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Gas Furnace.... Thermocouple???

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  • Gas Furnace.... Thermocouple???

    I have a gas furnace that you can hear running for about 30 seconds and then you can hear it cut off. It does this through out the entire cycle, on-off-on-off-on-off..... Im thinking its a bad thermocouple... Could this be the cause? If not, what could it be?

  • #2
    Re: Gas Furnace.... Thermocouple???

    Need make and model of the furnace to help you but does NOT sound like thermocouple. Flame sensor maybe...limit switch...

    Comment


    • #3
      Re: Gas Furnace.... Thermocouple???

      Originally posted by plumberscrack View Post
      Need make and model of the furnace to help you but does NOT sound like thermocouple. Flame sensor maybe...limit switch...

      I will get the model and make in a little while... Thanks.

      Comment


      • #4
        Re: Gas Furnace.... Thermocouple???

        Originally posted by plumberscrack View Post
        Need make and model of the furnace to help you but does NOT sound like thermocouple. Flame sensor maybe...limit switch...
        Dirty filter, blower wheel, evap coil if applicable. Sounds like a little housekeeping is in order.

        James

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        • #5
          Re: Gas Furnace.... Thermocouple???

          Gonna think out loud here

          -Thermocouple: Nope, it only looks for a pilot signal
          -Flame rod: Nope, The system would cycle in less then 30 seconds if the flame rod were dirty (10-15 seconds to verify flame)

          30 seconds sounds like a temp limit at first guess

          My vote is airflow restriction

          If it is due to a dirty filter, I'm gonna gamble and say the evap coil is growing fur as well... (this without any information at all...lol)

          (Brand and model number of furnace would be great as well as a picture if ya got it)

          Comment


          • #6
            Re: Gas Furnace.... Thermocouple???

            The unit is an Amana Model#ghe80dn-3. The filter was dirty, but I dont think dirty enough to cause any problems. I was wrong, its not 30 seconds. When you turn it on it takes about 60 seconds after the pilot lights for the burners to light. they stay let for 3-4 seconds and then turn right off. It keeps cycling like this for about 45 seconds or so and then they stay lit a little longer before going out (maybe 5-6 seconds). When they light the one on the left has a little bit of the flame shotting out towards where all the wiring is. Turns out, the reset will trip from time to time as well (sometimes it does sometimes it doesnt.)

            From what I gather I would guess its the flame sensor switch, but is there anyway to test this?
            Last edited by futuresweets10; 01-24-2010, 06:55 PM.

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            • #7
              Re: Gas Furnace.... Thermocouple???

              Originally posted by futuresweets10 View Post
              The unit is an Amana Model#ghe80dn-3. The filter was dirty, but I dont think dirty enough to cause any problems. I was wrong, its not 30 seconds. When you turn it on it takes about 60 seconds after the pilot lights for the burners to light. they stay let for 3-4 seconds and then turn right off. It keeps cycling like this for about 45 seconds or so and then they stay lit a little longer before going out (maybe 5-6 seconds). When they light the one on the left has a little bit of the flame shotting out towards where all the wiring is. Turns out, the reset will trip from time to time as well (sometimes it does sometimes it doesnt.)

              From what I gather I would guess its the flame sensor switch, but is there anyway to test this?
              Yes, A micro ammeter.

              I use a Fluke 16, hook the meter up in series with the flame sensor and see what it's reading.Usually a reading over 9 micro amps is good. You can also clean the flame sensor with steel wool and see if that will keep it running. I usually clean them before checking the current draw.

              James

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              • #8
                Re: Gas Furnace.... Thermocouple???

                Pull the flame rod and shine it up with some steel wool or in a pinch sand paper and see if that keeps the flame on... If not we can move on to the next step.

                (since I did not read James's post all the way through do what James says)

                Bill

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                • #9
                  Re: Gas Furnace.... Thermocouple???

                  Originally posted by OkieBill View Post
                  Pull the flame rod and shine it up with some steel wool or in a pinch sand paper and see if that keeps the flame on... If not we can move on to the next step.

                  (since I did not read James's post all the way through do what James says)

                  Bill
                  I pulled it out, but I went ahead and ordered a new one... Its not like one I have ever seen before. Took about 30min to get it out (had to disassemble alot). The sensor is what looks to be a normal flame sensor on one side, and the other end of the wire is a little box with a three prong plug that plugs into the ignition module. the wire that connects the two isnt like electrical wire... it is a single strand with no insulation that resembles a coat hanger wire. It was about 60 bucks for the new part.

                  I did try to polish it a little bit with it still in the unit and couldnt get to it easily. It pretty much ran the same with the exception of one cycle when it stayed on for about 25 seconds. This change tells me that the sensor was dirty.

                  The reason I opted to go ahead and get a new one was because I know there is alot more to this sensor than most flame sensors and with how labor intensive it was to take out I didnt want to waste my time with the old one.

                  If for some reason this doesnt fix it, what should I be looking for next?

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                  • #10
                    Re: Gas Furnace.... Thermocouple???

                    OK so I gotta see if I can remember the old amana you have...

                    Does this unit by chance have a standing pilot light and a honeywell or robershaw fan limit switch (google fan limit switch image to see what I'm talking about)

                    Comment


                    • #11
                      Re: Gas Furnace.... Thermocouple???

                      Originally posted by OkieBill View Post
                      OK so I gotta see if I can remember the old amana you have...

                      Does this unit by chance have a standing pilot light and a honeywell or robershaw fan limit switch (google fan limit switch image to see what I'm talking about)

                      Kinda looks like that... http://images.google.com/imgres?imgu...a%3DN%26um%3D1

                      Comment


                      • #12
                        Re: Gas Furnace.... Thermocouple???

                        Originally posted by OkieBill View Post
                        Gonna think out loud here

                        -Thermocouple: Nope, it only looks for a pilot signal
                        -Flame rod: Nope, The system would cycle in less then 30 seconds if the flame rod were dirty (10-15 seconds to verify flame)

                        30 seconds sounds like a temp limit at first guess

                        My vote is airflow restriction

                        If it is due to a dirty filter, I'm gonna gamble and say the evap coil is growing fur as well... (this without any information at all...lol)

                        (Brand and model number of furnace would be great as well as a picture if ya got it)
                        10-15 seconds of built up gas with a mis-adjusted spark rod would cause a big problem. 5 sec- 7 sec is more likely

                        Comment


                        • #13
                          Re: Gas Furnace.... Thermocouple???

                          Originally posted by James P View Post
                          Yes, A micro ammeter.

                          I use a Fluke 16, hook the meter up in series with the flame sensor and see what it's reading.Usually a reading over 9 micro amps is good. You can also clean the flame sensor with steel wool and see if that will keep it running. I usually clean them before checking the current draw.

                          James
                          I'd say 2 is more like the min, .2-.5 is a common drop out., I likw using a brass wire brush. Steelwool is messy to keep in my bag. IMO

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                          • #14
                            Re: Gas Furnace.... Thermocouple???

                            I changed the flame sensor switch today and it seems to be working alot better....

                            Comment


                            • #15
                              Re: Gas Furnace.... Thermocouple???

                              If the flame shot out, the burners may be dirty causing it not to crossover. Very dangerous
                              Also, it's good you replaced the sensor. Thats a mecury flame sensor. Not too much fun if you break it.
                              Have it cleaned before doing anything else your self. 75 bucks well spent I promise. THEN we can help you fix something. Get it safe first though please!

                              Comment

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