to be in good condition, and not the cause of this intermittent ignition problem. the only thing that I can come up with that could be the problem where it just doesn't light off is if one of, or both the gas flapper and the air flapper valve are not opening when the need to. i have never replaced either of the flapper valves on this furnace, and am posting this to see if anyone else has come across this problem where a pulse furnace every couple of cycles just doesn't establish ignition. if I locked out once a heating season, that could be lived with, but it is now more often not firing off then firing off. thanks to all I know this is a long post

Comments

  • unclejohn
    unclejohn Member Posts: 1,749
    Pulse

    I had a twenty four year old pulse that would not light at all this season. I replace both flapper valves that came it a tune up kit. It was the only way I could get them and it fired right up.
  • zepfan
    zepfan Member Posts: 315
    Thanks

    Thanks for the response, I ordered the tune up kit today and am going to install it as soon as the kit arrives.
  • zepfan
    zepfan Member Posts: 315
    Furnace still not firing

    I installed the tune up kit today, and the pulse furnace will not fire at all. With the old parts it would fire about once every three times, now it does not fire at all. You here the sparking and see manifold pressure, however it does not fire. I do not know what do at this point, as everything I have done to this point has resulted in a dead end. I can't think of anything else to chek or replace.
  • drhvac
    drhvac Member Posts: 189
    the end

    I'm all about and take pride in keeping a customers furnace well maintained and running as efficiently and safely as long as possible. But unless this is your own furnace and you want to try and keep it as long as possible, I would advise the customer that it is time for a new one. I'm sure if you keep at it, eventually you will find the problem and fix it, but how many times back and forth to this customers house will it take? How many times are the neighbors of this person going to see your truck coming back and forth to this customers house and saying " what's going on over there" does this guy know what he is doing? And little do they know you are going crazy to try and save this dinasaur of a furnace. Then the homeowner themselves must not be to happy with their heat going on and off. Its time my friend, you did your best, your in business to make money. You did what you could for the Pulse, and the customer, now its time for it to go.
  • FitterFreds
    FitterFreds Member Posts: 15
    ....

    Lennox pulse furnance should have the heat exchangers pressure checked each season! Lennox makes a test kit for this, or you can make up your own. I also agree with drhvac. Time to send this one down the road.
  • zepfan
    zepfan Member Posts: 315
    thanks

    thanks to all, i have the pulse furnace heat exchanger test kit and have used it several times.this furnace's heat exchanger seems to be ok, but i feel at this point replacement is the only option. i was able to get it to fire when i went back to measure up for a new furnace. this furnace's has become less reliable that replacement is the best option. the only saving grace is that this house has four systems, and when one is not working sometimes it is not even noticable.
  • unclejohn
    unclejohn Member Posts: 1,749
    Don't get me wrong

    But are you sure that the air flapper is installed faceing the right way. I only ask because the directions that come with the kit talk about the "kidney shaped holes" faceing one way or the other and I saw nothing that resembeled a kidney on that air flapper. So after several no light offs I turned it around and it worked just fine.
  • zepfan
    zepfan Member Posts: 315
    Holes face in

    The kidney shaped holes face in towards the heat exchanger, that the way they are facing. Thanks