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  1. #1
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    Inducer fan running upon start-up without call for heat.

    Hi. I was called out to work on Armstrong packaged unit pge10B36D075A-9B.

    After closing disconnect the furnace control board receives power, waits approx 20 seconds, and starts the inducer fan motor- without a call for heat. I checked voltage from c-w: 0v. Checked R/O and Hi limit circuit- maintains 27v back to furnace control board. Then checked pressure switch and it remains open until inducer fan kicks on, and then closes. No voltage across pressure switch unless c-w becomes energized and then voltage allowed through pressure switch, across aux switch on fan, and back to board.

    When the board receives power immediately LED flashes twice. The trouble codes are no longer on the furnace but from what i could find online seems to be - System lockout. Failed to detect or sustain flame. However, the gas valve never energizes and no spark is ever attempted.

    I've replaced the control board and removed the flame sensor from the board to see if the fault still exists, and still flashes twice. Any ideas?

  2. #2
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    from what i've experienced it sounds like a limit or rollout issue, check continuity on everything (including the limits on the blower).

  3. #3
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    My money is on the rollout. You may need a wind baffle for the flue outlet...AFTER you check the HX.
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    2 Tim 3:16-17

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  4. #4
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    in addition, there are sometimes limits inside of the heat exchanger compartment. that one stumped me for a couple hours once.

  5. #5
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    Thread Starter
    As a test, I bypassed all rollouts and the high limit switch on the heat exchanger. I still get the same code. If i disconnect any wire connected to the limit circuit the board flashes 4 times instead of two- High limit or rollout switch open.

  6. #6
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    Bypassing the rollout or limit is not as good a diagnostic as measuring the voltage at the terminals.
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    2 Tim 3:16-17

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  7. #7
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    Thread Starter
    voltage is 0 across each limit indicating a closed switch- 27v common to switch. continuity through each with power off. wiring schematic shows 1 roll-out, 1 high limit and then back to circuit board. fan housing limit is after pressure switch.

  8. #8
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    Quote Originally Posted by BKHVAC View Post
    voltage is 0 across each limit indicating a closed switch- 27v common to switch. continuity through each with power off. wiring schematic shows 1 roll-out, 1 high limit and then back to circuit board. fan housing limit is after pressure switch.
    If there is an interruption of voltage into the safety chain., you will still read zero volts across the switch.

    So...

    You need to confirm both sides of the switch to common, reading full voltage. When it comes to safeties, I use the Lo Z setting.

    Do you see how that is better?
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

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  9. #9
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    Jan 2009
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    Turn off furnace power supply. ( unplug the cord)
    Turn off the gas supply.
    Gain access to the furnace grounding lug, located inside the incoming power terminal box.
    Connect a volt meter between the grounding lug and the neutral side of the transformer secondary winding.

    Turn the power to the furnace ON.
    You may have to bypass the door switch or use a device (clamp) to push the switch closed.

    The voltage reading should be no more than 2VAC. Higher voltage indicates possible grounding problem.

  10. #10
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    Another possibility would be one of the limits have too high of a resistance. But since you bypassed everything, I guess that possibility is out the window.

  11. #11
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    Quote Originally Posted by Airczech View Post
    Another possibility would be one of the limits have too high of a resistance. But since you bypassed everything, I guess that possibility is out the window.
    This (jumping out the limits) would not be a good indicator of limits not being a factor, because if there is no voltage going out to the limits, jumping them out or measuring voltage across them means nothing.


    You first have to prove that voltage is being sent out to the safety chain before jumping out or checking for voltage drops becomes meaningful.

    Because the current in the safety loop can be very low, there may not be enough current to cause a meaningful voltage drop, unless the limit is entirely open.
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    2 Tim 3:16-17

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  12. #12
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    Quote Originally Posted by timebuilder View Post
    This (jumping out the limits) would not be a good indicator of limits not being a factor, because if there is no voltage going out to the limits, jumping them out or measuring voltage across them means nothing.


    You first have to prove that voltage is being sent out to the safety chain before jumping out or checking for voltage drops becomes meaningful.

    Because the current in the safety loop can be very low, there may not be enough current to cause a meaningful voltage drop, unless the limit is entirely open.
    Id like to add having a jumper left in place accidently can and does happen!

    Jumpers are required but as a last resort. Clip one lead of your meter on the supply to the safety's and test with the other, one limit at a time in order!

  13. #13
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    Quote Originally Posted by pecmsg View Post
    Clip one lead of your meter on the supply to the safety's and test with the other, one limit at a time in order!
    My suggestion is to clip one lead (I use the black lead, from my habits with DC) to COMMON, and I probe around with the red lead.

    This ensures that the voltage is at least present.

    My second level when doing this is to use the Lo Z function, which loads the circuit, causing an increase in the current used to make the reading, and that helps to reveal high resistances in the safety chain.


    I will typically pick a limit and look for voltage. If I have it, I look at the diagram to see where in the chain the voltage I just found is located. If that is the first break point from the board, I continue to check down the circuit to make sure it is never lost, and I find where it returns to the board, at its connector.
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    2 Tim 3:16-17

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