Furnace will not light.

Discussion in 'HVAC Heating & Cooling' started by pitteach, Oct 3, 2010.

  1. pitteach

    pitteach New Member

    Messages:
    27
    Location:
    Hamburg, NY
    I have a Lennox 75k Btu 90+ furnace that will not light. (Approx 15 yrs old)
    This is the first attempt of the season.

    Here's what I've got:
    Turn on tstat and call for heat.
    Exhaust blower kicks on.
    After a few seconds, the intermittent ignitor starts firing.
    It keeps firing for almost a minute and while it does, the main blower kicks on but it does not light.
    The main blower stays on for about a minute, shuts down and the ignitor stops.
    The exhaust blower stays running continuously.
    It seems like there is no gas coming to the manifolds.
    The gas valve is in the on position.

    Any ideas? I am surprised that the main blower would come on without any heat. Could this be a clue? The unit ran fine when shut down last winter and I have just replaced the tstat before this happened.

    Any ideas would be greatly appreciated.
  2. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    If there's voltage across your gas valve and current through it while your ignitor is sparking, it probably means the valve is stuck closed. This could cost you a half a kilobuck.
  3. pitteach

    pitteach New Member

    Messages:
    27
    Location:
    Hamburg, NY
    OK, where can I check voltage at the gas valve? There are 3 leads to the valve (mv, mv/pv, and pv).
    Am I looking to get 24v across all of these leads?
    Can I assume that the pressure switch is ok since the ignitor is coming on?
  4. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    After disconnecting the gas valve [some multimeters put out enough current on the ohms ranges that they will damage electronics] check voltage on and continuity between all the valve terminals, 3 measurements for each. Also check for continuity between the 3 terminals and the valve housing.

    I'd say 24v +/- a few volts is OK on the valve. The pull-in/drop-out coil specs for gas valves is very hard to get. Some work on millivolts.

    I don't know the logic diagram for your furnace but it sounds like the only thing necessary after the sparking is for the gas to flow.
    If you post a schematic, which may be found on the inside of a panel, some possibilities can be eliminated.

    For my '82 Bryant the manufacturer sent me a several page factory manual for free, including Schematics and Theory of Operation. That's how I diagnosed my gas valve problem.

    http://en.wikipedia.org/wiki/Furnace
    Last edited: Oct 4, 2010
  5. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,636
    Location:
    Cave Creek, Arizona
    The blower will come on in anticipation of the burner igniting, and when it does not it shuts down. There are MANY possible reasons for the valve not to open, and each has its own diagnostic procedure.
  6. pitteach

    pitteach New Member

    Messages:
    27
    Location:
    Hamburg, NY
    Success

    After multiple tries, the furnace is on.
    I'm still not sure what happened but after a bit of fidgeting, it fired up.
    The gas to the house had been shut off at the meter for a couple months, so maybe I had some air in the line.
    I disconnected the pressure switch hose at the blower and reinstalled it and after a few misfires, it lit and is working fine.
    Thanks for the help.
  7. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    You should take this opportunity to measure every voltage, current and resistance in your furnace while you still can. These values are extremely hard to come by for DIYers.
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