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    RobC's Avatar
    RobC Posts: 4, Reputation: 1
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    #1

    Dec 10, 2005, 08:06 PM
    Carrier split - heat burners cycle on and off
    Carrier split unit in heat mode appears to basically operate OK. Once the pilot is lit and the burners come on, the pilot remains lit but the burners will intermittently cycle on and off. This appears to sometimes delay the blower from turning on. Once the blower turns on, it heats to the set temp and then shuts off. My concern is if this is the beginning to a major issue. Any ideas? Thanks.
    labman's Avatar
    labman Posts: 10,580, Reputation: 551
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    #2

    Dec 10, 2005, 08:43 PM
    Some furnaces have 2 limit switches in them. One turns on the blower when it get hot enough. The other shuts off the gas valve if it gets too hot likely because the blower didn't start.

    There is also a third temperature sensor that tests to see if the pilot light is on or the main burner ignites. Ye olde systems had a pilot light that stayed on all the time. They had a thermocouple that shut everything off it it got cold. The newest systems don't have a pilot light. I don't know if your system has a sensor for both the pilot light and the main burner.

    Just finding all the temperature sensors and cleaning them and checking their connections may fix the problem. Otherwise, post back.
    RobC's Avatar
    RobC Posts: 4, Reputation: 1
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    #3

    Dec 11, 2005, 09:04 AM
    Carrier split - heat burners cycle on and off
    The pilot has an igniter. It clicks and the pilot lights OK. I have notice that the thermal sensing element gets red hot and after a few seconds, the burner jets light. The element appears red hot and the burners will still shut off. The pilot remains lit and I do not see or hear the igniter clicking which leads me to believe that the pilot sensor is OK, Moments later, the burner jets come back on. For simplicity, my first thoughts were to replace the pilot and /or sensor anyway. I was able to purchase the pilot assembly only to find that the ceramic sensing element and wire was not included. I have not installed it for this reason. I have noticed what appears to be two quick disconnect wires plugged into a nonmetallic square approximately 4 inches above the pilot. My guess is that this is the heat chamber sensor. Can this be removed and cleaned? Are there any test points or a schematic I can get my hands on to try and located and diagnose the operation of these temp sensors? Or is there a site I can log on to for this information. My unit model number is Carrier 58GS100-3. Thanks
    labman's Avatar
    labman Posts: 10,580, Reputation: 551
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    #4

    Dec 11, 2005, 11:40 AM
    What you described should be one of the temperature sensors. Most of them are an on/off switch. Check it cold, and then as the furnace cycles. If it is the high temperature shut off it should be closed when cool and open when hot. Could be backwards, but that is more how one to turn on the blower would work.

    It sounds like you have been in the furnace enough to have found the schematic or instructions that should be there. If not, you need to hunt up Carrier's website or hotline. You could try to talk to a local Carrier dealer. Most likely they will want to send out a technician in a week or 2. They could be busy enough to give you a book to get rid of you.
    RobC's Avatar
    RobC Posts: 4, Reputation: 1
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    #5

    Dec 11, 2005, 02:03 PM
    Carrier split - heat burners cycle on and off
    Thanks, when cold, across the sensor I read approx 0 ohms. Should I be reading a voltage drop instead of ohms? If I remove one of the leads from the sensor, the blower turns on. I have no problems with the blower so I guess that this sensor is not the issue. However, when the chamber is hot and the blower is on, the switch appears to have the same reading. Is there another sensor that monitors the burner jets to turn the gas valve on and off? I have not found any documentation in or around the unit. The control logic appears to be typical however without knowing what or where I am checking, I am just guessing. I have tried Carrier sites but to no avail. Worse comes to worse, I will have to call for service. Thatís one way to figure it out.
    labman's Avatar
    labman Posts: 10,580, Reputation: 551
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    #6

    Dec 11, 2005, 04:15 PM
    I don't have that wide of experience, and I am not sure what is happening. Can you pull the sensor out, clean it, and heat it with a hair drier and see if it ever opens when it gets hot enough? If the blower starts when you break the circuit, how does it start otherwise while the circuit is still connected?
    RobC's Avatar
    RobC Posts: 4, Reputation: 1
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    #7

    Dec 12, 2005, 03:34 AM
    Carrier split - heat burners cycle on and off
    With the exception that the burner jets cycle on and off, the operations of the unit appears to be OK. Maybe Iím missing somethingÖ. Are the burners supposed to cycle? I donít think they should or remember it doing it before. I guess my main concern is that the main valve assembly may be faulty. Sounds like a piece of change$$. Do you know if there is a burner flame sensor and where it would be located? Thanks.
    labman's Avatar
    labman Posts: 10,580, Reputation: 551
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    #8

    Dec 12, 2005, 06:11 AM
    The furnace could depend on the sensor for the pilot light, assuming if the pilot light is on, the main burner is going to light. Before we had reliable, modern controls like them, houses used to explode once in a while. If there is one, it is going to be near the main burner, but I can't say what it will look like. If it was slow to respond, the burners would come on and go off again fairly quickly. After several cycles, it might finally respond allowing a normal cycle. I am not sure how they work, and don't know if a hair drier would get hot enough to check one.

    Worse than the main gas valve, would be a problem on the control board. I paid $300 for one for my 10 year old Bryant.

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