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Lennox gas furnace fires sometimes

Discussion in 'Residential' started by wetbar, Jan 28, 2013.

  1. wetbar

    wetbar New Member

    Messages:
    12
    hey I have a Lennox furnace, Im having problems with. I was getting blinking error codes for limit switches. So I shorted all of then out and used a temp probe to make sure I didnt over heat it. It did get up to about 150 and shut off again with a error code of -"Improper main ground or line voltage below 75 volts; OR: Broken ignitor; OR:Open ignitor circuit."
    I checked the voltage and the ground. I'm not sure about the broken ignitor because it did ignite for 45 sec.

    So I think I have two problems: #1 it is over heating, I checked the fan and filters all seemed OK and the coil is clean. I'm not sure what else can cause over heating.
    I tried to check the gas pressure but the screw is stripped out and I could not remove it.

    problem # 2 I am just lost, any clues?
     
  2. CincyHVAC

    CincyHVAC Hoptimus Prime, Master of Beer-Fu

    Messages:
    46,995
    Location:
    Ohio's Southern Shoreline
    What's the model & serial # of the unit?
     
  3. wetbar

    wetbar New Member

    Messages:
    12
    Sorry forgot that it is 8ouhg3/4 100a-2
    s# 6398L 02813
     
  4. CincyHVAC

    CincyHVAC Hoptimus Prime, Master of Beer-Fu

    Messages:
    46,995
    Location:
    Ohio's Southern Shoreline
    Ohm out the ignitor. If it reads under 10 or over 20 ohms, replace the ignitor. Verify your incoming line voltage at 120v, double check all wiring harnesses that they are secure, including the wiring to the ignitor Double check that your safety circuit is properly wired..
     
  5. wetbar

    wetbar New Member

    Messages:
    12
    OK I will try that. The safety circuit was jumpered when I got the second error. So I don't think that could be it. I am still assuming the gas is correct. The flames looked great, but I can get the darn screw out to put my manometer in.
     
  6. CincyHVAC

    CincyHVAC Hoptimus Prime, Master of Beer-Fu

    Messages:
    46,995
    Location:
    Ohio's Southern Shoreline
    You may need to recommend a gas valve replacement due to the stripped out screw. You gotta be able to verify gas pressure and make adjustments. It's in the best interest of the customer.
     
  7. Hot Gas

    Hot Gas Woof! Woof!

    Messages:
    12,031
    Location:
    On a bucket somewhere
    If the plug is stripped out, can't depend on it to hold back NG when it is not being supervised. Valve needs to be replaced for safety. But.....

    I'm wondering if this screw is the one that covers the adjustment screw? Got a pic to post of what you are working on?
     
  8. jpb2

    jpb2 Dog Tard *

    Messages:
    47,725
    Location:
    within the bounds of reality
    Make sure it is on a dedicated circut back to the breaker box and verify ground.













    then get the pos lemmox out of there. even Jay will tell you that
     
  9. CincyHVAC

    CincyHVAC Hoptimus Prime, Master of Beer-Fu

    Messages:
    46,995
    Location:
    Ohio's Southern Shoreline
    Another thing to check is to make sure the flue is unobstructed. If the furnace is shutting down on high limit, and you've got the limit jumped out..........
     
  10. wetbar

    wetbar New Member

    Messages:
    12
    went and checked everything again. The flue seems fine even though it is very hot like 150 degrees. Checked all the wires again, checked the breaker, ground and ignotor wires. The ignitor was 16 ohms most of the time. There was one time the unit just shut down and it read 40 ohms, but came down after about 5 min. I replaced the ignitor anyway. It ran for about 1.5 hours. much better, but same error. I did talk the customer into changing the gas valve. so I will at least try to use a reverse out and get the plug out without damage.
     
  11. Craig Daly

    Craig Daly New Member

    Messages:
    30
    Location:
    Quincy Il
    I have seen a few of lenux boards go bad with the low voltage error. If you can get it to act up while testing. The ones I have seen have been intermitent.
     
  12. Puggsly 72

    Puggsly 72 New Member

    Messages:
    15
    Location:
    NW Indiana
    curious, what is your delta t?
     
  13. wetbar

    wetbar New Member

    Messages:
    12
    OK so I got the plug off and checked the pressure was 3.2. I brought it up to 3.5 and of course it does the same thing. It pops the limit switch just before the fan starts, but then after the fan run it will refire and run for 45 min or so and then give the low voltage error code. This time I ran it with the furnace doors open to make sure there was not a air problem. Im at a lost! the only thing I can think of is the board. Any thing else I should try. oh yeah I did check the exhaust and it was 148 just after the fan but went down quick as I checked further up the stack, so it is not blocked
     
  14. CincyHVAC

    CincyHVAC Hoptimus Prime, Master of Beer-Fu

    Messages:
    46,995
    Location:
    Ohio's Southern Shoreline
    148 degree stack temperature on an induced draft 80% furnace? Holy shit, son........:mrbill:
     
    knave likes this.
  15. CincyHVAC

    CincyHVAC Hoptimus Prime, Master of Beer-Fu

    Messages:
    46,995
    Location:
    Ohio's Southern Shoreline
    Induced draft 80% should be a little higher stack temp, otherwise it's gonna condense. Somewhere in the vicinity of supply air temperature PLUS 170 degrees.......
     
  16. CincyHVAC

    CincyHVAC Hoptimus Prime, Master of Beer-Fu

    Messages:
    46,995
    Location:
    Ohio's Southern Shoreline
    You've got a heat transfer problem.
     
  17. CincyHVAC

    CincyHVAC Hoptimus Prime, Master of Beer-Fu

    Messages:
    46,995
    Location:
    Ohio's Southern Shoreline
    What's your Delta T?
     
  18. wetbar

    wetbar New Member

    Messages:
    12
  19. wetbar

    wetbar New Member

    Messages:
    12
    I cant figure this one out! It cant have anything to do with air flow because it is before the fan comes on. I just knew the gas valve was going to be 6 or so. WTF? There are not to many things going on after they fire and before the fan comes on.
     
  20. CincyHVAC

    CincyHVAC Hoptimus Prime, Master of Beer-Fu

    Messages:
    46,995
    Location:
    Ohio's Southern Shoreline
    Temperature rise for this furnace, according to the specifications, should be between 45* and 75*.

    What's your altitude in NC? Manifold pressure should be:

    0−4500'.....3.5 "wc

    4500'−5500'.........3.4 "wc

    5500'−6500'..............3.3" wc

    6500'−7500'........... 3.2" wc

    Have you clocked the draft inducer? Is it possible that it may be failing? If it's running slower than it should, that could cause the heat to build up in the HX and trip the limit before the blower comes on. Riddle me this: If the main limit trips, the blower should come on automatically, right?