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  • Furnace blower comes on before ignition

    I was a funace tech years and years ago, back when standing pilots were the norm. I have a Lennox Furnace, when the tstat calls for heat the main blower comes on before the ignition cycle has finished. This makes for cool air to be circulated until it warms up. I'm curious if there is a timer or temp switch that could have failed or are they incorporated into the control board? Thanks.

  • #2
    Re: Furnace blower comes on before ignition

    Doesn't it run before ignition to clear any gas that may have accumulated in the fire box to prevent an unexpected explosion?

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    • #3
      Re: Furnace blower comes on before ignition

      Originally posted by ArizonaPlumber View Post
      Doesn't it run before ignition to clear any gas that may have accumulated in the fire box to prevent an unexpected explosion?
      You are referring to the draft inducer fan motor, he makes it sound like he is talking about the blower motor fan.
      AllurePlumbing.com
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      • #4
        Re: Furnace blower comes on before ignition

        - the ignition cycle should last no longer than 5 seconds on any furnace (from spark to flame). it may be up to 45 seconds for hot surface ignition
        - check for dirty pilots, orifices, burners
        - check for correct fuel pressure
        - check to make sure the board is not set to continuous blower
        - check for no jumper between W and G terminals
        - engage a call for heat at the wall with no thermostat attached (ensures wiring is ok, if fan problem stops replace tstat)
        - check the board to see when the timer engages the main blower, some are fixed and not resettable
        - some older lennox furnaces had an orange wire tied to the gas valve that traveled down to a fan timer board. if this wire is connected to the wrong terminal (on the pilot terminal) then the main blower may engage before the main burners turn on
        - the board may be bad

        what is the model of this furnace?
        Last edited by Plumber Punky; 02-14-2013, 03:08 PM.
        ~~

        ... it was plumbed by Ray Charles and his helper Stevie Wonder

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        • #5
          Re: Furnace blower comes on before ignition

          Ok, thanks for the replies. The combustion fan (or powered exhaust fan) comes on along with the main blower. This is when the tstat calls for heat. This is a hot surface ignition unit so it takes a bit to heat up plus there is a purge timer. The main blower should not be coming on right away, it should allow the air to become heated before it comes on so it doesn't circulate cold air. Model # G40UH-36B-090-03. This is a natural gas unit. I will try removing the tstat and jumping the red & white wires and see if the fan comes on (it is a Hunter touchscreen). I've heard about Hunter's being bad but I haven't had any issues with them, if it is the problem then I'll buy a Honeywell (but I have had lots of problems with them in the commercial environment). Thanks again!!

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          • #6
            Re: Furnace blower comes on before ignition

            Originally posted by Plumber Punky View Post
            - the ignition cycle should last no longer than 5 seconds on any furnace (from spark to flame). it may be up to 45 seconds for hot surface ignition
            - check for dirty pilots, orifices, burners Ok will check, flame color is bright blue
            - check for correct fuel pressure
            - check to make sure the board is not set to continuous blower not sure how to do this, any ideas?
            - check for no jumper between W and G terminals no jumper present
            - engage a call for heat at the wall with no thermostat attached (ensures wiring is ok, if fan problem stops replace tstat) will try this, this is a house built in the '60's, no air when new, when they added air they didn't run the correct color wires, all I have is wh and red, thinking of running new wire
            - check the board to see when the timer engages the main blower, some are fixed and not resettabledon't know where the timer is located
            - some older lennox furnaces had an orange wire tied to the gas valve that traveled down to a fan timer board. if this wire is connected to the wrong terminal (on the pilot terminal) then the main blower may engage before the main burners turn on there is an orange wire coming off the gas valve, can't see where it goes, will check it out closer tonight
            - the board may be bad

            what is the model of this furnace?
            Model # G40UH-36B-090-03

            Comment


            • #7
              Re: Furnace blower comes on before ignition

              Originally posted by coach73 View Post
              Model # G40UH-36B-090-03
              The G40UH line is rather old. Yours is a 3 ton, 90,000btu revision 3 model of 80% efficiency.

              However, if it is a newer install, this should have an integrated circuit board that has LED error codes. Are there any of those? What is the serial number? This will tell you the age of the furnace.

              Your heat blower on-time is 45 seconds and not adjustable. If the furnace blower is engaging before the main flame is on there is something wrong with the ignition sequence.

              Based on your answers to some of the questions and pointers above it is my humble suggestion you engage the services of a qualified service technician.
              ~~

              ... it was plumbed by Ray Charles and his helper Stevie Wonder

              Comment


              • #8
                Re: Furnace blower comes on before ignition

                Thanks for the reply, unfortunately the service techs around here are not much more qualified than I am, a little but not much. And at $100/hr it's not worth it to me. I don't know how much a board is but I would think it wouldn't be much more than $100.

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                • #9
                  Re: Furnace blower comes on before ignition

                  Originally posted by coach73 View Post
                  but I would think it wouldn't be much more than $100.
                  You would be wrong. It's Lennox; it's proprietary and gold plated.

                  Because there is a Lennox installed there must be an authorized dealer. From the Lennox web site I found the one dealer in your area:

                  Dale Plumbing & Heating

                  306 First Ave N
                  Glasgow, MT 59230
                  Phone: 406-228-8245

                  If you have a unit that no one can service you have 3 options:

                  - Replace parts until the problem is corrected (need authorized Lennox tech)
                  - Call the corporation and have a service rep fix it
                  - Replace the furnace with a non-Lennox unit
                  ~~

                  ... it was plumbed by Ray Charles and his helper Stevie Wonder

                  Comment


                  • #10
                    Re: Furnace blower comes on before ignition

                    Well I found out it's in the Tstat. I have a Hunter touch screen, I'll have to decide if I want to live with it or buy a different one. Thanks for all your help!!

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                    • #11
                      Re: Furnace blower comes on before ignition

                      No problem.
                      ~~

                      ... it was plumbed by Ray Charles and his helper Stevie Wonder

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