No announcement yet.

Furnace Trouble.

  • Filter
  • Time
  • Show
Clear All
new posts

  • Furnace Trouble.

    Please note that I'm not a HVAC pro, just a homeowner looking for some help.

    The other day I turned on our furnace, and the blower came on, but no heat.

    I went down and looked at the unit, it's a Lennox model G16Q3X-75-3.

    I shut of the power, then powered it up with the thermostat still set to heat on. Looking at the pilot I noticed no igniter spark.

    With everything 'on' I held a match by the pilot, the pilot lit and stayed lit, and the gas flowed to the elements and the heater was working.

    So I think both the pilot and main gas valves are ok, and pilot circuit is also okay. Which makes me think the igniter is at fault.

    I check things out and the ohm meter shows that there is no resistance between the tip of the igniter and the control module, a Robert Shaw #18G9101. Likewise the ground to the pilot is ok. I hook up the voltmeter to the ignition and to ground, but don't see any action, however I'm sure that the spark is pretty high voltage, and very quick, so I don't know if my meter would measure it. I do hear a faint "click" a few seconds after I switch it on, but I don't know if that's an ignitor, or something else. I would guess that the igniter circuit would be a series of clicks and sparks until the pilot lights and heats the thermocouple, at least that's how our gas stovetop works.

    A new control unit costs around $200, and I'd hate to replce it only to find that it wasn't the issue. Am I on the right track here?


  • #2
    Sounds right so far. Are there error indicator LEDs on the main board? The wiring schematic should be on the back of the blower access door and contain the flash sequece for the errors. While looking at the schematic check if there is a fuse for the ignitor then check the fuse. Your meter will not read the high voltage arc from the ignitor tip, in fact you stand a good chance of toasting your meter trying to read it. It will however tell you if you have 120V power to the ignitor module.


    • #3

      Are you by any chance near a farm supply store? If yes, you might see if they have a low cost, simple electric fence tester. You would connect it up in place of the igniter. The wire with an alagator clip would be connected to the frame of your furnace (bare metal part) and the part you contact the fence with would connect to the high voltage lead from your ignition coil. With hands in the clear turn power back on. The neon lamp in the tester should flash once or more times. If it stays lit a few seconds that's better yet. Such a tester will have instructions and a fence charger (zapper) sends high voltage pulses to the insulated fence wire. I have used a "Test Spark Plug" (with the gap closed down to around 0.015 - 0.020) to check output of fence chargers. You might try one of them in place of the igniter. The fence tester will flash at a lower voltage than will a spark jump the gap of a tester spark plug and thus for the furnace check would be a better choice if you can find one in your area.

      Look at the one near the bottom of this page. It may help you find the problem.

      Here are a few more models you might check out.

      After reviewing my posting, I realized that I kind of messed up my wording. The test spark plug is for use with gasoline or gasious fueled engines and not electric fences, but it will work with a fence charger if the gap is set down to about 0.015 inches. With the wide gap 0.060 or larger that such normally is used the spark won't jump the gap. For your furnace, I would recommend the neon lamp tester. You might just replace the igniter if it's not too costly.

      Please note that my "Test Spark Plug" is one that I made using an 18mm industrial engine spark plug such as AC C87 or Champion D16.
      Last edited by Woussko; 12-30-2006, 11:10 PM.


      • #4
        is it gas or diesel?

        transformer or photocell

        or possibly electrodes
        Last edited by subiemech85; 12-30-2006, 12:11 AM.


        • #5
          Note to myself: Always work up a posting in a Word or WordPerfect. Then print it out triple spaced and mark corrections. Next edit it and print it again. Only when I have it right, do I copy & paste it into a post.

          Actually I think that I may need a new pair of "Computer Glasses".