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-   -   The blower doesn't start on "Auto" (https://www.askmehelpdesk.com/showthread.php?t=15105)

  • Nov 21, 2005, 12:10 PM
    ravithas
    The blower doesn't start on "Auto"
    Please someone help me.

    My furnace Lennox G20Q3/4E-100-5 behaves as follows:
    1) Fan is on "Auto"
    2) Thermostat calls for heat
    3) Green light comes "On" on IPI module and then burner comes "On"
    4) Flame occurs
    5) Blower doesn't come "On". After a while, the burner stops due to safety. It retries again, and same thing happens.
    6) When the burner is "On", I put the blower on "Fan On" mode and the furnace keeps running until it meets the thermostat setpoint. Then the burner goes "Off". But, the blower keeps running. Because it's on "Fan On" forced mode.

    Please note the A/C worked fine during the summer when the fan was on "Auto" mode.

    Please advice me. I am thanking you in advance for your assistance.

    Thanks, Ravi :confused:
  • Nov 21, 2005, 12:36 PM
    labman
    The blower operates completely differently in heating mode than On or A/C. Usually there is a lower speed coil in the blower motor controlled by a different relay which in turn is control by a limit switch. Check the limit switch first. It should have power either from the white wire from thermostat or directly from the red wire on the transformer. Power should flow to it and back once the furnace is up to temperature. It then activates the blower relay, powering the low speed coil. Often as not, the problem is the relay. Unfortunately, the newer ones are on a circuit board, challenging to diagnose and replace. The board is expensive to replace. You want to be very sure the low speed blower coil and the limit switch are working before replacing the board. If the board is bad, you can save $50-100 by not having a technician to come out and check it and replace it.
  • Nov 21, 2005, 01:06 PM
    ravithas
    Hi labman,

    Thanks for your immediate response. Don't you suspect the flame sensor or the thermostat to be bad?

    Please advise me.

    Thanks, Ravi :confused:
  • Nov 21, 2005, 02:09 PM
    labman
    There are 2 limit switches or maybe one double one. The lower temperature one completes the circuit to start the low speed blower. It may be wired to the coil of the relay, or to a logic circuit on the board. If the blower fails to start, the temperature rises until higher temperature one shuts the gas off. As long as the gas comes on and stays on for a few minutes, the thermostat and flame sensor are working fine. I am not sure about the time limit, but if the flame sensor does not detect a flame, it shortly kills the gas. Running the fan on On wouldn't bypass that problem.
  • Nov 26, 2005, 09:40 AM
    ravithas
    Please help me
    Hi Labman,

    I found the limit switch just above the heat exchangers. There are two thermocouples connected with a one cent coin size tab inserted inside and the they are connecte through two blue wires to the control board.

    I measured the voltage while kept the fan on "Auto", Before the burner comes "On", I was reading around 28 Volts across the limit switch. Once after the burner came on, there is almost none voltage reading, it was something like 0.012.
    Is limit switch the problem?

    Please note that I also run the furnace with the "Fan on" through thermostat. I also notice the blower goes "off" once in a while. Why is that?

    Please help me.

    Thanks, Ravi :confused:
  • Nov 26, 2005, 10:23 AM
    labman
    That sounds more like the flame detector. Try disconnecting one lead and see if the burner goes off almost as soon as it lights.

    This is becoming confusing with 3 different, separate threads on similar problems. As I explained yesterday at https://www.askmehelpdesk.com/showth...7&page=3&pp=10

    ''As furnaces become more complex, they become a bigger challenge to the DIY, even the HVAC technician. The old limit switches had a bimetalic spring or snap disk that closed or opened contacts. If those are really thermocouples, you would need to connect a sensitive volt meter across the right contacts on the circuit board to measure the voltage. You would then need to know the typical voltage at the temperature. Best you may be able to do is to go to where they connect to the board, and check for conductivity with one lead disconnected. If the leads don't seem to disconnect, they could be thermisters, no easier to check. Thermisters and thermocouples are less likely to go bad than limit switches. May be time to gamble on a new board or a visit from a technician.''

    You may be able to get a good voltage reading where the thermocouple leads connect to the board if they are exposed. The voltage should be very low with the heat off. As soon as the burner comes on, the voltage should start to rise, but maybe only tenths of a volt. If instead, there is a measurable voltage there, it is a thermister, but the voltage should still go up as it heats. If you do get increasing voltage readings however low, likely the problem is on the board, logic or the blower relay. Although somebody did report taking the board to work and fixing it, board repair is beyond most of us.

    Maybe if I think about it for a while, I can explain the blower going off sometimes.
  • Nov 26, 2005, 11:18 AM
    ravithas
    Hi Labman,

    I strongly believe that with yours and others, I will be able to fix the problem. I am lucky in one way that I am able to use the furnace in "Fan On" mode. My wife has already started bugging me when I am going to hire the professional to fix the problem.

    Labman, I am sorry that I wrote as the blower. I meant to type the burner. OPPS... The blower runs continuously. The burner turns off once in a while.

    What do you think about the voltage reading across the limit switch?

    Thanks, Ravi :confused:
  • Nov 28, 2005, 06:25 AM
    ravithas
    :) Dear labman,

    I believe, I have done some home work after everyone's advice.

    I found the Heat Tab on the control board. When the thermostat on "Auto", after the burner came on, I checked the voltage between Heat Tab and Neutral and I found "Zero" voltage.

    I wanted to make sure whether I was using the meter right. When the fan was on "Fan On" mode and when the fan was running, I checked the voltage between Cool Tab and Neutral and found 120 V.

    It lookd like the control board is bad.

    (Please note, that I was able to measure 24 V by the limit switch when the thermostat is on "Auto").

    Can I now come to a conclusion the board is bad? Will I able to replace the control board?

    Thanks again for your assistance.

    Thanks, Ravi
  • Nov 28, 2005, 07:57 AM
    labman
    Replacing the board on my furnace isn't too bad other than the $300. There are a lot of wires on it, but duh, red on R, white on W, etc. Some of the other stuff were plugs that only fit one place. Look it over first and make notes of anything not glaringly obvious where it goes. Disconnect the wires, remove the screws or clips and pull it out. Take it to your dealer and pop a nitroglycerine tablet. Take the new one home and check your notes and pop it in. You have at least saved the $50-100 for a service call.
  • Nov 29, 2005, 08:46 AM
    ravithas
    Hi Labman,

    I do have a happy ending story. Yes, I was able to replace the control board with a new one and it's working like brand new. When I removed the old one, I was able to see one of the relay was burnt, which shall be for the low speed blower for heat.

    Thanks again for yours and all the others assistance for me to get this work well done.

    Thanks, Ravi :)
  • Nov 29, 2005, 11:18 AM
    ravithas
    Hi Labman,

    I read your comments. The cost wasn't that bad at all. I paid only $138 for the board. But, the service technician wanted to charge around $560 for replacing the board. He didn't sound firm either. He also included replacing the valve in his estimate, which is not necessary.

    Thanks, Ravi :)

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