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jeffkarp Member Posts: 2
I have a miller oil furance, and the burn will come on, heat the fire box and then the blow will run for 3 to 4 minutes then it goes into reset mode.  I check the cad cell and its getting 1800 ohms when it see light.  replace the high limit switch and check the aux switch.  I bypassed the filter and ran number1 heating oil to it.  And I also replaced the nossle.  can someone pleassseeee help


  • chapchap70
    chapchap70 Member Posts: 139
    I am having trouble understanding what you are trying to describe

    1800 ohms seems to be too much resistance when the cad cell sees the flame but I am not understanding what is going on.

    Is the oil burner running for 30-45 seconds then locking out on safety? 

    Is the burning running for a few minutes while the blower comes on then then shutting off  because the fan/limit control reaches high limit then blower continues to run until the plenum is cooled; turning the burner on again?

    Why did you change out the "high limit switch"?

    If the oil burner produces a flame, what would bypassing the oil filter or changing the nozzle do to help the problem?
  • jeffkarp
    jeffkarp Member Posts: 2

    The furance starts like its surpose to, then after it heats up,  the blower kicks on and starts blow heat throught out the house.  After about 4minutes of running the burner shuts its self off and goes into lockout mood.  I ran a straight line  to the furance to bypass the filter and such to make sure that the problem wasnt there.  Then I changed the high limit switch, because I thought that might be the problem, Then I checked the cad cell, and then I checked the filter in the furance pump to make sure that it wasnt clogged.  But it does the same thing every time.
  • icesailor
    icesailor Member Posts: 7,265

    You are far enough long to use an ohm meter. 1800 is way too high and at the end of what most controls can handle without dropping out. What you have changed is not part of the problem.

    Put the ohm meter on and start the burner. See if the resistance is OK, like 500 or 600 ohms. But below the 1800. If it is lower, and the reading goes up when the fan starts, you probably have a cracked heat exchanger and the blower is pushing air into the exhaust. You can't see it with your eye but the "eye" can see it.

    You have a combustion problem. Excessively high draft or too much air through the burner can cause it.

    You need a combustion analyzer on it.
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